Question

The amount of zinc required to produce $224{\rm{mL}}$ of ${H_2}$ at STP on treatment with dilute ${H_2}S{O_4}$ will be:
(A) $65g$
(B) $0.065g$
(C) $0.65g$
(D) $6.5g$

Answer 1

Hint: Zinc on reaction with sulphuric acid undergoes displacement reaction to form a salt of $ZnS{O_4}$ along with the removal of hydrogen gas. On the basis of the stoichiometric coefficients of the reactants in the reaction, we can compare the number of moles of each compound formed in the reaction and can thus, calculate the amount of a particular compound formed.

Complete Step by Step Solution:
The reaction of zinc with dilute sulphuric acid is a displacement reaction and can be represented with the help of a chemical equation as follows:
$Zn + {H_2}S{O_4} \to ZnS{O_4} + {H_2}$

As per the chemical reaction, one mole of zinc is reacting with one mole of sulphuric acid to give one mole of hydrogen. And we know that at STP, 1 mole of hydrogen $ = 22.4\;L\; = 22400 {\rm{mL}}$
So we can say that, $22400{\rm{ mL }}{{\rm{H}}_2}$ is formed on reaction of 1 mole of zinc.
Therefore, the number of moles required for the formation of $224{\rm{mL }}{{\rm{H}}_2} = \frac{{1 \times 224}}{{22400}} \Rightarrow 0.01\;{\rm{moles}}$

The number of moles can be defined as the ratio of the mass used to the atomic mass of the compound. So, the formula can be mathematically represented as follows:
$n = \frac{m}{M}\;\; - (1)$
We know that the atomic mass of zinc is $65g$. Substituting all the values in the equation$(1)$:
$0.01 = \frac{m}{{65}}$
$ \Rightarrow m = 0.01 \times 65 = 0.65g$
Hence, the mass of zinc consumed to produce $224{\rm{mL}}$ of ${H_2}$ at STP is $0.65g$. Thus, the correct answer is option (C).

Note: It is important to note that the reaction needs to be balanced before comparing the stoichiometric coefficients of respective reactants and products as this may lead to variation in the result. For the case of above reaction, there was no need of balancing as the chemical reaction was already balanced, but if we consider the reaction of zinc with hydrochloric acid i.e., $Zn + 2HCl \to ZnC{l_2} + {H_2}$. Thus, such a reaction needs to be balanced because the complete answer is based on the number of moles of reactants reacting and number of moles of products forming within a reaction.