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Calcium hypochlorite tablets are added to swimming pools to kill microorganisms.

Quantitative Chemistry

Quantitative chemistry turns reactions into numbers. Practise moles, masses, yields and concentrations, and learn how equations help you calculate what you need in experiments and industry.

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Fascinating Fact:

Concentration in solutions is often measured in mol ÷ dm³. Use c = n ÷ V, with volume in dm³.

In GCSE Chemistry, quantitative chemistry is about using the mole to link particles, mass and reacting amounts. You use balanced symbol equations to find mole ratios, convert between mass and moles using relative formula mass, and calculate things like percentage yield, atom economy and solution concentration.

Key Terms

Mole: A counting unit for particles, 1 mole contains 6.02 × 10²³ particles of a substance.
Relative formula mass (Mr): The total of the relative atomic masses in a formula, used to convert between mass and moles.
Concentration: How much solute is dissolved in a given volume of solution, often measured in mol ÷ dm³.

Frequently Asked Questions

How do I calculate moles from mass in GCSE Chemistry?

To calculate moles from mass, use n = m ÷ Mr. The mass (m) must be in grams, and Mr is the relative formula mass from the periodic table.

How do I work out concentration of a solution in mol ÷ dm³?

Use c = n ÷ V, where c is concentration in mol ÷ dm³, n is moles of solute, and V is volume in dm³. Convert cm³ to dm³ by dividing by 1000.

How do you calculate percentage yield?

Percentage yield = (actual yield ÷ theoretical yield) × 100. Theoretical yield comes from moles and the balanced equation, while actual yield is what you make in the real experiment.

Try These Related Quizzes

1 .

Sometimes athletes take drugs to enhance their performance. One such drug is ephedrine which has the formula C₁₀H₁₅NO. What is the molecular mass of ephedrine?

55 amu

165 amu

40 amu

164 amu

Total carbon = 10 x 12 = 120. Total hydrogen = 15 x 1 = 15. Total nitrogen = 14 x 1 = 14. Total oxygen = 16 x 1 = 16. Add together = 165 amu

2 .

Amphetamine, another drug, has the formula C₉H₁₃N. The molecular mass is 135 amu. Calculate the percentage by mass of hydrogen in amphetamine.

9.6%

80%

10.4%

0.7%

Remember, be methodical. There are 13 hydrogen atoms in the molecule so they contribute 13 x 1 =13 amu to the total. To work this out as a percentage:13 /135 = 0.096 x 100 = 9.6%

3 .

A substance used as a cosmetic by the Ancient Egyptians has been analysed. It was found to contain 76% lead, 13% chlorine, 2.2% carbon and 8.8% oxygen. Calculate the empirical formula of this compound.

PbCl₆CO₄

Pb₄Cl₄C₂O₆

PbClCO₃

Pb₂Cl₂CO₃

Divide the % by RAM for each individual element. Divide those answers by the smallest which will give you your final ratios

4 .

Some toothpastes contain sodium fluoride. This compound has the formula NaF. Calculate the relative formula mass of this compound.

42 amu

30 amu

20 amu

The value of option 2 is also numerically correct, but it is very important to include the units in all your answers

5 .

Calculate the % by mass of fluorine in sodium fluoride.

55%

45%

48%

0.45%

19/42 x 100 = 45%. You can check this by calculating the percentage by mass of sodium - this gives 55%. 55 + 45 = 100%

6 .

A tube of toothpaste contains 1.5 g of sodium fluoride. Calculate the mass of sodium in this tube of toothpaste.

0.675 g

0.825

0.72 g

0.825 g

Again you MUST remember your units. The quick way to this answer is to use the answer to the previous question. 55% of NaF is sodium. 55% of anything is 0.55 so 1.5 x 0.55 gives you the correct answer

7 .

Calcium hypochlorite tablets are added to swimming pools to kill microorganisms. The formula of calcium hypochlorite is CaCl₂O₂. Calculate the relative formula mass of calcium hypochlorite.

143 amu

91.5 amu

45 amu

70 amu

40 + 2 x 35.5 + 2 x 16 = 143 amu

8 .

Calculate the percentage by mass of oxygen in calcium hyopchlorite.

22%

88%

11%

78%

2 x 16 /143 x 100 = 22%

9 .

Calculate the mass of oxygen in a 50g tablet of calcium hypochlorite.

39%

5.5 g

11 g

44 g

From the previous question - 22% of calcium hypochlorite is oxygen, so 0.22 x 50 gives the correct answer

10 .

Car airbags inflate when an impact causes a mixture of chemicals to react. The mixture of chemicals contains sodium azide (NaN₃) which undergoes thermal decomposition to produce sodium and nitrogen. Calculate the mass of sodium that would be produced when 170 g of sodium azide decomposes.

109.8 g

60.2 g

105.6 g

58.4 g

Remember - be methodical and first figure out what calculations would be helpful. RMM of sodium azide is 65 amu therefore by mass, there is 35.4% of Na present. The mass of Na produced from 170 g NaN₃ is 170 g x(35.4/100) = 60.18 g which rounds to 60.2 g

You can find more about this topic by visiting BBC Bitesize - Calculations in chemistry (Higher)

Author: Kate Gardiner (Chemistry Educator & GCSE Quiz Writer)

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