Question

A piece of brass (alloy of copper and zinc) weights $12.9g$ in air. When completely immersed in water it weighs $11.3g$. What is the mass of copper contained in the alloy? Specific gravities of copper and zinc are $8.9$ and $7.1$ respectively

• A. $1.67g$
• B. $7.61g$
• C. $6.11g$
• D. $7.16g$

Answer 1

The correct option is B $7.61g$

Weight of brass in air = 12.9 g

weight of brass in water = 11.3 g

Weight of water displaced by brass = 12.9-11.3 = 1.6 g

Relative density of copper = 8.9= density of substance/density of water

Density f water = 1 g/cc

So, density of copper = 8.9 g/cc= Mass/ Volume

So, density of zinc = 7.1 g/cc

Volume of alloy = 1.6cc

Mass of copper in the alloy = x

Volume of alloy= Volume of copper + Volume of zinc

$1.6=\frac{x}{8.9}+\frac{12.9-x}{7.1}$

Solving the above equation, x= 7.61 g