Posted by ryan on .
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= 8.3log(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)?

chemistry lab 
drbob222,
ryanDid you post earlier under the name of Matt? I didn't answer that question because a TRIS buffer is made differently. But IF you have a solution of the acid form of TRIS, only (no base present), then 4.80 is the correct answer, I believe.
2. With that same understanding of the problem (no acid present in problem 2), then I found 10.48 for the pH instead of 10. Treat it as you would an ammonia solution (NH3).
RNH2 + H2O ==>RNH3^+ + OH^
Kb = (RNH3^+)(OH^)/(RNH2) = 2E6
(OH^) = sqrt(RNH2)*2E6 = sqrt(0.045*2E6)= 3E4
pOH = 3.52 and pH = 10.48
Check my thinking. Check my arithmetic.
3. I believe the way to approach this problem is as follows:
RNH3^+ + OH^ ==> RNH2 + H2O
pH = pKa + log (base/acid)
So lets start with 100 mL of the acid. That means we have 0.1 L x 0.05 M = 0.005 mols acid initially. If we add y mols NaOH, we will produce y mols of RNH2 and we will have remaining 0.005y mols RNH3^+.
9.3 = 8.3 + log(y/0.005y)
I have y = 0.00455 mols NaOH which is 4.55 mL (which I would round to 4.5 mL) for the volume. That SOUNDS reasonable but check my work. Check my thinking.