AQA GCSE Single Science Chemistry: Quantitative Chemistry

What does the law of conservation of mass state?

• No atoms are lost or made during a chemical reaction so the mass of the products equals the mass of the reactants.

Why might there appear to be a change in mass in a reaction?

• A reactant or product is a gas and its mass has not been taken into account.

What is the Avogadro constant?

• The Avogadro’s constant is the number of atoms, molecules or ions in a mole of a given substance

• The value of this is 6.02 x 10²³ per mole

What is a limiting reactant? How does it affect the mass of product formed?

• The reactant that is completely used up in a reaction is called the limiting reactant because it limits the amount of products.

• The mass of product formed in a reaction depends upon the mass of the limiting reactant. This is because no more product can form when the limiting reactant is all used up.

Why might it not always be possible to obtain the calculated amount of a product?

• The reaction may not go to completion because it is reversible

• Some of the product may be lost when it is separated from the reaction mixture

• Some of the reactants may react in ways different to the expected reaction.

How can you calculate percentage yield?

• Percentage yield = (Mass of product actually made/maximum theoretical mass of product) × 100

What is atom economy?

• Atom economy = (Relative formula mass of desired product/sum of relative formula masses of all reactants) × 100

• A measure of the amount of starting materials that end up as useful products.

• It is important for sustainable development and for economic reasons to use reactions with high atom economy.

What is the volume of any gas at room temperature and pressure?

• 24 dm³