Brass is an alloy of copper and zinc. A sample of brass weighing `5.793` g, when treated with excess of dil. `H_(2)SO_(4)` gave 336 mL of hydrogen gas at ` 136.5` K and 785 mm pressure. If the hydrogen gas is collected over water and pressure of water vapour under the given conditions is 25 mm, find out the percentage weight of copper in the alloy.

Explanation

Among Cu and Zn, only Zn displaces `H_(2)" from "H_(2)SO_(4)`
`Zn+H_(2)SO_(4) to ZnSO_(4) + H_(2)`
Pressure = 785 - 25 = 760 mm
Temperature = 136.5 K, volume = 336 mL
`(P_(1)V_(1))/T_(1) = (P_(2)V_(2))/T_(2)`
`(760 xx 336)/(136.5) = (760 xx V_(2))/(273)`
` V_(2) = 672` mL
`(V_(2))` Vol. of `H_(2)` liberated at STP = 672 mL
Weight of zinc required = `(65.5 xx 672)/(22400) = 1.965` g
Weight of copper in brass = `5.793 - 1.965 = 3.828` g
% of Cu in the alloy = `(3.828)/(5.793) xx 100 = 66%`