Grade 10 Physical Sciences · Chemical Change

Use conservation of mass

Show that the total mass or number of atoms is the same before and after a reaction (no atoms are lost or gained).

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Question 1.3 · May/June 2025 · Paper 2 · National (DBE) · 2 marks

1.3 A compound

CH_{3} (CH_{2})_{5} CH_{3}

undergoes the following reaction:

CH_{3} (CH_{2})_{5} CH_{3} \to R + CH_{3} (CH_{2})_{2} CH_{3}

Which ONE of the following combinations CORRECTLY describes the type of reaction and the IUPAC name of compound R?(2)

A Type: Elimination; IUPAC name: Propane
B Type: Addition; IUPAC name: Propene
C Type: Cracking; IUPAC name: Propane
D Type: Cracking; IUPAC name: Propene

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More past paper questions: use conservation of mass

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3. Question 4.1.2 · November 2023 · Paper 2 · National (DBE) · 3 marks
4.1 Consider the cracking reaction below.

$C_{16} H_{34} \to C_{6} H_{14} + C_{6} H_{x} + 2 C_{y} H_{z}$
4.1.2 Write down the values represented by x, y and z in the equation above.(3)
Show the memo (3 ticks)Memo
1. ✓
  Value of x $x = 12$
  1 mark · Use conservation of mass
2. ✓
  Value of y $y = 2$
  1 mark · Use conservation of mass
3. ✓
  Value of z $z = 4$
  1 mark · Use conservation of mass
4. Question 4.2.4 · May/June 2022 · Paper 2 · National (DBE) · 3 marks
4.2 The cracking of a long chain hydrocarbon, $C_{10} H_{22}$ , takes place in test tube A, as shown below.

Two STRAIGHT CHAIN organic compounds, X and Z, are produced in test tube A according to the following balanced equation:

$C_{10} H_{22} (ℓ) \to 2 X (g) + Z (g)$

Answer the questions that follow.
4.2.4 Write down the molecular formula of compound Z.(3)
Show the memo (1 tick)Memo
1. ✓
  Molecular formula of Z $C_{2} H_{6}$
  
  OR $C_{4} H_{10}$ OR $C_{6} H_{14}$
  3 marks · Use conservation of mass
5. Question 6.1.2 · November 2024 · Paper 2 · Provincial · 3 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 $^{- 1}$ .
6.1.2 Use the principle of conservation of mass and determine the value $P$ .(3)
Show the memo (3 ticks)Memo
1. ✓
  Mass of reactants $P (58) + 13 (32)$
  1 mark · Use conservation of mass
2. ✓
  Mass of products $= 8 (44) + 10 (18)$
  1 mark · Use conservation of mass
3. ✓
  Value of P $58 P + 416 = 532$
  $P = 2$
  1 mark · Use conservation of mass
6. Question 6.2.3 · November 2024 · Paper 2 · Provincial · 2 marks
6.2 The balanced equation for the reaction is:

$2 Pb(NO_{3})_{2} (s) \to 2 PbO(s) + 4 NO_{2} (g) + O_{2} (g)$

Answer the questions that follow.
6.2.3 When $6, 62$ g of lead nitrate [ $Pb(NO_{3})_{2}$ ] is heated, $4, 46$ g of solid is produced. What is the mass of gas that will be produced?(2)
Show the memo (2 ticks)Memo
1. ✓
  Subtraction $6, 62$ g $- 4, 46$ g
  1 mark · Use conservation of mass
2. ✓
  Answer $= 2, 16$ g
  1 mark · Use conservation of mass

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