Chemistry Labs

Problem 3

A volume of 200 cm3^3 of a 2-normal sodium chloride solution (ρ=1.10\rho = 1.10 g cm−3^{-3}) was electrolysed at permanent stirring in an electrolytic cell with copper electrodes. Electrolysis was stopped when 22.4 dm3^3 (at STP) of a gas were liberated at the cathode. Calculate the mass percentage of NaCl in the solution after electrolysis. Relative atomic masses: Ar(H)=1A_r(\text{H}) = 1; Ar(O)=16A_r(\text{O}) = 16; Ar(Na)=23A_r(\text{Na}) = 23; Ar(Cl)=35.5A_r(\text{Cl}) = 35.5; Ar(Cu)=64A_r(\text{Cu}) = 64.
Step 2 of 3: What happens to the chlorine
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ClX2+Cu→CuClX2;CuClX2+2 NaOH→Cu(OH)X2+2 NaCl\ce{Cl2 + Cu -> CuCl2};\quad \ce{CuCl2 + 2NaOH -> Cu(OH)2 + 2NaCl}
Analysis

The copper anode is attacked: anodic Cl2 forms CuCl2 which, in the stirred solution, precipitates Cu(OH)2 with the cathodic NaOH and regenerates NaCl. Net effect: the NaCl mass is unchanged, only water decomposes.