In which case pH will not change on dilution

Q: In which case pH will not change on dilution

(A) 0.01M CH3COONa + 0.01M CH3COOH buffer

(B) 0.01M CH3COONH4

(C) 0.01M NaH2PO4

(D) in all cases

Solution: Mixture of sodium acetate and acetic acid is a buffer of pH value equal to pKa so its buffer capacity is very high and hence its pH will not change significantly while CH3COONH4 is a salt of weak acid CH3COOH and weak base NH4OH whose magnitude of Ka and Kb are equal. So its pH does not depend upon concentration. Further more, NaH2PO4 is, in fact, a single solute buffer

Ans:(D)

If Kb for CN– at 25°C is 4.7 the pH of 0.5 M aqueous NaCN solution is

Q: If Kb for CN at 25°C is 4.7 the pH of 0.5 M aqueous NaCN solution is

(A) 12

(B) 10

(C) 11.5

(D) 11

Solution: CN + H2O → HCN + OH

pKb = 4.7

pKa = 9.3

$\large [OH^-] = C h = C \sqrt{\frac{K_h}{C}} $

$\large = \sqrt{K_h C} = \sqrt{\frac{K_w C}{K_a}}$

$\large = \sqrt{\frac{10^{-14} \times 0.5}{10^{-9.3}}}$

$\large [OH^-] = \sqrt{10^{4.7} \times 0.5}$

pOH = 11.4

Ans: (C)