# Homework Help: How to solve buffer calculations-Biochemistry

1. Sep 9, 2010

### brittley96

1. The problem is need to prepare 400.0 mL of a .200M buffer having a pH of 7.69

Given .5 M of dihydrogen Phosphate at pKa of 6.82

2. Equation to work from is Henderson-Hasselbalch pH=pKa-log of (A-)/(HA)
where (A-) is weak acid and (HA) is the conjugate base

3. I know 400mL is .4 x .2 will give .08 the total for the buffer amount and that (A-) and (HA) will equal that, or x+y= 0.08 so can say x (the weak acid) = 0.08-y

I also know 10 to the (pH-pKa) is 10 to .087 or 7.413

So 7.413=(0.08-y)/y

I need to know how to solve for y and where do I go from there and then how to set up and solve for an ICE table.

2. Sep 9, 2010

### Staff: Mentor

You don't need ICE table to solve buffer questions, Henderson-Hasselbalch equation is enough.

Solving for unknown is a very basic algebraic skill, while I can show you how to deal with this specific case, it will be much better if you will spend some time working on this skill in general, as you will need it quite often.

Last edited by a moderator: Aug 13, 2013
3. Sep 9, 2010

ok thanks.