[calvin coolidge]

So in the lab, we were assigned to create a 100mL, 100mM buffer w/pH of 6.8..which we did, no problem. Our measured pH for the buffer was 6.6. We added 1.0mL of .1M NaOH and 1.0mL of .1M HCl to 50mL of water, in separate beakers, and got pH of 11.51 for NaOH+H2O and 2.18 for HCl+H2O. We calculated the the pH for these two at 11.31 for NaOH+H2O and 2.69 for the HCl+H2O. For the post lab report, we're to account for the variation for the buffer's pH and the acid or base + H2O. The variation for these values seem quite small and the only thing that comes to mind for a cause would be either the measurements of the Na2HPO4 and/or NaH2PO4 hydrates for the buffer creation, or the pH measurement probe (which was calibrated prior to taking any measurements)..so....am I missing something else that could be contributing to the differences?

Thanks for any helpful input

~CC

[Borek]

http://www.chembuddy.com/?left=pH-calculation&right=ionic-strength-activity-coefficients

[calvin coolidge]

 ....wow, Borek.....this is for a chem112 lab, and that's a lot of info I've never seen in my text, lab manual, or heard in lecture...clearly, I'm playing Sorry and you're playing Chess 

So, from what I read from the info from your link, not taking the Ionic strength of the reactants into account would cause the variation between the pH measurements and calculations?


Thx,

~CC

[Borek]

So, from what I read from the info from your link, not taking the Ionic strength of the reactants into account would cause the variation between the pH measurements and calculations?

Exactly - and always. This is a real problem, especially when it comes to concentrated solutions - in fact, apart from specific cases, we can't correctly calculate pH of solutions with ionic strength above 0.1.

[calvin coolidge]

Thanks, Borek. This will give me something else to cover in my lab report for variation OTHER than the all too oft used, "the balance used to measure the grams of component needs calibrated..."...I HATE having to use that as a reason for variation as it feels like a "cop out" answer.

Thanks again!

~CC