In the iodometric estimation of copper, 2Cu^2+ + 4I^- -> 2CuI + I2, the number of moles of Na2S2O3 consumed in the later titration per mole of Cu^2+ is

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In the iodometric estimation of copper, $2 Cu X^{2 +} + 4 I X^{-} 2 CuI + I X_{2}$ , the number of moles of $Na X_{2} S X_{2} O X_{3}$ consumed in the later titration per mole of $Cu X^{2 +}$ is

2 mol of

Cu X^{2 +}

liberate 1 mol of

I X_{2}

, and 1 mol of

I X_{2}

needs 2 mol of thiosulphate. Hence 2 mol thiosulphate per 2 mol

Cu X^{2 +}

, i.e. 1 mol per mol.