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NVAEasyJEE 2023Stoichiometry & Calculations

JEE Chemistry 2023 Question with Solution

Zinc reacts with hydrochloric acid to give hydrogen and zinc chloride. The volume of hydrogen gas produced at STP from the reaction of 11.5 g11.5 \, \text{g} of zinc with excess HCl is \hspace{2cm} L (Nearest integer).

Given: Molar mass of Zn=65.4 g mol−1,Molar volume of H2 at STP=22.7 L/mol\text{Molar mass of Zn} = 65.4 \, \text{g mol}^{-1}, \quad \text{Molar volume of H}_2 \, \text{at STP} = 22.7 \, \text{L/mol}

Answer

Correct answer:4

Step-by-step solution

Standard Method

Given: Mass of Zn\text{Zn} is 11.5 g11.5 \, \text{g}. Molar mass of Zn\text{Zn} is 65.4 g mol−165.4 \, \text{g mol}^{-1}. Molar volume of H2\text{H}_2 at STP is 22.7 L/mol22.7 \, \text{L/mol}.

Find: Volume of H2\text{H}_2 produced at STP.

The balanced chemical equation is

Zn+2HCl→ZnCl2+H2↑\text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2 \uparrow

From the equation, 11 mole of Zn\text{Zn} produces 11 mole of H2\text{H}_2.

Moles of Zn\text{Zn} used:

Moles of Zn=11.5 g65.4 g/mol=0.176 mol\text{Moles of Zn} = \frac{11.5 \, \text{g}}{65.4 \, \text{g/mol}} = 0.176 \, \text{mol}

Therefore, moles of H2\text{H}_2 evolved = 0.176 mol0.176 \, \text{mol}.

Volume of H2\text{H}_2 at STP:

Volume of H2=0.176 mol×22.7 L/mol=3.99 L\text{Volume of H}_2 = 0.176 \, \text{mol} \times 22.7 \, \text{L/mol} = 3.99 \, \text{L}

Nearest integer = 44. Therefore, the required volume of hydrogen gas is 4 L4 \, \text{L}, so the answer is 44.

Using stoichiometric ratio explicitly

Given: Zn\text{Zn} reacts with excess HCl\text{HCl} and forms H2\text{H}_2. Mass of zinc is 11.5 g11.5 \, \text{g}.

Find: The nearest integer value of the volume of H2\text{H}_2 at STP.

First convert the given mass of zinc into moles:

11.565.4=0.176\frac{11.5}{65.4} = 0.176

Now use the reaction

Zn+2HCl→ZnCl2+H2\text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2

The mole ratio Zn:H2=1:1\text{Zn} : \text{H}_2 = 1:1, so moles of hydrogen formed are also 0.1760.176.

At STP, volume of hydrogen is

0.176×22.7=3.99 L0.176 \times 22.7 = 3.99 \, \text{L}

Hence the nearest integer is 44.

Common mistakes

  • Using the coefficient of HCl\text{HCl} to conclude that hydrogen formed is twice the moles of zinc is incorrect. From the balanced equation, 11 mole of Zn\text{Zn} gives 11 mole of H2\text{H}_2. Always read the product-side stoichiometric ratio carefully.

  • Using the mass 11.5 g11.5 \, \text{g} directly in the gas-volume calculation is wrong because gas volume at STP is based on moles, not grams. First convert zinc to moles, then use molar volume.

  • Writing the answer as 3.993.99 without applying the instruction 'Nearest integer' is incomplete. After calculating the volume, round it to the nearest integer to get the final numerical answer.

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