Question: 79. 1 gram of sodium hydroxide was treated with 25 mL of 0.75 M HCL solution, the mass of sodium hydroxide left unreacted is equal to
(1) 750 mg
(2) 250 mg
(3) Zero mg
(4) 200 mg
Answer: Option (2)
Explanation:
The balanced chemical reaction between sodium hydroxide and hydrochloric acid is
\mathrm{NaOH} + \mathrm{HCl} \rightarrow \mathrm{NaCl} + \mathrm{H}_{2}\mathrm{O}Number of moles of \mathrm{NaOH} initially present is
\text{Moles of NaOH}=\dfrac{1}{40} = 0.025Number of moles of \mathrm{HCl} used is
\text{Moles of HCl} = 0.75 \times \dfrac{25}{1000} = 0.01875From the reaction equation, \mathrm{NaOH} and \mathrm{HCl} react in a 1:1 molar ratio.
Thus, 0.01875 moles of \mathrm{NaOH} will react completely with \mathrm{HCl}.
Moles of \mathrm{NaOH} left unreacted are
0.025 - 0.01875 = 0.00625Mass of unreacted \mathrm{NaOH} is
0.00625 \times 40 = 0.25 \text{ g} 0.25 \text{ g} = 250 \text{ mg}Therefore, the mass of sodium hydroxide left unreacted is 250 \text{ mg}.