Question

Calculate the molar hydronium ion concentration in a solution containing 0.23M hypochlorus acid (HOCl), a monoprotic weak acid used in bleach solutions. For HOCl, Ka=2.9x10^-8

Answer 1

write the balence equation
HOCl <---> H⁺ + OCl^-
for each mole of HOCl that dissociates 1 mole of H⁺ and 1 mole of OCl^- is produce

if x moles of HOCl disassociate ...

do an ice table
I. ...................0.230. . . . 0. . . ..0
C ....................-x............x ........x.
E.................(0.230 - x).... x. . . . x..

write the Ka expression
ka = [H⁺][OCl^-] / [HOCl]

2.9x10^-8 = [x] [x] / [0.23 - x] ......{x << 0.23 so x neglect }
2.9x10^-8 = x² / 0.23

x² = (2.9 x10^-8 x 0.23 ) = 6.67x10^-9
x= 8.167x10^-5
[H+] = 8.167 x 10^-5

pH = -log[H+]

pH = -log[8.167x10^-5] = 4.08