Page 21 - Q & A STPM 2022 Chemistry

Chemistry Term 1 STPM Chapter 1 Atoms, Molecules and Stoichiometry
Question 9

When an aqueous solution of hydrogen peroxide is added to an aqueous
solution of potassium manganate(VII), KMnO 4 , oxygen gas is evolved. At the
same time, KMnO 4 is decolourised. The incomplete half-equations for the
reactions are shown below.
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MnO 4 + H + e : Mn + H 2 O
+
–
2+
–
H 2 O 2 : H + O 2 + e
–
+
In an experiment, it was found that 22.80 cm of acidified KMnO 4 was required
3
to react completely with 25.0 cm of 0.20 mol dm hydrogen peroxide.
–3
3
(a) Complete the two half-equations above.
Term
(b) Write a balanced chemical equation to represent the reaction above.
1 (c) Calculate the concentration of the acidified KMnO 4 solution.
(d) What is the volume of gas evolved at s.t.p.?
Answer:
(a) MnO 4 + 8H + 5e : Mn + 4H 2 O
–
+
2+
–
H 2 O 2 : 2H + O 2 + 2e –
+
– + 2+
(b) 2MnO 4 + 6H + 5H 2 O 2 : 2Mn + 8H 2 O + 5O 2
a
(c) ——– = —
M a V a
M b V b b
M a = Concentration of MnO 4
–
V a = Volume of MnO 4
–
M b = Concentration of H 2 O 2
V b = Volume of H 2 O 2
M a (22.80) 2
——–—— = —
0.2(25.0) 5
Concentration of KMnO 4 , M a = 0.088 mol dm –3
(d) The number of moles of O 2 = The number of moles of H 2 O 2
MV
= ——–
1000
0.20 × 25.0
= ——–——–
1000
= 0.005
Volume of O 2 = 0.005 × 22 400
= 112 cm 3

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