Question

1 gram of sodium hydroxide was treated with 25 ml. of 0.75 M HCI solution, the mass of sodium hydroxide left unreacted is equal to

• A. 750mg
• B. 250 mg
• C. Zero mg
• D. 200 mg

Answer 1

Answer: (B)

250 mg

Answer 2

1. The reaction is: $NaOH + HCl \rightarrow NaCl + H_2O$.

2. Moles of $HCl = 0.75 \, M \times 0.025 \, L = 0.01875 \, \text{mol}$.

3. Moles of $NaOH = \frac{1}{40} = 0.025 \, \text{mol}$.

4. $HCl$ reacts with $0.01875 \, \text{mol}$ of $NaOH$, leaving $0.025 - 0.01875 = 0.00625 \, \text{mol}$ of $NaOH$.

5. Mass of unreacted $NaOH = 0.00625 \times 40 = 250 \, \text{mg}$.