[Khemikooligan]

So I performed a lab over the last two weeks where I dissolved 2.5g of an unknown diprotic acid in 250mL of distilled water, transferred 10mL of that acid solution to a beaker and dillluted it with another 25mL of distilled water. I then titrated it with NaOH (I recorded the NaOH to have a pH of 12.21 so its Molarity should be around 0.016M), it took about 16mL to reach the second equivalence point. Also, I recorded pH and volume added after each addition. So my problem is I need to find the Molecular Weight of the unknown acid, I know it involves the volume of base from the second endpoint and the Concentration of the NaOH but I don't know what other value I'm supposed to use to determine the Molecular weight. I was thinking I would set the amount of moles of base = 2.5g divided by MW, but I am not sure if that works since I know my 10mL aliquot I withdrew did not have the full 2.5g in it. Does anyone know what I am supposed to do to determine MW?

[Borek]

Write the titration reaction equation (using H₂A for the acid).

http://www.titrations.info/titration-calculation

That should allow you to calculate number of moles of the acid - and you know mass of the sample (don't forget you were titrating only part of it). Knowing number of moles and mass you can easily find molar mass.

What I don't like about your description is that you used pH meter to determine NaOH concentration. Wasn't the concentration given to you? Finding it from pH is highly inaccurate, as you are ignoring effects of the ionic strength of the solution (my estimates suggest 0.19 M solution, not 0.16 M).

[Khemikooligan]

What I don't like about your description is that you used pH meter to determine NaOH concentration. Wasn't the concentration given to you? Finding it from pH is highly inaccurate, as you are ignoring effects of the ionic strength of the solution (my estimates suggest 0.19 M solution, not 0.16 M).

Yeah, I don't know why they didn't give us the concentration either. The professor who runs the labs purposely leaves the procedures we use to prepare for lab vague so we have to read our textbook and use our own wits to write a proper procedure.

There was no concentration listed in the bottles of NaOH on the counters when everyone came in so we just figured we would measure the pH to determine concentration. I honestly wouldn't be surprised if one of the other sections of our lab class didn't figure that out and are screwed on their calculations.

I was also curious about something else. When I did the lab my acid must have been either slightly impure or slightly insoluble because it took a good portion of time to dissolve it, even using heat and a magnetic stir bar. Because it took so long, I only got one titration done the first week and had to store the solution in a storage bottle with a cap and screw-on lid. When I went to do trials 2 and 3 the following week, my endpoints were taking about half as much of the volume of base. The first week, it took about 30mL, but trial 2 and 3 took both close to 16 mL to reach the second endpoint so I was wondering if anyone knew what could have happened to my solution to account for the difference?

The obvious answer would be that maybe my storage bottle wasn't clean but I was curious if it could have been something else like possible the diprotic acid dissociating a little in solution.

[Borek]

Generally speaking you should standardize your NaOH solution first: http://www.titrations.info/acid-base-titration-solution-standardization

the following week, my endpoints were taking about half as much of the volume of base. The first week, it took about 30mL, but trial 2 and 3 took both close to 16 mL to reach the second endpoint

Are you sure it was the same NaOH solution both weeks?