A seemingly simple titration problem

  • #1
zenterix
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Homework Statement
In your lab, you find an old bottle labeled “sodium hydroxide” with no concentration on it. To determine what the concentration is, 50.0 mL of the solution was diluted to 100. mL and titrated to the equivalence point with 15.4 mL of 1.0 M HCl(aq). What is the molarity of the sodium hydroxide solution in the bottle?
Relevant Equations
This seems to be a simple problem but my answer does not match the solution (on MIT OCW).
I tried to solve the problem (problem 1 on this problem set) by noting that titrating to the equivalence point means neutralizing all of the NaOH with the titrant HCl.

Since 15.4mL of HCl at 1.0M was used, then 0.00154 mol of HCl was used to neutralize the same amount of NaOH.

Therefore, in the original sample of 50mL which came from the bottle, the concentration was 0.00154mol/50mL which is 0.308M.

The available answer from MIT OCW is 0.62M.

Now, a few observations

- I know that I am not doing a good job with significant figures. I imagine my answer should be 0.31M, which means the answer from MIT OCW is double mine.

- Not sure why though. After all, whatever NaOH was neutralized in 100mL was present in the original 50mL.
 
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  • #2
Your solution looks OK, must be mistake on the site.
 
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  • #3
If 1000ml contains 1 mole
then 100ml contains 0.1 mole
and 10ml contains 0.01 moles.
So 15.4ml contains 0.0154 moles, not 0.00154 moles.
Which is a typo on your part, of course as 0.0154 moles matches your answer of 0.308M. Which is correct answer.

Did you get the correct answers for the other questions?
 
  • #4
DrJohn said:
If 1000ml contains 1 mole
then 100ml contains 0.1 mole
and 10ml contains 0.01 moles.
So 15.4ml contains 0.0154 moles, not 0.00154 moles.
Which is a typo on your part, of course as 0.0154 moles matches your answer of 0.308M. Which is correct answer.

Did you get the correct answers for the other questions?
Yes, I did.
 
  • #5
So their answer of 0.62M is one of two things.
a) Wrong
or
b) They altered the question to make it slightly different, and forgot to alter the answer.
Settle for you got it right!
 
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