JEE Main 2025 — Some Basic Concepts Of Chemistry Question with Solution

We are tasked with calculating the mass of magnesium required to produce 220 mL of hydrogen gas at STP with excess dilute HCl.

Step 1: Write the balanced reaction

$\text{Mg}+2\text{HCl}\to {\text{MgCl}}_2+{\text{H}}_2$

• 1 mole Mg gives 1 mole H₂.

Step 2: Molar volume at STP

At STP, 1 mole of an ideal gas occupies 22.4 L = 22,400 mL.

Step 3: Moles of H₂ produced

$n({\text{H}}_2)=\frac{\text{Volume of H₂}}{\text{Molar Volume}}=\frac{220}{22400}=0.00982\text{mol}$

Step 4: Moles of Mg required

From stoichiometry, 1 mole Mg produces 1 mole H₂, so:

$n(\text{Mg})=n({\text{H}}_2)=0.00982\text{mol}$

Step 5: Mass of Mg required

$m(\text{Mg})=n\times M(\text{Mg})=0.00982\times 24=0.236\text{g}$

$=236\text{mg}$

✅ Final Answer:

Option D: 236 mg