# Chemical Forums

## Chemistry Forums for Students => High School Chemistry Forum => Topic started by: aliok5 on April 29, 2012, 05:11:45 PM

Title: Calculating pH of a solution containing multiple acids and bases?
Post by: aliok5 on April 29, 2012, 05:11:45 PM
Consider a solution formed by mixing 37.0 mL of 0.100 M H2SO4, 35.5 mL of 0.100 M HOCl, 25.0 mL of 0.200 M NaOH, 25.0 mL of 0.100 M Ca(OH)2, and 10.0 mL of 0.150 M KOH. Calculate the pH of this solution.

I started by finding the moles of H+ and OH- present in the solution. I got: .106 M H+ and .192 M OH-. (Multiplied the volume by concentration to find moles, added the moles together and divided by total volume)

I at first thought the pH was just -log(.106) but that's not correct, and I know I'm missing something. I don't know how Ka comes into play either because of the multiple acids present. Thank you in advance.
Title: Re: Calculating pH of a solution containing multiple acids and bases?
Post by: Borek on April 29, 2012, 05:40:44 PM
Start assuming it is just a limiting reagent problem.

While your approach

(Multiplied the volume by concentration to find moles, added the moles together and divided by total volume)

Sounds OK, I can't reproduce neither the acid nor the base concentration.
Title: Re: Calculating pH of a solution containing multiple acids and bases?
Post by: aliok5 on April 30, 2012, 04:56:54 PM
I'm getting different answers now that I check my calculations...

(.037L x (2x.100M H2SO4)) + (.0355L x .100M HOCl) all divided by (.037L + .0355L) = .151 M H+

(.025L x .200M NaOH) + (.025L x (2x.100M Ca(OH)2)) + (.010Lx.150M KOH) all divided by (.025L + .025L + .010L) = .192 M OH-

I'm not sure what you mean by applying a limiting reagents approach to this type of problem.
Title: Re: Calculating pH of a solution containing multiple acids and bases?
Post by: Borek on April 30, 2012, 07:01:10 PM
I'm not sure what you mean by applying a limiting reagents approach to this type of problem.

Compare

http://www.chembuddy.com/?left=pH-calculation-questions&right=pH-of-mixture-q1

Note: if is is a weak acid (or base) that is in excess, problem gets more complicated. But even then stoichiometry is a first step.