Question

Brass is an alloy of Cu-Zn. A sample of brass weighing 6g when treated with excess of dil H2SO4 gives 328mL of dry hydrogen gas at 27◦C and 760mm Hg pressure. What is the % of copper by weight in the alloy? (Atomic mass of zinc is 65.4g, R= 0.082L atm K-1 mol-1)

Answer 1

Final Answer :About Cu : 85.5% , Zn : 14.5%

Assumptions :
=>Brass is purely made up of Copper and Zinc only.
=>H2 gas released is Ideal gas.

Steps and Understanding :
1) We know that, Only Zinc can displace Hygrogen from sulphuric acid as Zinc is more reactive than Hydrogen unlike Copper.

So, only Zinc will take part in reaction.

2) Reaction is shown in pic.

We will find moles of H2 gas released by Ideal Gas Equation.
Pressure, P = 760 mmHg = 1 atm
Volume, V = 328mL = 328 * 10^(-3) L
Temperature, T = 300K
R = 0.0821 units

n(H2) = PV/RT
= 1atm * 328 * 10^(-3) L /( 0.0821 * 300K)

$= \frac{328 \times {10}^{ - 3} }{3 \times 8.21} = 13.32 \times {10}^{ - 3} \: mole \:$

3) By Reaction mole of Zn = mole(H2)
= 13.32 * 10^(-3) mole

Mass of Zinc, M(Zn) = No. of moles * Molar mass (Zn)
=
$13.21 \times {10}^{ - 3} \times 65.4 \\ = 0.87g$


Since, Total mass = 6g
Therefore, Mass of Copper = 6 - 0.87 = 5.13g

% weight of Copper, = Mass of Copper / Total Mass
=
$\frac{5. 13}{6} \times 100 \\ = 85.5 \: percentage$


Therefore, We have
% weight of Copper = 85.5%
% weight of Zinc = 14.5%