Question

In order to prepare a buffer of pH= 8.26, the amount of (NH₄)₂ SO₄ required to be mixed with one litre of 0.1(M) NH₃ (aq), pK_b = 4.74 is

• A. 1.0 mole
• B. 10.0 mole
• C. 0.50 mole
• D. 5 mole

Answer 1

Answer: (C)

0.50 mole

Answer 2

pOH = pK_b + log $\frac { \left[ \mathrm { NH } _ { 4 } ^ { + } \right] } { \left[ \mathrm { NH } _ { 3 } \right] }$

5.74 = 4.74 + log $\frac { \left[ \mathrm { NH } _ { 4 } ^ { + } \right] } { 0.1 }$

or log $\frac { \left[ \mathrm { NH } _ { 4 } ^ { + } \right] } { 0.1 }$ = 1

or $\frac { \left[ \mathrm { NH } _ { 4 } ^ { + } \right] } { 0.1 }$ = 10

or $\left[ \mathrm { NH } _ { 4 } ^ { + } \right]$ = 1 (M)

\ $\left( \mathrm { NH } _ { 4 } ^ { + } \right) _ { 2 }$ SO₄ required = 0.5 mole