[plasticfood]

i am trying to follow an example that we've did in class to do this problem, but there's something i'm not too sure about.

i have a buffer system with 70mL of .45M HF and .35M NaF, Ka HF = 7.2 * 10^-4
the problem says to calculate the pH after .002 moles of NaOH is added.

i wrote this reaction: HF + OH ---> H2O+  +   F-
i know that next i'm supposed to find the limiting reagent, but what is the concentration of F- that i need to know to find moles? I know that NaF completely disassociates, but does F- have the same concentration as NaF even though it's a salt? if not, then what is my next step?

[Borek]

Concentration of F^- is that of the salt. It goes up when you add NaOH.

[plasticfood]

thanks just wanted to make sure.

also i have another question. i have a buffer system with 70mL of .3M NH3 / .45M NH4Cl, kb for NH3 = 1.8e-5, and i need to find the pH.

when doing this problem, does the volume matter? do i just ignore it and just use the concentration to solve for the [H3O+]? 

[Borek]

If you take half of the solution, it will have exactly the same pH.

[plasticfood]

hmm not sure if i understand what you mean..

[UG]

Just use the concentrations

[plasticfood]

thanks