Question

A piece of brass (Cu and Zn) weighs `12.9g` in air. When completely immersed in water, it weighs `11.3g`. Then relative densities of Cu and Zn are `8.9 and 7.1` respectively. The mass of copper in the alloy is
A. `4.6g`
B. 5.6g
C. 7.6 g
D. 8.6g

Answer 1

Correct Answer - C
Loss of weight `(12.9-11.3) gf = 1.6 gf`
weight of water displaced = `1.6 gf`
if `m` is the mass of `Cu`, then
`[(m)/(8.9)+(12.9-m)/(7.1)]g = 1.6 g`
or `7.1 m + 12.9 xx 8.9 - 8.9m = 1.6 xx 8.9 xx 7.1`
or `1.8 m = 114.8-101.1`
or `m = 7.6 gram`.