Question

Which of the following buffer solution turns invalid on addition of $10\, mL$ of $1.0\, M\, HCl$ ?

• A. $100\,mL$ having $0.15\,M\,NH_{3}$ and $NH_{4}Cl$ each
• B. $ 100\,mL$ having $0.2M\,NH_{3}$ and $NH_{4}Cl$ each
• C. $ 100\,mL$ having $0.2M\,NH_{3}$ and $0.1\,M\,NH_{4}Cl$ each
• D. $ 100\,mL$ having $0.05\,M\,NH_{3}$ and $NH_{4}Cl$ each

Answer 1

Answer: (D)

Millimoles of $NH _{3}=100 \times 0.05=5\, m\, mol$
Added $HCl =10 \times 1.0\, M =10\, m\, mol$
$NH _{3}$ gets completely neutralised, solution turns acidic that is now not the buffer solution. In all other cases, $m$ moles of $NH _{3}$ are higher as compared to $m$ moles of $HCl$. Thus, they remain as buffer.