[giustina53]

I mixed 50 mL of 0.1 M sodium dihydrogen phosphate (NaH2PO4) and 0.1 M disodium hydrogen phosphate (Na2HPO4) to make a buffer solution.

My teacher asked if the volume would go down and why? I didnt see it go down but I have a feeling volume decreases but I dont know why. Might it have something to do with more bonds being formed when the acid and base mix?

Then I placed 25 mL of the buffer solution in another beaker with 1 mL of 0.1 M NaOH. How do you calculate the pH of this new solution?

And lastly, I have to calculate the mol of A- (which here is the HPO4 2- molecule). He gives a hint saying that "rememeber the initial solution contained half A- and half HA." Im pretty sure the Henderson-Hasselbach. He also told me to find mol of NaOH cause it equal to A- and then subtract this from total mol of phosphate in the original soln to get the mol of acid (HA). This isnt working though. I think I'm doing something wrong.

Thanks in advance!!!!

[Babcock_Hall]

The volumes are additive in this case.  However, when you mix two unlike solvents, volumes are not always strictly additive.  With respect to the second half of your question, my general approach is the following:
Identify the acid and the base and write a chemical reaction.
Convert the concentrations into amounts in moles, mmoles, or µmoles
Treat this problem as a limiting reagent problem to calculate final amounts from initial amounts
If the problem asks for it, convert the final amounts into final concentrations.

[Babcock_Hall]

Although the Henderson Hasselbalch equation can be used to calculate the final pH in this problem (after addition of NaOH), one does not need it to get started.  The numbers of moles of NaH₂PO₄ and of Na₂HPO₄ are essentially unchanged upon mixing these two solutions.

[giustina53]

OK, thank you very much for your *delete me*

[giustina53]

^^^wow, i was trying to says thanks for the help. I dont know why 'delete me' came up?

lol thanks again

[Babcock_Hall]

There's a funny bug in the software; I can't quite recall what key brings that about.  Did you solve the problem?

[giustina53]

Yes I was able to solve it! Thank you very much!