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Question

5 g of a sample of brass were dissolved in 1 litre conc. H2SO4. 20 mL of this solution were mixed with KI and liberated iodine required 20 mL of 0.0327 N hypo solution for titration. Calculate the amount of copper in the alloy.

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Solution

When brass is extracted with concentrated H2SO4, it gives copper sulphate.
2[Cu+2H2SO4CuSO4+SO2+2H2O]
2CuSO4+4KI2K2SO4+2CuI2
2CuI2Cu2I2+I2
2Na2S2O3+I22NaI+Na2S4O6
2 mole Cu1 moleI12 mole hypo
20 mL of solution reacts with 20 mL of 0.0327 N hypo
1000 mL of solution will react with 1000 mL of 0.0327 N hypo
No, of moles of hypo used =MassMolecularmass(158)
=E×N×V1000×158
where, E=158,N=0.0327 given, V=1000 mL
No. of moles of hypo used =158×0.0327×10001000×158=0.0327
No. of moles of Cu= No. of moles of hypo =0.0327 mole
Mass of copper in brass =0.0327×63.5=2.07645
% of copper in brass =2.076455×100=41.529%.

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