Vapor Pressure | Raoult's Law | Solution Class 12
Summary
TLDRThis video explains key concepts related to vapor pressure and Raoult's law, starting with the distinction between volatile and non-volatile substances. It covers evaporation, vapor pressure in closed containers, and the equilibrium between evaporation and condensation. The video delves into how temperature and intermolecular forces influence vapor pressure. It introduces Raoult's law, describing how partial vapor pressure is directly proportional to mole fractions in a solution. The video concludes with applications of Raoult's law, solving problems on total vapor pressure using Dalton's law, and includes numerical examples for clarity.
Takeaways
- 🌡️ Volatile substances, such as water, gasoline, and ethanol, evaporate at temperatures above or below room temperature, whereas non-volatile substances like salt and sugar do not.
- 💧 Evaporation is the process where surface molecules of a liquid, which have higher kinetic energy, escape and become vapor.
- 🔄 The difference between evaporation and boiling is that boiling occurs at a fixed temperature, while evaporation can happen at any temperature.
- 💭 Vapor pressure is defined as the pressure exerted by vapors on the surface of a liquid in a closed container and is a result of evaporation within that container.
- 🔆 Temperature has a direct relationship with vapor pressure; as temperature increases, so does vapor pressure.
- 🔗 The nature of the liquid and its intermolecular forces inversely affect vapor pressure; liquids with weaker intermolecular forces have higher vapor pressures.
- 🌀 Raoult's Law states that the partial vapor pressure of a liquid in a solution is equal to the vapor pressure of the pure liquid multiplied by its mole fraction in the solution.
- 📉 Dalton's Law is used to calculate the total vapor pressure of a solution, which is the sum of the partial vapor pressures of each volatile component in the solution.
- 📊 Graphically, Raoult's Law can be represented by a straight line where the vapor pressure of a liquid is directly proportional to its mole fraction.
- 🧪 Numerical problems involving Raoult's Law involve calculating the partial vapor pressure of solvents in solutions using their mole fractions and pure vapor pressures.
Q & A
What is a volatile substance?
-A volatile substance is one that evaporates into a gas at room temperature or higher. Examples include water, gasoline, and ethanol.
What is the difference between volatile and non-volatile substances?
-Volatile substances evaporate at room temperature or below, such as water and ethanol, while non-volatile substances, like salt and sugar, do not evaporate at these conditions.
What is evaporation?
-Evaporation is the process by which a liquid transforms into vapor. It occurs when surface molecules with high kinetic energy escape from the liquid into the air.
How is evaporation different from boiling?
-Boiling occurs at a fixed temperature (e.g., 100°C for water), while evaporation can occur at any temperature, even as low as 0°C.
What is vapor pressure?
-Vapor pressure is the pressure exerted by the vapors of a volatile liquid on the surface of the liquid when in a closed container at a given temperature.
How does temperature affect vapor pressure?
-There is a direct relationship between temperature and vapor pressure. As temperature increases, vapor pressure also increases, such as water at 60°C having a higher vapor pressure than at 40°C.
What are the factors affecting vapor pressure?
-Two main factors affect vapor pressure: temperature (direct relationship) and the nature of the liquid (inverse relationship with intermolecular forces).
What is Raoult’s Law?
-Raoult’s Law states that the partial vapor pressure of a liquid in a solution is directly proportional to the mole fraction of that liquid in the solution.
How do you calculate total vapor pressure in a solution?
-Total vapor pressure of a solution can be found using Dalton’s Law, which states that it is the sum of the partial vapor pressures of all the volatile components in the solution.
What is the difference between Raoult's Law and Dalton's Law?
-Raoult’s Law explains the partial vapor pressure of a single volatile component in a solution, while Dalton’s Law is used to find the total vapor pressure of a solution by summing all partial pressures.
Outlines
💧 Understanding Vapor Pressure and Evaporation
This section introduces the fundamental concepts of vapor pressure and evaporation. It distinguishes between volatile and non-volatile substances, using water, gasoline, and ethanol as examples of the former, and salt, sugar, and urea as examples of the latter. The script explains evaporation as a process where surface molecules with higher kinetic energy escape from the liquid to become vapor. It contrasts evaporation with boiling, highlighting that boiling occurs at a fixed temperature, whereas evaporation can happen at any temperature. The concept of vapor pressure is then introduced as the pressure exerted by vapors on the surface of a liquid in a closed container, reaching equilibrium when the rate of evaporation equals the rate of condensation.
🔍 Factors Affecting Vapor Pressure
The second paragraph delves into the factors that influence vapor pressure. It establishes a direct relationship between temperature and vapor pressure, illustrating that water at 60 degrees Celsius has a higher vapor pressure than at 40 degrees Celsius. It also discusses the nature of the liquid and its intermolecular forces, showing an inverse relationship with vapor pressure. Using water and acetone as examples, it explains that acetone has a higher vapor pressure at 40 degrees Celsius due to its weaker intermolecular forces compared to water's strong hydrogen bonding. The paragraph reinforces the understanding that vapor pressure is also known as equilibrium vapor pressure and is independent of the shape of the container.
🌡️ Vapor Pressure of Pure and Binary Liquids
This part of the script teaches the concept of vapor pressure for pure liquids (P naught) and binary liquids. It describes how a pure liquid in a closed container will exert vapor pressure over time and how combining two liquids in one container results in a solution with each liquid exerting partial pressure. The difference between the vapor pressure of a pure liquid (P naught) and the partial pressure (P) of a liquid in a solution is clarified. The script then introduces Raoult's Law, stating that the partial vapor pressure of a liquid in a solution is directly proportional to its mole fraction in the solution. The law is applied to calculate the partial vapor pressures of both liquids in a solution.
📊 Graphical Representation of Raoult's Law
The fourth paragraph provides a graphical representation of Raoult's Law. It describes how to plot vapor pressure against mole fraction for two volatile liquids, with one being more volatile than the other. The script explains that as mole fraction decreases from left to right on the x-axis, the partial vapor pressure also decreases for the first liquid, while it increases for the second liquid. The resulting graph consists of two straight lines representing the partial vapor pressures of each liquid in the solution. The total vapor pressure of the solution is the sum of the partial vapor pressures of both liquids.
📚 Application and Numerical Problems of Raoult's Law
The final paragraph applies Raoult's Law to solve numerical problems related to vapor pressure. It presents a scenario where glucose is added to water and calculates the partial vapor pressure of water in the resulting aqueous solution. The calculation involves determining the number of moles of each component and applying Raoult's Law to find the partial vapor pressure of water. Another example involves an ideal solution of ethanol and methanol, where the total vapor pressure is calculated by summing the partial vapor pressures of each component, determined using their respective mole fractions and pure vapor pressures.
Mindmap
Keywords
💡Volatile Substances
💡Non-Volatile Substances
💡Evaporation
💡Vapor Pressure
💡Boiling Point
💡Equilibrium
💡Intermolecular Forces
💡Raoult’s Law
💡Partial Vapor Pressure
💡Dalton’s Law
Highlights
Definition of volatile substances that evaporate at room temperature or below.
Definition of non-volatile substances that do not evaporate at room temperature or below.
Explanation of evaporation as a process occurring in volatile substances like water.
Description of the kinetic energy difference between surface and bulk molecules in a liquid.
Evaporation defined as the transformation of a liquid into vapors due to high kinetic energy of surface molecules.
Difference between evaporation and boiling point clarified, with boiling occurring at a fixed temperature.
Introduction to vapor pressure as evaporation within a closed container.
Vapor pressure defined as the pressure exerted by vapors on the surface of a liquid at a given temperature.
Factors affecting vapor pressure include temperature and the nature of the liquid or intermolecular forces.
Vapor pressure does not depend on the shape of the container.
Concept of vapor pressure of pure liquids (P naught) explained.
Introduction to partial vapor pressure in solutions containing more than one volatile liquid.
Definition of partial vapor pressure as the pressure exerted by one liquid in a solution.
Explanation of Raoult's Law stating that the partial vapor pressure of a liquid in a solution is directly proportional to its mole fraction.
Graphical representation of Raoult's Law showing the relationship between vapor pressure and mole fraction.
Application of Raoult's Law in calculating the partial vapor pressure of liquids in a solution.
Difference between Raoult's Law and Dalton's Law in calculating total vapor pressure of a solution.
Numerical problem-solving using Raoult's Law to find the vapor pressure of aqueous solutions.
Numerical problem-solving using Raoult's Law to calculate the total vapor pressure of a solution containing ethanol and methanol.
Transcripts
vapor pressure and routes law firstly we
will learn some basic concepts like
volatile substances and non-volatile
substances a volatile substance is one
that evaporates into a gaze through
temperature or below for example water
gasoline ethanol Etc are all volatile
substances
while non-volatile substances is the one
that doesn't evaporate into a gaze at
room temperature or below for example
salt sugar
urea Etc are all non-volatile substances
the second concept is evaporation
remember that evaporation only occurs in
volatile substances like water now
consider this open object which contains
water we know that the molecules at the
surface are called surface molecules and
the molecule inside are at the bottom
are called bulk molecules the bulk
molecules have low kinetic energy due to
which they move slowly
while the surface molecules have high
kinetic energy due to which they move
fast now listen carefully after some
time the surface molecules would leave
the liquid and would Escape into the
vapors
let me repeat it after some time the
surface molecules would leave the liquid
and would Escape into the vapories this
process is known as evaporation
if you ask me why these surface
molecules leave the liquid the answer is
simple it is because they have high
kinetic energy due to which they leave
the liquid surface and become Vapors
therefore we Define evaporation as the
process by which a liquid is transformed
into Vapors is called evaporation
here let me ask you one important
question what is the difference between
evaporation and boiling point
well boiling occurs at fixed temperature
for example the boiling point of water
is fixed which is 100 degree Centigrade
while evaporation can occur at any time
pressure
for example water can even evaporate its
0 degree centigrade
just remember that every volatile liquid
evaporates at any temperature
now let me teach you the basic concept
of vapor pressure if I teach the
complete concept of vapor pressure in
one statement then I would say vapor
pressure is evaporation in a closed
container this statement explain the
whole philosophy of vapor pressure now
consider this close container which
contain a volatile liquid
we already know that volatile liquid
evaporates I mean these surface
molecules convert into vapor due to high
kinetic energy hence I write evaporation
occurs and which surface molecules
evaporate into vapories
after some time these Vapors would
condense and would fall back into the
liquid
hence I write condensation occurs in
which Vapors convert to liquid here
inside this close container evaporation
occurs I mean liquid converts to Vapor
secondly condensation occurs I mean
Vapors convert to liquid now a time will
reach when rate of evaporation would be
equal to the rate of condensation this
stage is called equilibrium
to make this concept more simple we say
that at equilibrium if 10 molecules of
liquid evaporates into vapories then 10
molecules of vapor condensed into liquid
now listen carefully here I am going to
teach you the best part of vapor
pressure which a lot of talented people
are missing
we know that these Vapors condense and
fall back into the liquid
when these Vapors fall back into the
liquid surface they exert force or unit
area of the liquid surface let me repeat
it when these Vapors fall back into the
liquid surface they exert force on unit
area of the liquid surface
we know that pressure is equal to force
on a unit area this pressure is exerted
by the vapors so we call it vapor
pressure
remember that vapor pressure is the
pressure of vapors on the surface of
liquid therefore we Define vapor
pressure as the pressure exerted by
Vapors on the surface of the liquid at a
given temperature is called vapor
pressure let me repeat it the pressure
exerted by Vapors on the surface of the
liquid at a given temperature is called
vapor pressure remember that vapor
pressure is also known as equilibrium
vapor pressure
hence notary down that the pressure of
vapors on the surface of liquid is
called vapor pressure
now let me teach you factors affecting
vapor pressure
first Factor affecting vapor pressure is
temperature there is direct relationship
between temperature and vapor pressure
for example water is 60 degree
Centigrade has more vapor pressure than
water at 40 degree centigrade
the second factor is nature of liquid or
intermolecular forces there is inverse
relationship between intermolecular
forces and vapor pressure
for example consider water and acetone
at 40 degree Centigrade the vapor
pressure of acetone is more than water
because acetone has weak intermolecular
forces like London dispersion forces
while the water has strong
intermolecular forces like hydrogen
bonding also remember that vapor
pressure doesn't depend upon the shape
of objects
hence noted down all these important
points
now we will learn the most important
concept of this lecture
firstly we will learn the vapor pressure
of pure liquid P naught remember that I
take volatile liquids now consider first
liquid in this closed container and
secant liquid in this close container
this container contains one type of
molecules and this container also
contains one type of molecules after
some time the volatile liquid will exert
vapor pressure on the surface of the
liquid
we write vapor pressure of pure liquid 1
is equal to P1 naught
similarly the volatile liquid in the
second container would exert vapor
pressure on the surface of the liquid
we write paper pressure of pure liquid 2
is equal to P2 naught
hence we learned that e naught is the
vapor pressure of pure liquid or the
vapor pressure of only one liquid
let me repeat this important point
P naught is the vapor pressure of pure
liquid or the vapor pressure of only one
liquid
hence noted down this very very
important point
now let me teach you the Weber pressure
of binary liquids are two liquids
consider liquid number one and liquid
number two enclosed container from the
previous example we know that vapor
pressure of pure liquid 1 is equal to P1
naught
and the vapor pressure of pure liquid 2
is equal to P2 naught now I combine
these two liquids in one closed
container I get a solution
this solution contains two volatile
liquids liquid number one and liquid
number two I mean this solution contains
two types of different molecules
now listen carefully and the solution
liquid one exert partial pressure P1 and
liquid to exert partial pressure P2 let
me repeat it
and the solution liquid 1 exert partial
pressure P1 and liquid to exert partial
pressure P2
here let me ask you one of my favorite
questions what is the difference between
P1 naught and P1 can you guess the
answer
well it is super easy P1 naught is the
vapor pressure of pure liquid one
for example P1 naught is the vapor
pressure of pure liquid and this
container while P1 is the partial
pressure of liquid 1 and a solution
for example E1 is the partial pressure
of liquid one and this container of
solution
similarly P2 naught is the vapor
pressure of pure liquid 2 and P2 is the
partial pressure of liquid number two
and a solution
now what is meant by partial vapor
pressure well partial vapor pressure
means pressure of one liquid and a
solution for example here are two
liquids in this solution
P1 is the partial vapor pressure of
liquid one and P2 is the partial vapor
pressure of liquid number two
hence noted down all these important
points
now let me ask you the most important
question of this lecture
how can we find the partial vapor
pressure of P1 and partial vapor
pressure of P2 and a solution
well to find partial vapor pressure in a
solution here comes the routes Baba he
states that partial vapor pressure of
any liquid is directly proportional to
mole friction of that liquid and a
solution
so we write partial vapor pressure of P1
in a solution is directly proportional
to its mole fraction X1
if you want to learn more about mole
friction then watch our video and its
link is given in the description
now to eliminate the sign of
proportionality we have to put some sort
of constant
let this Escape I write partial vapor
pressure of liquid 1 is equal to K into
its mole fraction let this is equation
number one
remember that mole friction of liquid 1
x 1 is equal to number of moles of
liquid 1 upon number of moles of liquid
1 less number of moles of liquid two now
we will find the value of constant k
let's consider only liquid 1 in this
container its mole friction X1 is equal
to 1. now the vapor pressure of pure
liquid is P1 naught is equal to K into
its mole friction
our P1 naught is equal to K N to 1 we
get P1 naught is equal to K
that this is equation number two now I
plug in equation number to n equation
number one I get P1 is equal to P 1
naught into its mole friction it means
that partial vapor pressure of liquid 1
in a solution is equal to vapor pressure
of pure liquid 1 and to its mole
friction
similarly partial vapor pressure of
liquid to NS solution is equal to vapor
pressure of pure liquid number 2 and to
add small friction in a solution
therefore we Define route's law as
partial vapor pressure of a liquid or
solvent and a solution is equal to the
vapor pressure of pure solvent and its
mole friction in a solution
let me repeat it
partial vapor pressure of a liquid are
solvent in a solution is equal to the
vapor pressure of pure solvent into its
mole friction in a solution
thus we learned that routes law explain
the partial pressure of a volatile
liquid in a solution
hence noted down this important concept
now let me teach you the application of
routes law
well consider the solution of liquid
number one and liquid number two from
the previous example we know that there
are two types of volatile liquids in
this solution we already learned that
route slow helps us to find the partial
vapor pressure of liquid number one in a
solution which is P1 is equal to P1
naught into its mole friction and routes
law helps us to find the partial vapor
pressure of liquid number 2 which is P2
is equal to P2 not into its small
friction
so let me ask you can routes law find
the total vapor pressure of a solution
the answer is no route slope cannot
explain or find the total vapor pressure
of a solution it can only find or
explain the partial vapor pressure of
one liquid or one component in a
solution
now the second question is then how can
we find the total vapor pressure of a
solution
well with the help of delton's law we
can find the total vapor pressure of a
solution the total vapor pressure of a
solution is equal to partial vapor
pressure of first Liquid Plus partial
vapor pressure of second liquid this
remember that route slow explains the
partial vapor pressure of one liquid in
a solution and delton's law explained
the total vapor pressure of a solution
hence noted down this basic difference
between routes law and deltan's law now
I will teach you graphical
representation of routes law let's
consider two volatile liquids liquid
number one and liquid number two
let liquid number one is more volatile
than liquid number two according to
routes law vapor pressure of quid number
one P1 is directly proportional to its
mole friction
similarly vapor pressure of quid number
two P2 is directly proportional to its
mole friction
now I will teach you the graph of routes
law using my personal method I draw two
vertical arrows or y-axis we take vapor
pressure on y-axis
secondly I draw a horizontal line or x
axis
we take more friction on x-axis
that this is the left side and this is
the right side of x-axis
let I take more friction of first liquid
at left side and more friction of second
liquid at the right side
now I will write some important points
believe me no one can teach you these
magic lines
I write from left to right along x axis
mole friction of first liquid decreases
we know that when mole friction
decreases partial vapor pressure P1 also
decreases
if more friction are first liquid X1 is
1 here then mole friction of first
liquid is 0 there because it is
constantly decreasing along x axis
now this is the vapor pressure of first
liquid which is P1 naught we know that
both friction of first liquid decreases
from left to right
here partial vapor pressure P1 also
decreases and we get this straight line
it is partial vapor pressure P1 of
liquid one
secondly from left to right mole
friction of secant liquid increases we
know that if more friction of second
liquid is 0 here then its mole friction
is 1 there
we also know that pure vapor pressure of
secret liquid P1 naught is here because
it is less volatile than first liquid
now from left to right mole friction or
second liquid increases hence partial
vapor pressure of second liquid also
increases
I draw this straight line it is the
partial vapor pressure P2 of liquid
number two now what about total vapor
pressure of the solution
well this is the total vapor pressure of
the solution e total is equal to T1 plus
P2
hence rotate down this graphical
representation of routes law
finally let me teach you some important
numerical problems from je main exam
consider this question 18 gram of
glucose is added to
178.2 gram water find the vapor pressure
of water and Tor for the aqueous
solution firstly I write the given data
the given mass of glucose is 18 gram and
the given mass of water is
178.2 gram here is one important fact
which you must know and it is not given
in the question the vapor pressure of
pure water PW naught is equal to
760 torr now according to routes law
partial vapor pressure of water in this
aqueous solution is PW is equal to its
pure pressure and to its mole friction
our PW is equal to PW naught N2 number
of moles of water upon number of moles
of water plus number of moles of glucose
here I will calculate the molar mass of
glucose and molar mass of water the
molar mass of glucose is equal to 6
carbon plus 12 hydrogen plus 6 oxygen R
6 into 12 plus 12 into 1 plus 6 into 16.
I get 180 gram per mole we know that the
molar mass of water is 18 gram per mole
now I will calculate the number of moles
the number of moles of glucose is equal
to given Mass which is 18 gram upon
molar mass which is 180 gram per mole
after calculation I get 0.1 mole
secondly the number of moles of water is
equal to
178.2 gram upon 18 gram per mole after
calculation I get
9.9 moles now we know that the vapor
pressure of pure water PW naught which
is 760 tall
secondly we know that the number of
moles of water and wo which is 9.9 mole
thirdly we know that the number of moles
of glucose NG which is 0.1 mole
so I will plug in all these three values
in this equation
after calculation I get
752.4 tar
hence the partial vapor pressure of
water and this solution is
752.4 tall
so note it down this important numerical
problem
finally consider this another question
from je main exam
the vapor pressure of ethanol and
methanol are 44.5 and 88.7 mmhg an ideal
solution is formed by mixing 60 gram of
ethanol and 40 gram of methanol at the
same temperature calculate the total
vapor pressure of the solution
well I write the given data the given
vapor pressure of pure ethanol p e
naught is equal to
44.5 mm he the given vapor pressure of
pure methanol PM naught is equal to
88.7 mm HG
the given mass of ethanol is 60 gram and
the given mass of methanol is 40 Gram
now the required value is the total
vapor pressure of solution we know that
total vapor pressure is equal to partial
pressure of ethanol plus partial
pressure of methanol
no I will find the molar mass of ethanol
and methanol we know that the molar mass
of ethanol is
c2h5oh two carbon plus 5 hydrogen plus
oxygen plus hydrogen 2 into 12 plus 5
into 1 plus 16 plus 1 I get 46 gram per
mole
secondly the molar mass of methanol is
carbon plus three hydrogen plus oxygen
plus hydrogen are 12 plus 3 into 1 plus
16 plus 1 which is equal to 32 gram per
mole now I will find the number of moles
the number of moles of ethanol is equal
to given Mass which is 60 gram upon
molar mass which is 46 gram per mole I
get 1.3 mole the number of moles of
methanol is equal to given Mass which is
40 gram upon molar mass which is 32 gram
per mole I get 1.25 mole now I will find
the partial vapor pressure of ethanol PE
which is equal to p e naught into its
mole friction
our PE is equal to p e naught number of
moles of ethanol upon total number of
moles
we know that vapor pressure of pure
ethanol is 44.5 mm-hg the number of
moles of ethanol is 1.25 upon total
number of moles
after calculation I get partial vapor
pressure of ethanol is equal to
21.8 mmhg
secondly I find the partial vapor
pressure of methanol PM which is equal
to PM naught and to its mole friction
RPM is equal to PM naught and to number
of moles of methanol upon total number
of moles we know that the vapor pressure
of pure methanol is 88.7 number of moles
of methanol is 1.3 upon total number of
moles
after calculation I get partial vapor
pressure of methanol is equal to
44.35 mm HG
finally I put partial vapor pressure of
ethanol and partial vapor pressure of
methanol and this equation number one I
write total vapor pressure of the
solution is equal to
21.8 mm-hg plus
44.35 mmhg after calculation I get
66.15 mmh GE thus the total vapor
pressure of this solution is
66.15 mmhg hence noted down the second
numerical problem
I hope that you have learned all about
vapor pressure and routes law
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