Chromium plating of an article was carried out by treating it as cathode in an electrolysis. If during electrolysis the weight of the article increased by 2.6 g and 0.56 dm3 of oxygen (corrected up to STP) evolves at inert anode, the oxidation state of the chromium ions being discharged must be:

Explanation

During chromium plating, the article acts as the cathode and gains mass as chromium ions are reduced and deposited. The evolution of oxygen at the inert anode provides information about the number of electrons involved in the process. Since 0.56 dm3 of oxygen evolves, it indicates that 0.025 moles of O2 are produced (using the molar volume of 22.414 dm3/mol). This corresponds to 0.05 moles of electrons (since each O2 molecule requires 4 electrons for reduction). The mass increase of the article is 2.6 g, which corresponds to 0.05 moles of electrons being used to reduce chromium ions. Given that the molar mass of chromium is 52 g/mol, this implies that the oxidation state of the chromium ions is +2, as 0.01 moles of chromium are deposited (2.6 g / 52 g/mol), consistent with the stoichiometry of the reduction reaction. Therefore, option B is correct, and the other options do not match the stoichiometry and calculations based on the provided data.