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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: blern on March 04, 2012, 09:40:14 PM

Title: pH of weak acid solutions
Post by: blern on March 04, 2012, 09:40:14 PM

100.0 ml of 0.010M chlorous acid (Ka = 1.2 x 10^-2) is mixed with 200.0 ml of 0.020M hypochlorous acid (Ka = 3.5 x 10^-8).  Calculate the pH of the resulting solution.

I am unsure of what to do with the volumes given.  I believe that I should find the moles of each acid present and then divide it by the total volume of the solution to find the initial concentrations.  So:

.100L (.010 mol HClO2/1 L) = .00100 mol HClO2 / .3000 L = .00333 mol HClO2/L

.200L (.020 mol HOCl/1 L) = .00400 mol HOCl / .3000 L = .0133 mol HOCl/L

Is this the correct way to find the initial concentrations to then make and ICE table and find H+ concentrations?

Title: Re: pH of weak acid solutions
Post by: AWK on March 05, 2012, 01:07:47 AM

If one of  K x c  is about 100 greater than other then use one acid approximation (neglect lower product).

Title: Re: pH of weak acid solutions
Post by: Borek on March 05, 2012, 03:44:32 AM

Quote from: blern on March 04, 2012, 09:40:14 PM

Is this the correct way to find the initial concentrations

Yes.