Homework Help: Chemistry

Posted by Cliff on Friday, December 10, 2010 at 5:37pm.

Hi,may I have your help?
I have posted a titration question that I'm trying to solve. The correct answer to the problem is 0.128 M, however,I have arrived at the answer: 0.000128 M. Here's the question:

If 38.30 mL of 0.250 M NaOH is used to titrate 25.0 mL of phosphoric acid,what is the molarity of the acid? The balanced equation is

H3PO4(aq)+3 NaOH(aq)--- Na3PO4(aq)+3H2O (l).

Here's how I've tried to solve for the answer:
38.3 mL solution * (0.250 mol NaOH/1000 mL)(1 mol H3PO4/3 mol NaOH)= 3.19 moles H3PO4

(3.19 mol H3PO4/25.0 mL solution)(1L\1000 mL)= 1.276*10^-4
=0.000128 M

I appreciate your help.

Chemistry - DrBob222, Friday, December 10, 2010 at 5:43pm
0.0128 M is correct.
Thanks for showing your work.
Your error is in the first line. Instead of 3.19 moles, you never divided by the 1000 mL in the second term, and it should be 0.00319 moles. Then 0.00319 moles/0.025L = 0.12767 = 0.128 M.

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