Chemistry
posted by Alice .
What is the pH of a solution made up of 50cm^3 of 1M Ethanoic Acid and 25cm^3 of 1M sodium ethanoate? Assume Ka for Ethanoic acid is 1.8 x 10^5 mol dm^3
I'm not quite sure how to do this. I was going to use the equation pH = pKa + log[A]/[HA] but it doesn't seem to work

You're right. The HendersonHasselbalch equation is the one to use. If you want to post your work someone will take a look and try to find the error. You should be substituting 25/75 for for the base and 50/75 for the acid. The denominators cancel, of course, and you have just 25 for (A^) and 50 for (HA)