Question

A buffer solution of volume 50.0 mL contains 12.5 mmol HNO2 and 9.25 mmol KNO2.

1. What is the pH of this solution?
2. If 1.00 mmol of Ca(OH)2 is added to the solution, what will be the new pH?
3. If 2.00 mL of 2.00M HCl is added to the original solution, what will be the new pH?

If you could explain it step by step I'd really appreciate it!

Answer 1

1. Use the HH equation.
pH = pKa + log (KNO2)/(HNO2)

2. Without values of pKa any thing I calculate won't agree with your calculations. Here is what you do.
.......HNO2 + OH^- ==> NO2^- + H2O
I......12.5....0........9.25
add............2............
C......-2.....-2........+2
E......10.5....0........+2

Substitute the E line into the HH equation and solve for the new pH.

3. Follow the steps in 3 but
.......NO2^- + H^+ ==> HNO2
I......9.25....0........12.5
add............4............
C......-4.....-4........+4
E......etc.