What is the new pH when 1mL of 1M HCl os added to 5mL of 0.1M Phosphate buffer (pH 7.4)?

Answer 1

I assume you are using pK2 for pKa. You need to look up the value in your text or notes. My OLD text lists pK2 as 7.21.
First,
7.40 = 7.21 + log (base/acid)
Solve for B/A = 1.549 which makes
B = 1.549*A
Then B + A = 0.1, solve these two equations simultaneously and I arrive at
A = 0.03923M and B = 0.06077M

5 mL of the buffer gives us
0.03923 x 5 mL = 0.196 mmoles A
0.06077 x 5 mL = 0.304 mmoles B.

...............Base + H^+ ==> Acid
initial mmols..0.304..0.......0.196
change mmols...-1.....1.........1
final mmols....0.204..0........1.196
Substitute into the HH equation. I obtain something like 6.3 or so but you need to confirm that. Check my work.