GCSE Chemistry - The Mole (Higher Tier) #25

Summary

This video explains the concept of a mole in chemistry and how it is used to measure the amount of a substance. A mole represents 6.02 x 10^23 particles, known as Avogadro's constant. The video covers the formula for converting between moles, mass, and relative formula mass, with examples using elements like carbon, oxygen, and compounds like CO2. It also shows how to calculate the mass of individual elements within compounds and discusses chemical equations in terms of moles, using ratios to predict reactions. The video aims to simplify mole conversions for students.

Takeaways

• 📏 A mole is a unit used to measure the amount of a substance, similar to how meters measure distance and seconds measure time.
• 🔢 One mole contains 6.02 x 10^23 particles, which can be atoms, molecules, ions, or electrons, depending on the substance.
• 🧪 Avogadro's constant (6.02 x 10^23) relates the number of particles to the substance's relative atomic or formula mass in grams.
• 🧮 The mass of one mole of a substance is equal to its relative atomic or formula mass in grams (e.g., one mole of carbon weighs 12 grams).
• 🌡 The number of moles in a sample can be calculated by dividing the mass of the substance by its relative formula mass (Mr).
• 💡 Example: 42.5 grams of ammonia (NH3) divided by its Mr (17) equals 2.5 moles of ammonia.
• ⚖ The mass of a substance can also be found if the number of moles is known by multiplying the number of moles by its Mr.
• 🔍 You can find the mass of individual elements within a compound by multiplying the number of moles by the atomic mass of the element.
• 🧩 In chemical equations, moles are used to describe the ratios of reactants and products, making it easier to calculate amounts.
• ⚛️ Stoichiometric ratios show that if you double the amount of one reactant, the other reactants and products also double in proportion.

Q & A

What is the definition of a mole in chemistry?

A mole is a unit used to measure the amount of a chemical substance. One mole of any substance contains 6.02 × 10^23 particles, which can be atoms, molecules, ions, or electrons, depending on the substance.

Why is Avogadro's constant significant in measuring substances?

Avogadro's constant (6.02 × 10^23) is significant because it allows us to relate the number of particles in a substance to its mass. The mass of one mole of a substance corresponds to its relative atomic or formula mass in grams.

How do you calculate the mass of one mole of a substance?

The mass of one mole of a substance is equal to its relative atomic or formula mass expressed in grams. For example, one mole of carbon weighs 12 grams because its relative atomic mass is 12.

What is the formula for calculating the number of moles in a given mass of a substance?

The number of moles is calculated by dividing the mass of the substance by its relative atomic or formula mass (Mr). The formula is: moles = mass ÷ Mr.

How many moles are in 42.5 grams of ammonia (NH3)?

To calculate the number of moles in 42.5 grams of ammonia, divide 42.5 by the relative formula mass of ammonia (NH3), which is 17. This gives 2.5 moles of ammonia.

How would you rearrange the moles formula to calculate mass if the number of moles is given?

To find the mass when the number of moles is given, multiply the number of moles by the relative atomic or formula mass (Mr). The formula is: mass = moles × Mr.

What is the mass of 3 moles of carbon dioxide (CO2)?

The mass of 3 moles of carbon dioxide is calculated by multiplying the number of moles (3) by the relative formula mass of CO2, which is 44. This gives a mass of 132 grams.

How do you calculate the mass of an element within a compound?

To calculate the mass of an element within a compound, multiply the number of moles by the relative atomic mass (Mr) of that element. For example, in 3 moles of CO2, the mass of carbon is 3 × 12 = 36 grams.

How do chemical equations relate to moles?

In a chemical equation, the coefficients represent the ratios of moles of reactants and products. For example, in the reaction Mg + 2HCl → MgCl2 + H2, one mole of magnesium reacts with two moles of hydrochloric acid to produce one mole of magnesium chloride and one mole of hydrogen gas.

What would happen if you started with 2 moles of magnesium in the reaction with hydrochloric acid?

If you started with 2 moles of magnesium, you would need 4 moles of hydrochloric acid to react, and this would produce 2 moles of magnesium chloride and 2 moles of hydrogen gas.