GCSE Chemistry Quantitive Chemistry

What is relative atomic mass (Ar)?

The average mass of an atom of an element compared to 1/12th the mass of a carbon-12 atom

What is relative formula mass (Mr)?

The sum of the relative atomic masses of all the atoms in a formula

How do you calculate the relative formula mass of a compound?

Add up the Ar values of every atom in the formula (e.g. H₂O = 1 + 1 + 16 = 18)

What is the law of conservation of mass?

Mass is neither created nor destroyed in a chemical reaction – total mass of reactants = total mass of products

Why can the mass appear to change in some reactions?

If a gas is produced and escapes, or if a gas from the air is a reactant (e.g. burning magnesium)

What is a mole?

The amount of substance that contains the same number of particles as there are atoms in 12 g of carbon-12 (6.02 × 10²³ particles)

What is the formula linking mass, moles and relative formula mass?

Mass (g) = moles × Mr   or   moles = mass ÷ Mr

What is Avogadro’s constant?

6.02 × 10²³ – the number of particles in one mole of any substance

How do you calculate the number of particles from moles?

Number of particles = moles × 6.02 × 10²³

What is concentration measured in?

g/dm³ or mol/dm³

What is the formula for concentration in g/dm³?

Concentration (g/dm³) = mass of solute (g) ÷ volume of solution (dm³)

What is the formula for concentration in mol/dm³?

Concentration (mol/dm³) = moles of solute ÷ volume of solution (dm³)

How do you convert cm³ to dm³?

Divide by 1000 (e.g. 25 cm³ = 0.025 dm³)

What is percentage yield?

(Actual mass of product ÷ Maximum theoretical mass of product) × 100

Why is percentage yield never 100% in real reactions?

Some product is lost during transfer, the reaction may be incomplete, or unwanted side reactions occur

What is atom economy?

(Mr of desired product ÷ Sum of Mr of all reactants) × 100

Why is a high atom economy important?

It means less waste is produced – better for the environment and more sustainable / profitable

What is a limiting reactant?

The reactant that is completely used up first and limits the amount of product that can be formed

How do you find the limiting reactant?

Calculate the moles of each reactant and compare to the mole ratio in the balanced equation

What is the theoretical yield?

The maximum mass of product that could be formed from the limiting reactant (calculated using the balanced equation)

In a titration, what is the purpose of the indicator?

To show when the reaction is complete (the end point) by changing colour

What is the concentration of a solution if 0.5 mol of solute is dissolved in 2 dm³ of solution?

0.25 mol/dm³ (moles ÷ volume)

What does a balanced chemical equation tell you about reacting masses?

The ratio of the number of moles of each substance that react or are produced

How do you calculate the mass of a product from a given mass of reactant?

1. Calculate moles of reactant  2. Use mole ratio from equation  3. Convert moles of product to mass

What is the difference between percentage yield and atom economy?

Percentage yield measures how much product you actually get compared to the maximum possible; atom economy measures how much of the reactants end up in the desired product