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April 26, 2015

Homework Help: chemistry

Posted by student on Sunday, February 3, 2008 at 2:49pm.

I have already posted this and never got a response, please help.

here is my data from lab:

Buffer1: HPO4- weight 3.412g; original pH
Buffer 2: HEPES wt: 1.090g original pH 10.08

buffer1:
pH of 0.1M: 7.5;
pH of 0.01: 7.72;
pH of 0.001M: 7.87

buffer2:
PH of 0.1M: 7.5
ph of 0.01 M: 7.42
pH of 0.001M: 7.42

The problem will be done for one of the buffers made.

A) What is the ratio of A-/HA in your buffer after you adjusted its pH to the required value? (pH goal: 7.5)

B) How many micromoles of A- and HA are present in the solution?

C) If you now add 3 mL of 1 M NaOH, will you still have a valid buffer?

If necessary, here are the pka values and molecular weights: HEPES: pka 7.55; mw= 238.3; phosphate (HPO4-) pka1: 2.12 pka2: 7.21; pka 3: 12.32; mw=142.0


HERE ARE HINTS I HAVE BEEN GIVEN:

A. Use the Henderson Hasselbach equation.
B. micromole = 0.001 mole
you know that the HA + A- have to equal number of moles in the 0.1 M solution
C. Will all of the HA form to the A- form be converted? Or will some of it remain?


Thank you very much!!!

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