pH and Ka

[rasen58]

The pH of a 0.10 M solution of an aqueous weak acid (HA) is 4.00. The Ka for the weak acid is.....

 

I know the expression for Ka is [H+][A-]/[HA]

But I don't know what to do next.

[hypervalent_iodine]

Okay, well what is pH a measure of and what are the numbers you need to calculate Ka? I see you've written the equation for the equilibrium, but what do the values in the brackets represent and how would you go about getting those numbers?

[John Cuthber]

The other important thing to consider is where the H+ and A- come from,, and what that tells you about the relative size of their concentrations.

[Function]

you know the pH of a weak acid and you know its concentration: 0.1 M.

 

For a weak acid, the pH can be calculated as follows (you should've already seen this formula, else you couldn't solve this (easily)):

[math]pH=\frac{1}{2}\left(pK_a-\log{c_A}\right)[/math]

with [math]c_A[/math] the concentration of the acid. You should know what [math]pK_a[/math] is in function of [math]K_a[/math], so now it should not be too hard to calculate [math]K_a[/math]

Edited May 1, 2014 by Function