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Difficulty: HardHalogens: Chlorine, Hydrogen Chloride, and Oxoacids
When chlorine gas is bubbled into a hot, concentrated solution of potassium hydroxide (KOH\text{KOH}), it undergoes disproportionation according to the chemical equation:
3Cl2(g)+6KOH(aq)→5KCl(aq)+KClO3(aq)+3H2O(l)3\text{Cl}_2\text{(g)} + 6\text{KOH(aq)} \rightarrow 5\text{KCl(aq)} + \text{KClO}_3\text{(aq)} + 3\text{H}_2\text{O(l)}
What mass of potassium trioxochlorate(V) (KClO3\text{KClO}_3) is produced when 6.72 dm36.72\text{ dm}^3 of chlorine gas measured at s.t.p. reacts completely?
[Molar volume of gas at s.t.p. = 22.4 dm3mol−122.4\text{ dm}^3\text{mol}^{-1}; Molar masses: K=39 g/mol\text{K} = 39\text{ g/mol}, Cl=35.5 g/mol\text{Cl} = 35.5\text{ g/mol}, O=16 g/mol\text{O} = 16\text{ g/mol}]
  1. 12.25 g12.25\text{ g}Answer
  2. B
    36.75 g36.75\text{ g}
  3. C
    11.43 g11.43\text{ g}
  4. D
    9.05 g9.05\text{ g}

Answer

The correct mass of potassium trioxochlorate(V) produced is 12.25 g12.25\text{ g}.
The option specifying 12.25 g12.25\text{ g} is correct because 6.72 dm36.72\text{ dm}^3 of Cl2\text{Cl}_2 gas at s.t.p. corresponds to 0.30 mol0.30\text{ mol}. Based on the 3:1 stoichiometric ratio from the balanced equation (3Cl2→1KClO33\text{Cl}_2 \rightarrow 1\text{KClO}_3), 0.10 mol0.10\text{ mol} of KClO3\text{KClO}_3 is produced. Multiplying 0.10 mol0.10\text{ mol} by the molar mass of KClO3\text{KClO}_3 (122.5 g/mol122.5\text{ g/mol}) yields 12.25 g12.25\text{ g}.

Step-by-Step Solution

1
Calculate the amount of chlorine gas in moles at s.t.p.
n(Cl2)=6.72 dm322.4 dm3mol−1=0.30 moln(\text{Cl}_2) = \frac{6.72\text{ dm}^3}{22.4\text{ dm}^3\text{mol}^{-1}} = 0.30\text{ mol}
At s.t.p., 1 mole1\text{ mole} of any ideal gas occupies 22.4 dm322.4\text{ dm}^3.
2
Determine the moles of potassium trioxochlorate(V) (KClO3\text{KClO}_3) formed using stoichiometric ratios.
n(KClO3)=13×0.30 mol=0.10 moln(\text{KClO}_3) = \frac{1}{3} \times 0.30\text{ mol} = 0.10\text{ mol}
From the balanced equation, 3 moles3\text{ moles} of Cl2\text{Cl}_2 produce 1 mole1\text{ mole} of KClO3\text{KClO}_3.
3
Calculate the molar mass of KClO3\text{KClO}_3.
Molar Mass=39+35.5+(3×16)=122.5 g/mol\text{Molar Mass} = 39 + 35.5 + (3 \times 16) = 122.5\text{ g/mol}
Summing the atomic masses of one potassium, one chlorine, and three oxygen atoms.
4
Calculate the mass of KClO3\text{KClO}_3 produced.
Mass=0.10 mol×122.5 g/mol=12.25 g\text{Mass} = 0.10\text{ mol} \times 122.5\text{ g/mol} = 12.25\text{ g}
Mass is obtained by multiplying the number of moles by the molar mass.

Key Concept

Disproportionation reactions of halogens in hot concentrated alkalis and gas stoichiometry at s.t.p.
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Topics

Halogens: Chlorine, Hydrogen Chloride, and Oxoacids

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