Grade 12 Physical Sciences · Chemical Change

Complete an equilibrium table

Fill in a table of initial, change and equilibrium amounts using the mole ratio (if 1 mol N₂ reacts, 3 mol H₂ also reacts).

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Question 1.5 · November 2024 · Paper 2 · National (DBE) · 2 marks

Various options are provided as possible answers to the following questions. Each question has only ONE correct answer. Choose the answer and write only the letter (A–D) next to the question numbers (1.1 to 1.10) in the ANSWER BOOK, e.g. 1.11 E.

1.5 Initially, an equal number of moles of hydrogen gas,

H_{2}

(g), and iodine gas,

I_{2}

(g), are mixed in a closed container. The reaction reaches equilibrium at a constant temperature according to the balanced equation.

H_{2} (g) + I_{2} (g) ⇌ 2 HI(g)

Which ONE of the following is ALWAYS TRUE at equilibrium?(2)

A $[H_{2}] = [I_{2}]$
B $[HI] = [I_{2}]$
C $[HI] = 2 [H_{2}]$
D $[H_{2}] = [I_{2}] = [HI]$

Work it out on paper first. Then mark yourself with the official memo, one tick at a time.

More past paper questions: complete an equilibrium table

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3. Question 6.2.2 · May/June 2022 · Paper 2 · National (DBE) · 5 marks
6.2 The equation below represents the reversible reaction that takes place when $NO_{2}$ (g) is converted to $N_{2} O_{4}$ (g).

$2 NO_{2} (g) ⇌ N_{2} O_{4} (g)$

Initially, $x$ mol of $NO_{2}$ (g) is sealed in a $1$ dm $^{3}$ container at $350$ K. When equilibrium is established at this temperature, $0, 81$ mol $N_{2} O_{4}$ (g) is present in the container.

Answer the questions that follow.
6.2.2 Show that the equilibrium constant for this reaction is given by $\frac{0 , 81}{( x - 1 , 62 ) ^{2}}$ .(5)
Show the memo (5 ticks)Memo
1. ✓
  Change in N2O4 $Δ n (N_{2} O_{4}) = n (N_{2} O_{4})_{e q} - n (N_{2} O_{4})_{ini} = 0, 81 - 0 = 0, 81$ mol
  1 mark · Complete an equilibrium table
2. ✓
  Mole ratioRatio $n (NO_{2}) : n (N_{2} O_{4}) = 2 : 1$
  
  Change: $NO_{2}$ $1, 62$ mol used; $N_{2} O_{4}$ $0, 81$ mol formed
  1 mark · Complete an equilibrium table
3. ✓
  Equilibrium amount of NO2 $n (NO_{2})_{e q} = n (NO_{2})_{ini} - Δ n (NO_{2}) = x - 1, 62$ mol
  1 mark · Complete an equilibrium table
4. ✓
  Divide both by 1 dm³Equilibrium concentrations: $[NO_{2}] = \frac{x - 1 , 62}{1} = x - 1, 62$ mol·dm $^{- 3}$ ; $[N_{2} O_{4}] = \frac{0 , 81}{1} = 0, 81$ mol·dm $^{- 3}$
  1 mark · Calculate concentration of a solution
5. ✓
  Correct Kc expression $K_{c} = \frac{[ N _{2} O _{4} ]}{[ NO _{2} ] ^{2}} = \frac{0 , 81}{( x - 1 , 62 ) ^{2}}$
  1 mark · Write the Kc expression
4. Question 6.3 · May/June 2025 · Paper 2 · National (DBE) · 6 marks
The reaction between powdered carbon, C(s), and carbon dioxide gas, $CO_{2}$ (g), takes place in a closed $3$ dm $^{3}$ container according to the balanced equation below.

$C(s) + CO_{2} (g) ⇌ 2 CO(g)$ $Δ H < 0$

Equilibrium is reached at temperature T °C.

The graph below, not drawn to scale, shows the curves for the mass of the reactants in the container against time.
6.3 Calculate the value of X shown on the graph.(6)
Show the memo (6 ticks)Memo
1. ✓
  (b) Change in mass of carbon $Δ m (C) = 14 - 4, 44 = 9, 56$ g
  1 mark · Complete an equilibrium table
2. ✓
  (a) Substitute 44 $n (CO_{2})_{initially} = \frac{m}{M} = \frac{41 , 2}{44} = 0, 94$ mol ( $0, 936$ )
  1 mark · Calculate number of moles
3. ✓
  (c) Substitute 12 $n (C)_{used} = \frac{m}{M} = \frac{9 , 56}{12} = 0, 80$ mol ( $0, 797$ )
  1 mark · Calculate number of moles
4. ✓
  (d) Mole ratio 1:1 $n (CO_{2})_{used} = n (C) = 0, 80$ mol ( $0, 797$ )
  1 mark · Use the mole ratio
5. ✓
  (e) Equilibrium amount $n (CO_{2})_{eq} = n (CO_{2})_{initially} - n (CO_{2})_{used} = 0, 94 - 0, 80 = 0, 14$ mol
  1 mark · Complete an equilibrium table
6. ✓
  (f) Final answer $n (CO_{2}) = \frac{m}{M}$ : $0, 14 = \frac{m}{44}$
  $X = m (CO_{2}) = 6, 16$ (g)
  1 mark · Calculate number of moles
5. Question 6.2.2 · May/June 2023 · Paper 2 · National (DBE) · 1 mark
One mole of pure hydrogen iodide gas, HI(g), is sealed in a $1$ dm $^{3}$ container at $721$ K. Equilibrium is reached according to the following balanced equation:

$2 HI(g) ⇌ H_{2} (g) + I_{2} (g)$

It is found that $0, 11$ moles of $I_{2}$ (g) are present at equilibrium.
6.2 Determine the number of moles of EACH of the following at equilibrium:
6.2.2 HI(g)(1)
Show the memo (1 tick)Memo
1. ✓
  Moles of HI at equilibrium $n (HI)_{used} = 2 n (I_{2}) = 2 (0, 11)$
  $n (HI)_{eq} = 1 - 0, 22 = 0, 78$ (mol)
  1 mark · Complete an equilibrium table
6. Question 6.2.3 · November 2025 · Paper 2 · National (DBE) · 8 marks
6.2 Initially $70$ g of $NH_{4} HS$ (s) is placed in a $3$ dm $^{3}$ container at $250$ °C. The container is sealed and the reaction is allowed to reach equilibrium according to the balanced equation:

$NH_{4} HS(s) ⇌ NH_{3} (g) + H_{2} S(g)$

The $K_{c}$ values for the decomposition of $NH_{4} HS$ (s) at different temperatures are given in the table below.

Answer the questions that follow.
6.2.3 Calculate the mass of $NH_{4} HS$ (s) that will be present at equilibrium at $250$ °C.(8)
Show the memo (8 ticks)Memo
1. ✓
  (a) Moles of NH4HS $n = \frac{m}{M} = \frac{70}{51} = 1, 37$ mol
  1 mark · Calculate number of moles
2. ✓
  (b) Change row using the ratio 1 : 1 : 1Table (mol): $NH_{4} HS$ : initial $1, 37$ , change $x$ ; $NH_{3}$ : initial 0, change $x$ ; $H_{2} S$ : initial 0, change $x$ .
  Equilibrium: $NH_{4} HS$ : $1, 37 - x$ ; $NH_{3}$ : $x$ ; $H_{2} S$ : $x$
  1 mark · Complete an equilibrium table
3. ✓
  (c) Equilibrium concentrations (divide by 3) $[NH_{3}] = [H_{2} S] = \frac{x}{3}$ mol·dm $^{- 3}$
  1 mark · Calculate concentration of a solution
4. ✓
  (d) Kc expression $K_{c} = [NH_{3}] [H_{2} S]$
  1 mark · Write the Kc expression
5. ✓
  (e) Substitute Kc $18 \times 1 0^{- 2} = (\frac{x}{3})^{2}$
  $x = 1, 27$
  1 mark · Calculate Kc
6. ✓
  (f) Equilibrium moles of NH4HS $n (NH_{4} HS)_{e q} = 1, 37 - 1, 27 = 0, 1$ mol
  1 mark · Complete an equilibrium table
7. ✓
  (g) Substitute 51 in n = m/M $m (NH_{4} HS)_{e q} = n M = 0, 1 \times 51$
  1 mark · Calculate number of moles
8. ✓
  (h) Final answer $= 5, 1$ g
  1 mark · Calculate number of moles

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