Chemical Calculations

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FreeScienceLessons · AQA ChemistryMoles and relative formula mass

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Key facts

Relative formula mass (Mr)

Add up all the relative atomic masses (Ar) in the formula.
Example: H₂O = (2×1) + 16 = 18. CO₂ = 12 + (2×16) = 44.
NaCl = 23 + 35.5 = 58.5.
Ar values: H=1, C=12, N=14, O=16, Na=23, Mg=24, Al=27, S=32, Cl=35.5, K=39, Ca=40, Fe=56, Cu=64.

Moles

Number of moles = mass (g) ÷ Mr
Rearranged: mass = moles × Mr. Or: Mr = mass ÷ moles.
Avogadro's constant: 6.02 × 10²³ particles per mole.
In reactions: use molar ratios from balanced equation.
Example: CaCO₃ → CaO + CO₂. 1 mol CaCO₃ gives 1 mol CO₂.
If 50g CaCO₃: moles = 50 ÷ 100 = 0.5 mol → 0.5 mol CO₂.

Concentration

Concentration (mol/dm³) = moles ÷ volume (dm³)
Volume in dm³ = volume in cm³ ÷ 1000.
Rearranged: moles = concentration × volume.
Example: 0.25 mol/dm³ in 200 cm³: moles = 0.25 × 0.2 = 0.05 mol.

% yield and atom economy

% yield = (actual yield ÷ theoretical yield) × 100
Atom economy = (Mr of desired product ÷ total Mr of all products) × 100
High atom economy = greener chemistry (less waste).
Example: Mg + 2HCl → MgCl₂ + H₂. Making MgCl₂: atom economy = 95 ÷ (95+2) × 100 = 97.9%

Exam questions — 4 questions · 12 marks

1 markMr calculationAQA 8462 P1 style

What is the relative formula mass of calcium carbonate, CaCO₃? (Ca=40, C=12, O=16)

A 68

B 84

C 100

D 116

4 marksMoles calculationAQA 8462 P1 style

5.6 g of calcium oxide (CaO, Mr = 56) reacts with excess hydrochloric acid. Calculate the mass of calcium chloride (CaCl₂, Mr = 111) produced. Show all working. The equation is: CaO + 2HCl → CaCl₂ + H₂O (4 marks)

Hint: Step 1: moles = mass ÷ Mr. Step 2: use molar ratio from equation. Step 3: mass = moles × Mr.

+40 XP

3 marksConcentration calculationAQA 8462 P1 style

25.0 cm³ of sodium hydroxide solution exactly neutralises 20.0 cm³ of 0.10 mol/dm³ hydrochloric acid. Calculate the concentration of the sodium hydroxide solution. The equation is: NaOH + HCl → NaCl + H₂O (3 marks)

Hint: Step 1: moles HCl = concentration × volume (in dm³). Step 2: 1:1 ratio → same moles NaOH. Step 3: concentration = moles ÷ volume NaOH (in dm³).

+30 XP

1 markAtom economyAQA 8462 P1 style

Why is a high atom economy important in industrial chemistry?

A It means the reaction is faster

B It means fewer reactants are wasted and less waste is produced, making the process more economical and sustainable

C It means the product has a higher purity

D It means the reaction is endothermic

Chemical Calculations

Relative formula mass (Mr)

Moles

Concentration

% yield and atom economy

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