Question

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

• A. $7.61g$
• B. $6.61g$
• C. $5.61g$
• D. $4.61g$

Answer 1

The correct option is A $7.61g$

let the weight of copper be a,
Let the weight of zinc be b.
Mass of the brass$\left(alloy\right)$ in air $=a+b=12.9g$.............1
when complete immersed it loses$(12.9-11.3)g$ of weight,
So,
volume of liquid displace$=$volume of the brass$=1.6c{m}^3$
so,
According to question,$\Rightarrow \frac{a}{8.9}+\frac{b}{7.1}=1.6$.....................2
solving 1 and 2, we get ,
Mass copper$\left(a\right)=7.61g$
Mass of brass$\left(b\right)=5.29g$