[amplive]

I could use some help with this problem if anyone could please help me.  

a 10 mL buffer solution consists of 0.20 M HA (K_a = 1.4x10^-5) and 0.30 M KA.  Using the buffer solution, calculate the pH of the buffer after the addition of 4.0 mL of 1.0 M HCl solution.

I get stuck pretty quickly, because I see that not all of the HCl will be used, since I have .002 mol HA, .003 mol KA, and .004 mol HCl.  That leaves a negative value for KA which doesn't make sense.

[Borek]

After addition of the HCl you have a mixture of two acids - strong one and a weak one. It is no longer a buffer solution.

[amplive]

After addition of the HCl you have a mixture of two acids - strong one and a weak one. It is no longer a buffer solution.

So what exactly does this mean? Do I still just use the Henderson Equation?

[Borek]

No, it is not a buffer, so you don't use HH equation.

First approximation is to assume only excess HCl is responsible for the observed pH.

Second approximation is to calculate how much of the weak acid will dissociate in the presence of H⁺ from HCl and to sum H⁺ from both sources.