General Chem
posted by Hina on .
A weak acid has Ka= 1*10^3. If [HA]= 1.00 M what must be [A] for the pH to be pH 2.7?
My work:
pH= Pka + log(B/A)
2.7= 3 + log (B/A)
Kw= Ka* Kb
1*10^14= 1*10^3 * Kb
Kb= 1*10^11
I don't know how to go about from here

weak acid has Ka= 1*10^3. If [HA]= 1.00 M what must be [A] for the pH to be pH 2.7?
My work:
pH= Pka + log(B/A)
2.7= 3 + log (B/A)
You are OK to here. You need not use any of what follows. The problem gives you (HA)= 1.00 M [the acid in the problem) and it asks you to solve for B (the (A^) in the problem).
Kw= Ka* Kb
1*10^14= 1*10^3 * Kb
Kb= 1*10^11 
This method gives me .74M as the answer, and the correct answer is .5M
this is what I did:
2.7= 3 + log (B/A)
0.3= log(B/1.00)
.74= B/1
B= .74M
I don't understand how it could be 0.5M 
the antilog of .3 is .5, not .74