Equilibrium Question 507
Question: A buffer solution with $ pH $ 9 is to be prepared by mixing $ NH_4Cl $ and $ NH_4OH $ . Calculate the number of moles of $ NH_4Cl $ that should be added to one litre of $ 1.0,M\ NH_4OH., $
$ [K_{b}=1.8\times {10^{-5}}] $ [UPSEAT 2001]
Options:
A) 3.4
B) 2.6
C) 1.5
D) 1.8
Show Answer
Answer:
Correct Answer: D
Solution:
$ pH=-logK_{b}+log\frac{[\text{ Salt }]}{[\text{ Acid }]} $
$ pH=-log,[1.8\times {10^{-5}}]+log\frac{[\text{ Salt }]}{1\text{.0}} $
$ 9=4.7+\log \frac{[\text{ Salt }]}{1.0} $ ; $ \log \frac{[\text{ Salt }]}{1.0}=4.7-9=-4.3 $
$ \frac{[\text{ Salt }]}{1.0}=Antilog\frac{1}{4.3};[S\text{alt }]=1.8 $
