polyprotic ph problem


by Zealduke
Tags: polyprotic
Zealduke
Zealduke is offline
#1
Nov17-09, 06:49 PM
P: 5
A .05 M sample of K2HPO4 was dissolved in 800 mL of water, and the pH was adjusted to 7 by using a 1 M solution of H3PO4 followed by water bringing the volume up to 1 L. How many mL of H3PO4 were added? (On this problem charge and mass balance have to be used.)

My problem is just in the very beginning, as my book is terrible, Im not entirely sure on how to approach this problem.

Im assuming the mass balance for H3PO4 will be:

[H3PO4] + [H2PO4- ] + [HPO42-] + [PO43-] = 1 M

And the charge balance for K2HPO4 would be:

[H+] + [K+] = [OH-] + [H2PO4-] + 2*[HPO42-] + 3*[PO43-]


I'm really uncertain whether I'm missing anything or what the next step should be.
Phys.Org News Partner Science news on Phys.org
SensaBubble: It's a bubble, but not as we know it (w/ video)
The hemihelix: Scientists discover a new shape using rubber bands (w/ video)
Microbes provide insights into evolution of human language
Borek
Borek is offline
#2
Nov18-09, 02:53 AM
Admin
Borek's Avatar
P: 22,710
Please try these lectures on pH calcualtion as a starting point.

At pH 7.0 you can ignore presence of H3PO4 and PO43-. I have a gut feeling that this question can be solved just using Henderson-Hasselbalch equation and stoichiometry.

--


Register to reply

Related Discussions
Pulley problem involves Friction static and Kinetic problem Introductory Physics Homework 1
Polyprotic base Biology, Chemistry & Other Homework 4
One Kinematic Problem, One Pendulum Problem, One Wave Problem Introductory Physics Homework 7
classic E&M problem: point charge and a charged sphere, how to analyze this problem Advanced Physics Homework 1
Solid mechanics problem (pretty much a static problem) Introductory Physics Homework 1