Calculate the pH of the following aqueous solutions?

Calculate the pH of the following aqueous solutions?

a) 100 mL of 0.1 M solution of ammonium chloride to which 100 mL of
0.1 M hydrochloric acid solution.

b) 100 mL of 0.1 M solution of ammonium chloride to which 100 mL of
0.1 M ammonia solution.

Assume that all volumes are additive. Kb (ammonia) = 1.8 x 10-5

1 Answer
anor277
Apr 30, 2018

Well, in the first instance you have diluted your solution by HALF....

Explanation:

...however, you have added stoichiometric hydrochloric acid. #HCl(aq)# is a strong acid...for which the given equilibrium lies strongly to the right....

#HCl(aq) +H_2O(l) rarr H_3O^+ + Cl^-#

And so ......#[H_3O^+]=(100*mLxx10^-3*L*mL^-1xx0.1*mol*L^-1)/(0.100*L+0.100*L)#

#=(0.010*mol)/(0.200*L)=0.050*mol*L^-1#...

#pH=-log_10[0.050]=-(-1.30)=+1.30#...the small contribution made by protonolysis of the ammonium cation, is not worth considering.

And for #b.# you have prepared a buffer, for which....

#pH=pK_a+underbrace(log_10{[[NH_3(aq)]]/[[NH_4^+]]})#

#"....but the underbraced is"log_(10)1=0#...so #pH=pK_a=-log_10(1.80xx10^-5)=4.75#.

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