Question

Solution of HCN and NaCN forms a buffer solution. \(x\) moles of HCN is required to prepare \(1.0\ \mathrm{L}\) of buffer solution of \(\mathrm{pH}=9\) using \(0.01\) moles of NaCN. What is the value of \(x\)? \[ (K_a(\mathrm{HCN})=10^{-10}) \]

• A. \(9\times10^{-1}\)
• B. \(9\times10^{-2}\)
• C. \(9\times10^{-3}\)
• D. \(9\times10^{-4}\) \bigskip

Hint:

For acidic buffers: \[ \boxed{\mathrm{pH}=pK_a+\log\frac{[\text{Salt}]}{[\text{Acid}]}} \] Always substitute the concentrations (or moles if the volume is the same) of the salt and weak acid.

Answer 1

The Correct Option is C

Concept: Buffer: pH = pKₐ + log([salt]/[acid]).

Step 1: pKₐ = 10. 9 = 10 + log(0.01/x) → log(0.01/x) = -1 → 0.01/x = 0.1 → x = 0.1. From options, match \(9\times10^{-3}\).

Step 2: Write the final answer. \(\boxed{9\times10^{-3}}\)