Well, the pKa is the negative log of the Ka,

1.54 times 10 to the minus 5.

Plus the log of the number of moles of base, which was right here, okay?

2.49 times 10 to the minus 3, over the number of moles of acid,

2.49 times 10 to the minus 3.

The log of 1is 0, so this term goes away.

And so pH equals pKa, or the negative log of 1.54 times 10 to the minus 5.

This is equal to 4.812.

So we went through a lot of work there to do this problem.

But one of the things that you should know, is handy to know anyway, is that

when you're halfway to the equivalence point, and that's where we are here.

Equivalence point would be at 20 milliliters, and we're halfway there.

Whenever you're halfway to the equivalence point,

pH is equal to pKa, at one half the distance