Question

A buffer solution is prepared in which the concentration of nh 3 is 0.30 m and the concentration of 4 nh is 0.20 m. if the equilibrium constant, k b for nh 3 equals 1.8 × 10–5, what is the ph of this solution?

Answer 1

First calculate pKb by using this equation
pKb = -logKb
= - log [1.8 × 10-5]
= 4.774
Then calculate the pOH using Henderson–Hasselbalch equation
pOH = pKb + log (salt/base)
pOH = 4.774 + log (0.20/ 0.30)
= 4.774 + log 0.20 - log 0.30
= 4.774 -0.698-(-0.522)
= 4.598
Since, pOH + pH = 14
pH = 14 - pOH 
​pH = 14 - 4.598
pH = 9.402

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