Problem 3

A volume of 200 cm

^3

of a 2-normal sodium chloride solution (

\rho = 1.10

g cm

^{-3}

) was electrolysed at permanent stirring in an electrolytic cell with copper electrodes. Electrolysis was stopped when 22.4 dm

^3

(at STP) of a gas were liberated at the cathode. Calculate the mass percentage of NaCl in the solution after electrolysis. Relative atomic masses:

A_r(\text{H}) = 1

;

A_r(\text{O}) = 16

;

A_r(\text{Na}) = 23

;

A_r(\text{Cl}) = 35.5

;

A_r(\text{Cu}) = 64

Step 1 of 3: Cathode gas

n(\ce{H2}) = \dfrac{22.4}{22.4} = 1\ \text{mol}

Analysis

At the cathode water is reduced: $\ce{2H2O + 2e- -> H2 + 2OH-}$ , so 22.4 dm3 at STP is exactly 1 mol of H2 and 2 mol of H2O are consumed.