Question

Consider the following data on some weak acids and weak bases: - K acid name formula hydrocyanic acid HCN 4.9 x 10-10 acetic acid HCH,CO2 1.8 x 10-5 base name formula ammonia NH, 1.8 x 10-5 ethylamine C,H,NH2 6.4 x 10-4 Use this data to rank the following solutions in order of increasing pH. In other words, select a 'l' next to the solution that will have the lowest pH, a '2' next to the solution that will have the next lowest pH, and so on. solution PH 0.1 M NaNO3 choose one 0.1 M NaCN choose one choose one 0.1 MC2H5NH3CI choose one 0.1 M KCH3CO2 choose one x 6 ?

Answer 1

0.1 M NaNO₃

solution is neutral since it is salt of strong acid ( HNO3 ) and strong base ( NaOH) it’s P^H is  7 .

0.1 M NaCN

NaCN is salt of weak acid HCN and strong base NaOH .So CN^— ion will hydrolyse and form hydroxide ion.

Kb ( CN^- ) = K_W / K_a = 10^-14 / 4.9 x 10^-10 = 2.04 x 10^{-5 .}

P^H will be greater than 7 solution will be basic

0.1 M C₂H₅NH₃Cl

It is salt of strong acid ( HCl ) and weak base ( C₂H₅NH₂ ) so C₂H₅NH₃⁺ ion will hydrolyse and produce proton in solution .solution will be acidic and P^H will be less than 7.

0.1 M CH₃COOK

It is salt of weak acid ( CH₃COOH ) and strong base ( KOH ).solution will be basic due to hydrolysis of acetate ion . And P^H will be more than 7.

K_b ( CH₃COO^- ) = K_W / K_a = 10^-14 / 1.8 x 10^-5 = 5.6 x 10^-10

In NaCN and CH₃COOK more basic salt is NaCN due to large K_b.

P^H order - 0.1M C₂H₅NH₃Cl < 0.1 M NaNO₃ < 0.1M CH₃COOK < 0.1M NaCN