How do you solve this chemistry problem? Need HELP?

How do you solve this chemistry question?

• I'm in freshman college chemistry. This is my quiz review I'm working on. I have a big test after Spring Break. I don't even know where to start to solve this problem. "Calculate the concentration of the formate ion (CHO2-) to formic acid (HCHO2) in a buffer with pH of 4.00. (Hint: Use the Henderson-Hasselbach equation.) HCHO2=>H+ + CHO2- Ka=1.7*10^-4 My book says this is the Henderson-Hasselbach equation: pH=Ka + log ([conjugate base]/[acid]) I need to know how to solve this problem. It will be on the test that I really need to do well on. I need more than the answer- the steps to solve it. Thanks for any help.

• Answer:

  Actually, the Henderson-Hasselbalch equation is: pH = pKa + log([CHO2-]/[HCHO2]) The pKa = -log(Ka), defined similarly to pH = -log[H+]. Just substitute: 4.00 = -log(1.7*10^-4) + log([CHO2-]/[HCHO2]); calculate the pKa: 4.00 = 3.77 + log([CHO2-]/[HCHO2]); 0.23 = log([CHO2-]/[HCHO2]); 10^0.23 = 10^ log([CHO2-]/[HCHO2]); 1.70 = [CHO2-]/[HCHO2], which is the ratio that was asked for. We can't figure the actual concentrations without more information.