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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: Tunechi on February 21, 2013, 06:31:57 PM

Title: What is the [H3O+] for a solution labeled “0.10 M H3PO3(aq)” if pKa1 = 2.00 and
Post by: Tunechi on February 21, 2013, 06:31:57 PM

Q11. What is the [H3O+] for a solution labeled “0.10 M H3PO3(aq)” if pKa1 = 2.00 and pKa2 = 6.59?
A. 0.027 M
B. 0.054 M
C. 0.20 M
D. 0.015 M
E. 0.15 M

ok so i made an ice table and used the quadratic equation, subbed in the ka value for pka1 (10^-pka1) which is 0.01 and i ended up with 0.027. But i haven't used the pka2 value at all. do i need to? why is it provided? thanks

Title: Re: What is the [H3O+] for a solution labeled “0.10 M H3PO3(aq)” if pKa1 = 2.00 and
Post by: Borek on February 22, 2013, 04:14:16 AM

In most cases when the difference between consecutive values of pKa is larger than 3, you can ignore the second dissociation step.