Question

The maximum mass of copper sulphate crystals $CuS{{O}_{4}}.5{{H}_{2}}O$ , obtained from 20 g of a sample of brass containing only zinc and copper is 37.2 g. What mass of zinc was present in 20 g of brass?
[ Atomic masses of Cu=63.5; S=32; Zn=65]

Answer 1

To solve this question we will first find the molecular weight of copper sulphate crystal, then will calculate the number of moles of copper sulphate by the formula of:
$Number\text{ }of\text{ }moles=\dfrac{given\text{ }weight}{molecular\text{ }weight}$

Complete Solution :
- We can write the reaction that takes place as:
$Cu+{{H}_{2}}S{{O}_{4}}\to CuS{{O}_{4}}+{{H}_{2}}$

- We are being provided with the value of mass of $CuS{{O}_{4}}.5{{H}_{2}}O$ is 37.2 g.
- We can see in the above reaction that from one mole of copper one mole of copper sulphate will be formed. And we can find the molecular weight of copper sulphate crystal as:
$\begin{aligned} & CuS{{O}_{4}}.5{{H}_{2}}O \\\ & = 64 + 32 + (4\times 16)+5\times (2+16) \\\ & = 44 +\left( 64 \right)+\left( 90 \right) \\\ & = 160 + 90 \\\ & = 250 amu \\\ \end{aligned}$
- So, the molecular weight of copper sulphate crystal is 250 u.
- Now, we can calculate the number of moles of copper sulphate by the formula of given weight divided by the molecular weight. So, we can say that the moles of copper sulphate given are:
$Number\text{ }of\text{ }moles = \dfrac{given\text{ }weight}{molecular\text{ }weight}$
- Now, putting the values in the above equation we get:
37.2/250 = 0.1488 mole.
- Also the mole of copper will be 0.1488 So, we can write the mass of copper as 0.1488 $x$ 63.5 = 9.448.
- As, in the alloy the mass of copper is 9.448, so the mass of zinc will be 20 - 9.448 = 10.5 gm.
- Hence, we can conclude that 10.5 gm mass of zinc was present in 20 g of brass.

Note: - We should note here that copper doesn’t react with dilute sulphuric acid, as its reduction potential is higher than that of hydrogen. But, as concentrated sulphuric acid is an oxidising agent, so it will react with it.