Question

Can anyone help me with this pH question?

A buffer solution is prepared by mixing 76.5 mL of 0.0530 M benzoic acid with 28.6 mL of 0.0767 M sodium benzoate.


Calculate the pH (to two decimal places) of the buffer solution after the addition of 0.171 g of sodium benzoate (NaC6H5COO) to the buffer solution above.

Assume 5% approximation is valid and that the volume of solution does not change.



Here is a table of pKa values:
http://chemresources.chemistry.dal.ca/firstyear/acid_base.html





Please show your steps. Thank you.

Answer 1

the henderson hesselbach equation can be deduced for pf as if any generic weak acid dissociate like
HA = H^+ +A^-
Ph= Pka+ log(base/acid)

Answer 2

Thank you. But I still don't know what to do with the mass added beyond converting to moles.

Answer 3

Once you have converted them to moles, it will simply add to the amount/concentration of benzoate ions in the solution.

Answer 4

Ok, so I add those moles to the benzoate moles to form a new concentration? And does the bnzoic acid value change in any way?

Answer 5

No, because it is written: "Assume 5% approximation is valid and that *the volume of solution does not change*.

Answer 6

Alright. So I just need to get the new base concentration, and leave the acid concentration alone?

Answer 7

And then use the H-H equation to get the final answer?

Answer 8

Yep!