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 $



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