Question

The chief ore of $Zn$ is the sulphide, $ZnS$. The ore is concentrated by froth floatation process and then heated in air to convert $ZnS$ to $ZnO$.
$2ZnS+3O_2\stackrel{80\%}{\to }2ZnO+2S{O}_2$
$ZnO+H_{2}S{O}_4\stackrel{100\%}{\to }ZnS{O}_4+H_{2}O$
$2ZnS{O}_4+2H_{2}O\stackrel{80\%}{\to }2Zn+2H_{2}S{O}_4+O_2$
The number of moles of $ZnS$ required to produce 2 moles of $Zn$ will be:

• A. 3.125
• B. 2
• C. 2.125
• D. 4

Answer 1

The correct option is A 3.125

Let the moles of $ZnS$ taken is $n$

In the first step 80% of reaction is completed.
Then, amount OF $ZnO$ formed is
$n\times \frac{80}{100}=0.8n$

In the second step 100 % of reaction is completed.
Then, amount of $ZnS{O}_4$ formed is
$0.8n\times \frac{100}{100}=0.8n$

In the third step 80 % of reaction is completed.
Then, amount of $Zn$ formed is
$0.8n\times \frac{80}{100}=0.64n$

Given
0.64n must be equal to the 2 moles of $Zn$
$i.e0.64n=2$
$n=3.125$