Question

Suppose you add 2.0 mL of 0.10 M HCl to 100 ml of buffer having 0.10 M HA and 0.20 M NaA.
a. Which species will react strongly with one another?

I believe the answer is Cl- and Na+ to form NaCl.

b. What will be the change in pH? pKa = 4.82
I am not sure how to find the answer to this one.

Answer 1

a is not right.
You are adding an acid to a buffer. The base will be the one reacting with the acid and the base is A^-.

b. In chemistry the first two thoughts about working a problem are as follows:
1. mols
2. ICE table
In a buffer solutiion, I usually do these by millimoles and not by M (concentrations).
mmols HA = 100 mL x 0.1 M = 10
mmols A^- = 100 mL x 0.2 M = 20
mmols HCl added = 2 mL x 0.1M = 0.2

.......A^- + H^+ ==> HA
I.....20.....0.......10
add.........0.2................
C....-0.2..-0.2.....+0.2
E.....19.8...0.......10.2

Substitute the E line into the HH equation and calculate pH then take the difference from the initial pH to find the change.

Answer 2

I got pH = 6.76