Posted by alexa on Saturday, April 21, 2007 at 2:49pm.
A buffer is formed by adding 500mL of .20 M HC2H3O2 to 500 mL of .10 M NaC2H3O2. What would be the maximum amount of HCl that could be added to this solution without exceeding the capacity of the buffer?
A. .01 mol
B. .05 mol
C. .10 mol
D. .15 mol
E. .20 mol
[I know the correct answer is B, but I don't know why. Any help would be great.]
just tell me where to start? please?
I don't get 0.05 mol but it's close.
Calculate pH of the buffer.
I have pH = pKa + log [(base)/(acid)]
pH = 4.76 + log [(0.05/0.1) = 4.46
The buffer capacity is the number of mols of strong acid or strong base that can be added to make the buffer change by 1.00 pH units. So adding strong acid we want the pH to be 1.00 less than 4.46 or = 3.46
3.46=4.76 + log (b/a)
-1.3 = log (b/a)
0.05 = (b/a)
If we add x mols of a strong acid, then
mols base = what we start with minus strong acid = 0.05 mols acetate - x mols strong acid.
mols acetic acid = what we start with + mols strong acid = 0.1 mols acetic acid + x mols strong acid.
0.05 = (0.05-x)/(0.1+x)
solve for x mols strong acid.
I found 0.043 mols which doesn't round to 0.05. In fact, I don't think any of the choices are right although 0.05 mols is the closest. However, putting 0.05 mols stong acid gives this for the pH.
acetate = 0.05 - 0.05 = 0
acetic acid = 0.10 + 0.05 = 0.15
and pH = 4.76 + (0.0/0.15) and we run into trouble because of the zero concentration of the base. The buffering capacity is gone if we have no acetate left. I hope this helps.
yah, i wasn't allowed a calculator so we had to guess and estimate things, but thanks so much that really cleared it up.
Answer This Question
More Related Questions
- Chemistry - A buffer solution is prepared by adding 14.37g of NaC2H3O2 and 16....
- Chemistry - A solution is prepared by dissolving of pure HC2H3O2 and of ...
- Chemistry - A solution is prepared by dissolving 25.0 g of pure HC2H3O2 and 25....
- Chemistry - A solution is prepared by dissolving 50.0 g of pure HC2H3O2 and 20.0...
- chemistry - suppose you made a buffer solution that was 0.050M in both HC2H3O2 ...
- Chemistry - I actually posted this up before and Gk helped out but i don't ...
- chemistry - A buffer is made by adding 0.300 mol HC2H3O2 and 0.300 mol NaC2H3O2 ...
- Chemistry - Calculate the pH of 75 ml of the buffer composed of .1M HC2H3O2 with...
- chem 2 - For 520.0mL of a buffer solution that is 0.140M in HC2H3O2 and 0.125M ...
- Chemistry - Buffer capacity is a measure of a buffer solution\'s resistance to ...