[meenu]

Calculate the concentration of HA and A- forms of cysteine required to yield and overall .25M solution at pH 7.8

pKs of cysteine are 1.9, 10.70, and 8.37

 i dont know where to start  what is given & which equation to use

[renge ishyo]

Here's a general outline of steps for this particular problem:

1. You are going to use the Henderson Hasselbach equation.

2. You have the pH already and have to choose the pKa out of the three that would be most appropriate to use with your equation at this pH (which one would you guess that would be?).

3. Then you solve for the ratio of [A-]/[HA] from the HH equation.

4. Now you have [A-]/[HA] = some number. You have two unknowns now, [A-] and [HA], which means you need a second equation. In this case the second equation is [HA] + [A-] = .25M. Solve for either variable and plug into one of your variables in the first equation to solve for the concentration.