Question

What volume of hydrogen at STP is produced when $2.5 \cdot g$ of zinc react with an excess of hydrochloric acid in the reaction $Zn + 2HCl \to ZnC{l_2} + {H_2}$ ?

Answer 1

STP is defined as the standard temperature and pressure. At STP, one mole of gas contains $22.4L$ of that gas. For the given problem, first we need to figure out the number of moles of zinc present and then using stoichiometry, we need to find out the volume of hydrogen gas released.

Complete answer:
One mole of zinc has the mass of $65.39g$ or we can say, the molar mass of zinc is $65.39g/mol$ .
Now, first we will find the number of moles of zinc in $2.5 \cdot g$ of the zinc sample. i.e.
Number of moles of zinc $= \dfrac{{2.5g}}{{65.96g/mol}} = 0.038mol$
From the stoichiometry of the chemical equation, the amount of hydrogen gas produced is same as the amount of zinc consumed (i.e. $0.038mol$ )
Now, as one mole of ideal gas at STP contains $22.4L$ of volume. So, $0.038mol$ of hydrogen gas will contain –
\eqalign{ & Volum{e_{hydrogen}} = 0.038g/mol \times 22.4L \cr & Volum{e_{hydrogen}} = 0.86L \cr}
Hence, at STP, when $2.5 \cdot g$ of zinc react with an excess of hydrochloric acid in the reaction
$Zn + 2HCl \to ZnC{l_2} + {H_2}$ , the amount of hydrogen gas produced is $0.86L$ .

Note:
The production of hydrogen gas in the laboratory is done by the given chemical reaction between zinc and hydrochloric acid. The product also contains a salt called zinc chloride. The reaction is exothermic and produces heat.