S1.4.2 Relative atomic mass and molecular mass
Summary
TLDRThis video by MSJ Chem explores the concepts of relative atomic mass (RAM) and relative molecular mass (RMM). RAM is the weighted average mass of an element's isotopes, referenced to carbon-12, and is dimensionless. Elements' RAMs, like hydrogen (1.01) and iron (55.85), are compared to carbon-12 to understand their mass relationships. The video demonstrates calculating RAM using isotopes' abundances, such as for magnesium, yielding 24.32. RMM is the sum of an element's RAMs in a molecule, like H2O's 18.02. The script also introduces relative formula mass, used for ionic compounds like NaCl, calculated at 58.44, emphasizing the importance of these concepts in chemistry.
Takeaways
- 🌟 The relative atomic mass is the weighted average mass of an element's naturally occurring isotopes, compared to 1/12 the mass of a carbon-12 atom.
- 📊 Relative atomic masses are dimensionless and are based on the carbon-12 isotope, which is exactly 12 atomic mass units (amu).
- 🔍 Elements' relative atomic masses can be found on the periodic table, with examples given such as hydrogen (1.01), magnesium (24.31), chlorine (35.45), and iron (55.85).
- 🌐 Relative atomic masses are not whole numbers due to being a weighted average of isotopes, reflecting the natural isotopic distribution of elements.
- ⚖️ Relative atomic mass values allow for the comparison of atomic masses to carbon-12 and to each other, as demonstrated with hydrogen, helium, magnesium, phosphorus, and chlorine.
- 🧪 The calculation of an element's relative atomic mass involves multiplying each isotope's mass by its relative abundance, then summing and normalizing these values.
- 🔢 For magnesium, the relative atomic mass is calculated using the abundances of its isotopes, resulting in a value of 24.32 amu.
- 💧 Relative molecular mass is the sum of the relative atomic masses of the atoms within a molecule, such as H2 (2.02 amu), H2O (18.02 amu), and C2H6 (30.08 amu).
- 🌐 Similar to relative atomic mass, relative molecular mass is also a dimensionless quantity and does not have units.
- 🧩 The term 'relative formula mass' is used for ionic compounds and other substances that do not form discrete molecules, such as sodium chloride (NaCl) with a relative formula mass of 58.44 amu.
- 📚 The video concludes with an introduction to the concept of molar mass, which will be the focus of the next video.
Q & A
What is the definition of relative atomic mass?
-Relative atomic mass is the weighted average mass of the naturally occurring isotopes of an element, relative to 1/12 the mass of an atom of carbon-12.
Why is carbon-12 used as the reference for relative atomic mass?
-Carbon-12 is used as the reference because it has a mass of exactly 12 atomic mass units (amu), providing a convenient and precise standard for comparison.
What is the relative atomic mass of hydrogen?
-The relative atomic mass of hydrogen is 1.01.
How do relative atomic masses help in comparing the masses of different atoms?
-Relative atomic masses allow us to compare the masses of different atoms by providing a dimensionless scale where each element's mass is relative to 1/12 the mass of a carbon-12 atom.
Why are relative atomic masses not usually whole numbers?
-Relative atomic masses are not usually whole numbers because they represent a weighted average of the naturally occurring isotopes of an element.
How can you calculate the relative atomic mass of an element like magnesium?
-You calculate the relative atomic mass of an element by multiplying the mass of each isotope by its relative abundance, summing these values, and then dividing by 100.
What is the relative atomic mass of chlorine?
-The relative atomic mass of chlorine is 35.45.
What is the concept of relative molecular mass?
-Relative molecular mass is the weighted average mass of a molecule, relative to 1/12 the mass of an atom of carbon-12, and is calculated by summing the relative atomic masses of the atoms in the molecule.
How is the relative molecular mass of a molecule like H2O calculated?
-The relative molecular mass of H2O is calculated by adding the relative atomic masses of two hydrogen atoms (1.01 each) and one oxygen atom (approximately 16.00), resulting in a total of 18.02.
What is the difference between relative molecular mass and relative formula mass?
-Relative molecular mass is used for molecules, while relative formula mass is used for compounds that do not form molecules, such as ionic compounds.
Can you provide an example of calculating the relative formula mass of an ionic compound?
-Yes, the relative formula mass of sodium chloride (NaCl) is calculated by adding the relative atomic masses of sodium (approximately 22.99) and chlorine (35.45), resulting in 58.44.
Outlines
🔍 Introduction to Relative Atomic and Molecular Mass
This paragraph introduces the concepts of relative atomic mass (RAM) and relative molecular mass (RMM). RAM is defined as the weighted average mass of an element's naturally occurring isotopes, referenced to 1/12th the mass of a carbon-12 atom, which is exactly 12 atomic mass units (amu). The RAM values of elements like hydrogen, magnesium, chlorine, and iron are provided as examples, showing they are not whole numbers due to being a weighted average. The paragraph also explains that RAM values are dimensionless and can be used to compare the masses of different atoms to carbon-12 and to each other. The process of calculating RAM using the isotopes of magnesium is briefly mentioned, and the concept of RMM is introduced as the sum of RAMs of atoms in a molecule, with examples of H2, H2O, and C2H6.
📚 Calculation and Application of Relative Formula Mass
The second paragraph delves into the calculation of relative atomic mass for elements like magnesium, using the abundance and mass of its isotopes. It clarifies that RMM is the sum of the RAMs of all atoms in a molecule, and provides examples of how to calculate the RMM of simple molecules like H2, H2O, and C2H6. The paragraph also introduces the term 'relative formula mass,' which is used for compounds that do not form discrete molecules, such as ionic compounds. The relative formula mass of sodium chloride (NaCl) is given as an example, calculated by adding the RAMs of sodium and chlorine. The video concludes with a mention of the next topic to be covered: molar mass.
Mindmap
Keywords
💡Relative Atomic Mass
💡Isotopes
💡Periodic Table
💡Weighted Average
💡Dimensionless
💡Relative Molecular Mass
💡Diatomic Molecule
💡Ionic Compound
💡Relative Formula Mass
💡Molar Mass
Highlights
The video discusses the concept of relative atomic mass and relative molecular mass.
Relative atomic mass is defined as the weighted average mass of an element's naturally occurring isotopes, relative to 1/12 the mass of a carbon-12 atom.
The relative atomic mass scale is based on carbon-12, which has an exact mass of 12 atomic mass units (amu).
Relative atomic masses of elements are dimensionless and can be found on the periodic table.
Examples of relative atomic masses include hydrogen at 1.01, magnesium at 24.31, chlorine at 35.45, and iron at 55.85.
Relative atomic masses are not whole numbers due to being a weighted average of isotopes.
The concept of comparing atomic masses using their relative atomic masses is introduced.
Relative atomic mass values allow for comparisons of atomic masses to carbon-12 and to each other.
A method to calculate the relative atomic mass of an element using its isotopes and their abundances is explained.
The calculation example for magnesium's relative atomic mass is provided, resulting in 24.32.
Relative molecular mass is the weighted average mass of a molecule, also relative to 1/12 the mass of a carbon-12 atom.
Relative molecular mass is calculated by summing the relative atomic masses of the atoms in a molecule.
Examples given for calculating relative molecular masses include H2, H2O, and C2H6.
Relative molecular masses, like relative atomic masses, are unitless.
The term 'relative formula mass' is used for compounds that do not form molecules, such as ionic compounds.
The calculation of relative formula mass for sodium chloride (NaCl) is demonstrated as an example.
The video concludes with a look forward to the next topic, molar mass, in the subsequent video.
Transcripts
this is msj chem in this video i'll be
looking at
relative atomic mass and relative
molecular mass
so we'll start with the definition of
relative atomic mass
relative atomic mass is the weighted
average mass of the naturally occurring
isotopes of an element
relative to 1 12 the mass of an atom of
carbon 12
the relative atomic mass scale is based
on the isotope carbon-12
which has a mass of exactly 12 amu
on the left we can see the symbol for
the isotope carbon-12
it is to this isotope that the masses of
other atoms
are compared the relative atomic masses
of
elements can be found on a periodic
table
hydrogen has a relative atomic mass of
1.01
magnesium has a relative atomic mass of
24.31
chlorine has a relative atomic mass of
35.45
and iron has a relative atomic mass of
55.85
as you can see relative atomic masses
are not usually whole numbers
this is because they are a weighted
average of the naturally occurring
isotopes of that element
we'll look at how to calculate relative
atomic mass later
in the video note that because this is a
relative scale
relative atomic masses are dimensionless
in that they have no units
next we look at how the masses of atoms
can be compared
using their relative atomic masses in
this table we have the element
its relative atomic mass and its mass
compared to the isotope carbon-12
the first example is hydrogen which has
a relative atomic mass of 1.01
this means that an atom of hydrogen is
approximately
12 times lighter than an atom of carbon
12.
next is helium which has a relative
atomic mass of 4.00
which means that the helium atom is
approximately three times lighter
than an atom of carbon 12. next is
magnesium
with a relative atomic mass of 24.31
which means that a magnesium atom is
approximately two times heavier than an
atom of carbon 12.
next is phosphorus with a relative
atomic mass of 30.07
and is approximately 2.5 times heavier
than an atom of carbon
12 and finally we have chlorine with a
relative atomic mass of 35.45
which is approximately three times
heavier than an atom of carbon 12.
we can also use relative atomic masses
to compare the masses of different atoms
for example an atom of helium is
approximately four times heavier than an
atom of hydrogen
and an atom of magnesium is
approximately six times heavier
than an atom of helium so from this
table
we can see that relative atomic mass
values can be used to compare the masses
of different atoms
to carbon-12 as well as each other
next we look at how to calculate the
relative atomic mass of an
element in this table we have the three
naturally occurring
isotopes of magnesium they are magnesium
24
magnesium 25 and magnesium 26.
in the second column we have their
relative abundances
so we'll use these values to calculate
the relative atomic mass of magnesium
to do this we multiply the mass of each
isotope by its relative abundance
we then add these together and divide by
100
to get the mass of one atom so using
these values
we get a relative atomic mass for
magnesium
of 24.32 note that because it's a
relative scale
there are no units in this slide we'll
look at relative molecular mass
relative molecular mass is the weighted
average mass of a molecule
relative to 1 12 the mass of an atom of
carbon 12.
the relative molecular mass of a
molecule is the sum of the relative
atomic masses
of the atoms in the molecule so in this
table we have some examples
starting with hydrogen which is a
diatomic molecule consisting of two
hydrogen atoms bonded together
each hydrogen atom has a relative atomic
mass of 1.01
so we multiply this value by 2 to get a
relative molecular mass
of 2.02 the next example is h2o
which is composed of two hydrogen atoms
and one
oxygen atom if we add up the relative
atomic masses of the atoms
we get a relative molecular mass of
18.02
and finally we have c2h6 which is ethane
a molecule of ethane is composed of two
carbon atoms
and six hydrogen atoms if we add up the
relative atomic masses of the atoms
we get a relative molecular mass of
30.08
note that like relative atomic masses
relative molecular masses
do not have units we'll end the video
by looking at relative formula mass the
term
relative formula mass is mostly used for
compounds
that do not for molecules such as ionic
compounds
on the left we can see the structure of
an ionic compound
because ionic compounds do not form
molecules
we use the term relative formula mass
instead of
relative molecular mass for example the
relative formula mass of sodium chloride
which is nacl is 58.44
we calculate this by adding together the
relative atomic masses of
sodium and chlorine so that's all from
this video
in the next video i'll be looking at
molar mass
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