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October 20, 2014

Homework Help: Chemistry/pH- Weak Acid

Posted by Mary on Sunday, August 17, 2008 at 6:01am.

Hi again!
I have a new question, Can you help me?

Consider 50.0 mL of a solution of a weak acid HA (Ka = 1.00.E-6), which has a pH of 4.000. What volume of water must be added to make the pH = 5.000?

My Calculations:
To calculate the concentration of x, I take the pH value ->
pH = 4,000=> x=[H+]= 1.E-4=0.0001M=
x=1.E-4M
The equilibrium reaction is.

HA H+ + A-
Initial x ~0 0
Final x-1.E-4 x-1.E-4 x-1.E-4

Ka=(1,00.E-4)^2/(x-1*E-4)=1,00.E-6;
solving x = 0,0101M

To calculate the quantity of moles in the solution, I do the following:
(50,0*10-3L)* (0,0101M) = 5,05*10-4 mol HA.

I donīt know what to do now!

Thanks!

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