Problem 1

An amount of 20 g of potassium sulphate was dissolved in 150 cm

^3

of water. The solution was then electrolysed. After electrolysis, the content of potassium sulphate in the solution was 15 % by mass. What volumes of hydrogen and oxygen were obtained at a temperature of 20 °C and a pressure of 101 325 Pa?

Step 1 of 3: Water decomposed

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m(\text{solution}) = \dfrac{20}{0.15} = 133.3\ \text{g} \Rightarrow m(\ce{H2O})_{\text{lost}} = 170 - 133.3 = 36.7\ \text{g}

Analysis

The 20 g of K2SO4 are unchanged; if they now represent 15 % of the solution, the solution mass is 133.3 g. Since the initial solution was 20 + 150 = 170 g, 36.7 g of water were decomposed: $n(\ce{H2O}) = 36.7/18 = 2.04$ mol.