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3.2 Relative formula mass and percentage by mass

5 exam questions worth 11 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.

0 / 11

1
Multiple choice [1 mark]

What is the relative formula mass of carbon dioxide, CO2? (Ar: C = 12, O = 16)

Choose one answer.

Correct Not quite: the answer is 44

Mark scheme

C

Model answer

C: 12 + (2 × 16) = 44.

2
Calculation [2 marks]

Calculate the relative formula mass of magnesium hydroxide, Mg(OH)2.

A_\mathrm{r}: Mg = 24, O = 16, H = 1

Mark scheme

  • 24 + 2 × (16 + 1) (1)
  • = 58 (1)

Model answer

M_\mathrm{r} = 24 + 2 × 17 = 58.

Your mark
3
Calculation [3 marks]

Calculate the percentage by mass of iron in iron(III) oxide, Fe2O3.

A_\mathrm{r}: Fe = 56, O = 16

Mark scheme

  • M_\mathrm{r} = (2 × 56) + (3 × 16) = 160 (1)
  • (112 ÷ 160) × 100 (1)
  • = 70% (1)

Model answer

M_\mathrm{r} = 112 + 48 = 160. Percentage of iron = 112 ÷ 160 × 100 = 70%.

Your mark
4
Calculation [2 marks]

Calculate the relative formula mass of ammonium sulfate, (NH4)2SO4.

A_\mathrm{r}: N = 14, H = 1, S = 32, O = 16

Mark scheme

  • 2 × (14 + 4) + 32 + (4 × 16) (1)
  • = 132 (1)

Model answer

M_\mathrm{r} = 36 + 32 + 64 = 132.

Your mark
5
Calculation [3 marks]

Calculate the percentage by mass of nitrogen in ammonium nitrate, NH4NO3.

A_\mathrm{r}: N = 14, H = 1, O = 16

Mark scheme

  • M_\mathrm{r} = 80 (1)
  • (2 × 14) ÷ 80 × 100 (1)
  • = 35% (1)

Model answer

M_\mathrm{r} = 14 + 4 + 14 + 48 = 80. Percentage of nitrogen = 28 ÷ 80 × 100 = 35%.

Your mark
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