Chemical Forums

Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: ciel on October 21, 2005, 05:08:36 AM

Title: buffer and activity coefficient
Post by: ciel on October 21, 2005, 05:08:36 AM

hello,
I've done this lab and I have to do calculations.
This is the procedure.

Highest capacity buffer : Prepare 100 mL buffer solution. Acid = KH2PO4, base = K2HPO4.3H2O. It should be 0.1 M in conjugate acid and at the requated pH (7). The mass of the acid that I used = 1.3613 g, the mass of base = 1.2413 g.

The question : What pH did you calculate using the activity coefficient?


pH = pKa + [A-]activity coeff / [HA]activity coeff

-log act coeff = 0.51 z^2 [.]
    z is the ion of the interest.

My question is : What is z for [HA] ?
HA isn't ion... I don't know how to find z. Could someone help me?

For [A-] times act coeff --> z is 1 right? (from H2PO4)

Title: Re:buffer and activity coefficient
Post by: Borek on October 21, 2005, 07:53:56 AM

http://www.chembuddy.com/?left=pH-calculation&right=ionic-strength-activity-coefficients (http://www.chembuddy.com/?left=pH-calculation&right=ionic-strength-activity-coefficients)

z is a charge. What charge has HA?

Title: Re:buffer and activity coefficient
Post by: ciel on October 21, 2005, 09:38:13 AM

Is it zero?

But then the denominator would be zero and it would be weird.

Title: Re:buffer and activity coefficient
Post by: Borek on October 21, 2005, 11:26:19 AM

Quote from: ciel on October 21, 2005, 09:38:13 AM

Is it zero?

Yes.

Quote

But then the denominator would be zero and it would be weird.

No. Insert numbers and check for yourself. z of HA is 0, [HA] has some known value.