Not sure if i posted this but

Question

Need help with my pH calculations?

0.10 mol of solid sodium hydrogen carbonate and 0.20 mol of solid sodium carbonate are
dissolved in the same beaker of water, transferred to a volumetric flask and made to 250.0
mL. The Ka for HCO3
– is 4.7 x 10–11.

a) What is the pH of the resulting buffer?
i got pH i calculated here is 10.63

b) What is the pH of solution after 20.00 mL of 0.050 mol L–1 hydrochloric acid solution is
added to 25.00 mL of the original solution?

c)What is the pH of the resulting buffer after 0.040 g of solid sodium hydroxide is added
to 25.00 mL of the original solution?

thx

Answer 1

a is ok.

b. You have how much in solution.
100 millimols Na2CO3 x (25 mL/250 mL) = 10
200 millimols NaHCO3 x (25/250) = 20
You are adding 20 mL x 0.050M = 1 millimol HCl.

.........CO3^2- + H^+ ==> HCO3^-
I........10........0.......20
added.............1..............
C........-1.......-1.......+1
E.........9........0........21
Substitute the E line into the HH equation and solve for pH.

c is done the same way.
........HCO3 + OH^= ==> CO3^2- + H2O
etc
etc