Question

40 ml of 0.1 M ammonia solution is mixed with 20 mL of 0.1 M HCl. What is the pH of a mixture? ($p{K}_b$ of ammonia solution is 4.74)

• A. 4.74
  
• B. 2.26
  
• C. 9.26
  
• D. 5

Answer 1

The correct option is C 9.26


At half stage of titration, $\left[salt\right]=\left[base\right]$
Use the Henderson - Hasselbalch Equation:
$pOH=p{K}_b+log\left[\frac{\left[\text{salt}\right]}{\left[\text{base}\right]}\right]$
$pOH=p{K}_b+log\left[\frac{\left[N{H}_{4}Cl\right]}{\left[N{H}_3\right]}\right]$
$pOH=p{K}_b=4.74(\because [salt]=[base\left]\right)$
$\therefore pH=14-pOH$
$pH=14-4.74=9.26$