Grade 10 Physical Sciences · Chemical Change

Determine empirical and molecular formulae

Find the simplest whole-number ratio of atoms in a compound from masses or percentages, then the molecular formula (CH₂O for glucose).

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Question 1.3 · May/June 2024 · Paper 2 · National (DBE) · 2 marks

1.3 Which ONE of the following is the EMPIRICAL formula of ethyl ethanoate?(2)

A $C_{2} H_{4} O$
B $C_{2} H_{2} O$
C $C_{4} H_{8} O$
D $C_{4} H_{8} O_{2}$

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More past paper questions: determine empirical and molecular formulae

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3. Question 1.5 · November 2024 · Paper 2 · Provincial · 2 marks
1.5 What is the percentage of hydrogen in a molecule of hydrogen peroxide with molar mass of $34$ g·mol $^{- 1}$ , if one mole of hydrogen reacts with one mole of oxygen?(2)
1. A $72, 72$ %
2. B $94, 12$ %
3. C $11, 11$ %
4. D $5, 88$ %
Show the memo (1 tick)Memo
1. ✓
  Correct optionHydrogen peroxide is $H_{2} O_{2}$ : one mole of $H_{2}$ ( $2$ g) with one mole of $O_{2}$ ( $32$ g).
  % H $= \frac{2}{34} \times 100 = 5, 88$ %
  
  Answer: D
  2 marks · Determine empirical and molecular formulae
4. Question 7.3 · November 2024 · Paper 2 · Provincial · 2 marks
7.3 A learner heats $5$ g of a hydrated salt until all the water of crystallisation has evaporated, leaving $3$ g of the anhydrous salt. Determine the percentage of water of crystallisation in the hydrated salt.(2)
Show the memo (2 ticks)Memo
1. ✓
  Mass of water $m (H_{2} O) = 5 - 3 = 2$ g
  1 mark · Determine empirical and molecular formulae
2. ✓
  Percentage% $H_{2} O = \frac{mass of water}{mass of hydrated salt} \times 100 = \frac{2}{5} \times 100$
  $= 40$ %
  1 mark · Determine empirical and molecular formulae
5. Question 6.1.3 · November 2024 · Paper 2 · Provincial · 5 marks
6.1 Compound Q ( $C_{X} H_{Y}$ ) reacts with oxygen according to the balanced equation:

$P C_{X} H_{Y} + 13 O_{2} \to 8 CO_{2} + 10 H_{2} O$

The molar mass of compound Q is $58$ g·mol $^{- 1}$ .
The percentage composition of compound Q is:
6.1.3 Determine the molecular formula of compound Q.(5)
Show the memo (5 ticks)Memo
1. ✓
  OPTION 1: % C divided by M(C)Mol C : Mol H
  $\frac{82 , 76}{12}$ = $6, 90$
  1 mark · Determine empirical and molecular formulae
2. ✓
  OPTION 1: % H divided by M(H) $\frac{17 , 24}{1}$ = $17, 24$
  $6, 90 : 17, 24$
  1 mark · Determine empirical and molecular formulae
3. ✓
  OPTION 1: Simplest mole ratio $1 : 2, 5$
  $2 : 5$
  Empirical formula: $C_{2} H_{5}$
  1 mark · Determine empirical and molecular formulae
4. ✓
  OPTION 1: RatioFormula mass $= 2 (12) + 1 (5) = 29$
  Ratio $= \frac{58}{29} = 2$
  1 mark · Determine empirical and molecular formulae
5. ✓
  OPTION 1: Molecular formulaMolecular formula $= C_{4} H_{10}$
  1 mark · Determine empirical and molecular formulae
6. Question 2.4.2 · November 2023 · Paper 2 · Provincial · 4 marks
Use the table of substances below to answer the following questions.
2.4 The experimental setup below is used to dry $10$ g hydrated blue copper(II) sulphate crystals, $CuSO_{4} \cdot x H_{2} O$ , to form $6, 4$ g anhydrous salt.

Answer the questions that follow.
2.4.2 Determine the number of moles, $x$ , of water of crystallisation.(4)
Show the memo (4 ticks)Memo
1. ✓
  Mass of water $CuSO_{4} \cdot x H_{2} O \to CuSO_{4} + x H_{2} O$
  $10$ g $\to$ $6, 4$ g
  $Δ m = 10 - 6, 4 = 3, 6$ g
  1 mark · Determine empirical and molecular formulae
2. ✓
  Moles of anhydrous $CuSO_{4}$ $n (CuSO_{4}) = \frac{m}{M} = \frac{6 , 4}{159 , 5} = 0, 04$ mol
  1 mark · Determine empirical and molecular formulae
3. ✓
  Moles of water $n (H_{2} O) = \frac{3 , 6}{18} = 0, 2$ mol
  1 mark · Determine empirical and molecular formulae
4. ✓
  Value of $x$ $n (CuSO_{4}) : n (H_{2} O)$
  $0, 04 : 0, 2$
  $1 : 5$
  $∴ x = 5$
  1 mark · Determine empirical and molecular formulae

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