[labrat42]

Solution A is a monoprotic acid that is 4.0 M.  When diluted to 0.27 M and added to 87.0 mL of NaOH (molarity between 0.07-0.1 M), the resulting solution has a pH of 1.08. 

Solution B is also a monoprotic acid of unknown concentration, also with a pH of 1.08.  (Chances are it is the same solution.)

Can one safely assume that the concentration of solution B is also 0.27 M, and is there a way to determine this based on the pH alone?  Say, if there is not enough NaOH to titrate to the equivalence point?

Thanks in advance 

[Borek]

Any information about volume of the solution A?

Can you use the information given to judge if the solution A contains a weak, or a strong acid?

[labrat42]

My questions above are about a recent lab, where we were to find the concentration of an unknown acid (solution B), using whatever method we could figure out & the equipment out in the lab.

I decided to titrate a sample of solution A, a 4.0 M acid with w solution of NaOH, then use more of the NaOh to titrate solution B in order to find the concentration.  We weren't allowed to ask questions or talk during the lab, and I was the only one who appeared to be addressing it that way.  (My classmates were not titrating at all.). We didn't have color indicators but we had done a similar experiment using KHP to standardize the NaOH & a pH meter.

basically i titrated twice, neither time was I able to reach anything near the equivalence point before running out of NaOH.  When the lab was over all I had accomplished was raising the pH of solution A to 1.08 which was the same pH as my unknown.

my teacher seemed to approve of the fact that I chose titration, and I am not sure what to do about the write up.  It was a practical exam for a chunk of my grade and I am hoping to come up with something other than I screwed up, twice.

looking back I realize i should have either made a stronger NaOH solution or made more of it, but having no idea what concentration or how much to make I thought I'd be ok patterning it after the previous lab.

or maybe I should have standardized the NaOH with a KHP solution, but at the time I thought using the acid of known molarity would be a faster way to go.

[labrat42]

I don't know whether it is a strong or weak acid, but my guess is that it was either KHP used in a previous experiment, or HCl.

and the reason the standard acid was diluted is because at the last minute when I was getting desperate & running out of NaOH I diluted it hoping to get the pH closer to 7. 

I know that is probably bad form & makes earlier data points irrelevant, but I was almost out of time & couldn't figure out what else to do. 

[Borek]

Looks like a mess.

Titration was probably the right idea, but you should start trying to estimate amount of base required to get to the endpoint (from what you are saying you did know the acid concentration, so it was a simple stoichiometry). If you didn't get there, your data looks rather useless to me.

I understand you did the potentiometric titration, measuring pH of the solution?

[labrat42]

Thanks for your response. And for bearing with the stupid questions, as I am really trying to learn here...

potentiometric titration - I haven't heard that term but we used one electrode to measure the pH of our solution while titrating.  New to our school this term is the Vernier system & LabQuest app.  It is a little handheld computerized device that is wonderful for collecting data but only saves to a thumb drive about half the time.

To be sure i understand, since the standard was a 4.0 M concentration (unknown, but monoprotic) I could have estimated that I would need 4 moles of base to reach my equivalence point. 

for a NaOH solution of 4 M I would have weighed out 26.4 grams NaOH to make 150 mL.  (just guessing how much solution I would need, to titrate both the standard and my unknown.  The buret holds 50 mL so that seems like a good amount to make )

I don't think my teacher would want me to take 26 grams so I could have measured out 2.937 g NaOH and diluted to 250 mL to make a 0.267 M NaOH solution.  Then I could have measured out 10 mL of the 4.0 M acid standard, diluted that to 150 mL and had a standard of 0.267 M to titrate with.  I would have used 150 mL of the NaOH, leaving me 100 mL of NaOH to work with for my unknown.  Am I right so far?

how can i know if that was enough remaining NaOH? My teacher did tell me that my unknown was more dilute than the standard (after I'd been titrating for awhile, that is when I realized I would need to start over.)

am I right that in both titrations I needed to go past a pH of ~7 in order to reach the equivalence point?

I am still not sure how I should have known how much solution to make.  Am i on the right track?

[Borek]

potentiometric titration - I haven't heard that term but we used one electrode to measure the pH of our solution while titrating.

That's a potentiometric titration. Or pH-metric titration.

I don't think my teacher would want me to take 26 grams so I could have measured out 2.937 g NaOH and diluted to 250 mL to make a 0.267 M NaOH solution.  Then I could have measured out 10 mL of the 4.0 M acid standard, diluted that to 150 mL and had a standard of 0.267 M to titrate with.  I would have used 150 mL of the NaOH, leaving me 100 mL of NaOH to work with for my unknown.  Am I right so far?

With 50 mL burette in general you should aim for about 40 mL of NaOH solution, see http://www.titrations.info/titrant-and-sample-volume. From there - knowing titrant concentration - you should work backward, to find volume of the acid that should be titrated. Perhaps it would require an additional dilution step.

Alternatively, if you know what volumetric glass you can use, you could start from there, knowing volume of the acid you can calculate concentration of NaOH that will require around 40 mL of titrant.

You don't have to use all your acid, quite the opposite. What is important is the concentration of titrated sample and titrant, I would aim for both to be somewhere around 0.1 M.

am I right that in both titrations I needed to go past a pH of ~7 in order to reach the equivalence point?

Yes.