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.

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?

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.

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, 201411 yr by Function