Question

1. Calculate the pH of a 1.09×10-2 M solution of the decongestant ephedrine hydrochloride if the pKb of ephedrine (its conjugate base) is 3.86.

Answer 1

$pK_a+pK_b=14\\pK_a=14-3.86=10.14$

pH of a weak acid solution is $-\log[\sqrt{CK_a}]=-\frac12(\log C+\log K_a)=-\frac12 \log C+\frac12pK_a=-\frac12(-1.96)+5.07=6.05$