Question

A buffered solution contains 0.25 M NH3 (Kb = 1.8 x 10-5) and 0.40 M NH4Cl. a. Calculate the pH of this solution. b. Calculate the pH of the solution that results when 0.10 mole of gaseous HCl is added to 1.0 L of the buffered solution from part a.

Answer 1

$pOH=pK_b+log[\frac{[NH_4^+]}{[NH_4OH]} ]$

$pOH=-log(1.8×10^{-5})+log[\frac{0.4}{0.25} ]$

$pOH=-log(1.8)+5+log(1.6)$

$pOH=5+log(\frac{1.6}{1.8})$

$pOH=5+log(16)-log(18)$

$pOH=5+4log(2)-log(9×2)$

$pOH=5+4log(2)-log(2)-2log(3)$

$pOH=5+3×0.301-2×0.477$

$pOH=4.949$

$pH=9.051$

(b)$pOH=pK_b+log[\frac{[NH_4^+]}{[NH_4OH]} ]$

$pOH=-log(1.8×10^{-5})+log[\frac{0.5}{0.15} ]$

$pOH=-log(1.8×10^{-5})+log[\frac{0.5}{0.15} ]$

$pOH=5-log(1.8)+log[\frac{10}{3} ]$

$pOH=5-log(1.8)+log[10 ]-log(3)$

$pOH=5-log(18)+2-log(3)$

$pOH=5-log(2)+2-3log(3)$

$pOH=5.27$

$pH=8.73$