Question

Here are 2 questions I worked out, but need them checked. Also, can't get answer to another question correct unless question 1 is correct. Would you check the first 2 and help me with the third?

1) What is the pH of a 0.05 M solution of TRIS acid (pka = 8.3)?

My answer: I used the formula pH= (pka- log (HA)} /2 then I plugged in:
pH= 8.3-log(0.050)/2
pH= 4.8

2) What is the pH of a 0.045 M solution of TRIS base?

My answer: I used the formula PH=pka+14+log (base)/2

Then i plugged in:
pH=8.3+14+log(0.045M)/2 =
pH=10

3) How many total mL of 1 M NaOH can you add to the solution in problem 1 and still have a good buffer (that is, within 1 pH unit of the pka)?

Thank you.