Chemistry (molarity) (sorta an emergency)

posted by Ray

I'm doing a titration lab and writing a lab report where I'm sort of stuck on how exactly to find the concentration of HCl. These are the following info I have from the lab,
Equation:
HCl(aq)+NaOH(aq)-> H2O(l)+NaCl(aq)

-Calculated molarity of NaOH solution is .103M
-Volume of NaOH solution used was 15.95mL
-Volume of HCl used was 20.01mL

Now the question is what is the molarity of HCl in this case?

Knowing the molarity of NaOH, I converted from the volume of NaOH solution used to moles of NaOH.
(0.01595L NaOH)(0.103M NaOH)~0.00164mol NaOH
Since the balance equation is a 1 to 1 ratio between NaOH and HCl, does that mean I also have ~0.00164mol of HCl as well??

I assumed that that was the right answer and proceeded to calculate the molarity of HCl by (0.00164mol HCl)/(0.02001L HCl)~0.0821 [HCl]

Am I doing this correctly or am I missing an important step? Thanks in advance!

  1. bobpursley

    The titration equation is a lot easier to use (normality of both solutions is same as Molrity)

    Macid*Vacid=Mbase*Vbase

    Macid=Mbase*Vbase/Vacid

    = .103M*15.9ml/20.01ml which is not exactly the same as you got.

  2. Ray

    It looks like it's the same answer, but how we got to it was different. With your equation, that's what I would take on for this lab, but my chemistry teacher warned us a million times to NOT do it that way, don't know why, but she wanted us to do it the way I described above, more confusing in a way, but was just double checking if I'm doing it correctly or not, thanks

  3. DrBob222

    Your way is correct but I would suggest you not use the approximations but actual calculated values and keep the number of significant figures correct.
    Bob Pursley is absolutely correct, also. The main reason today's teachers don't use that method is because normality is not taught in the classroom anymore. Mr. Pursley and I agree that's a huge mistake; however, normality and chemical factors (along with a few other concepts) have been abandoned. Pity. But progress? moves along. We "old" guys are about the only ones now that know how to use those abandoned concepts.

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