Chromium plating can involve the electrolysis of an electroyte of an acidified mixture of chromic acid and chromium sulphate. If during electrolysis the article being plated increases in mass by 2.6 g and 0.6 dm³ of oxygen are evolved at an inert anode, the oxidation state of chromium ions being discharged must be (assuming Cr = 52 and 1 mole of gas at toom temperature and pressure occupies a volume of 24 dm³)