# Chemistry

posted by .

Strong base is dissolved in 675 ml of 0.200 m weak acid (ka=3.25x10^-5) to make a buffer with a ph of 3.95. Assume that the volume remains constant when the base is added.
HA + OH ---> H2O + A^-

calculate the pka value of the acid and determine the number of moles of acid initially present
-so i got pka=4.488 and 0.135 mol HA

When the reaction is complete , what is the concentration ratio of conjugate base to acid.
i got [A^-]/[HA]=0.2897

Then it says , How many moles of strong base were initially added?... All i know the strong base has to be less than the number of moles of acid but i have no clue how to get the moles of OH^-.

Can you guys help me? Thank you so much!

• Chemistry -

Note: m stands for molal. M stands for molar.
0.135 moles HA is correct.
4.88 is correct for pKa.
But (A^-)/(HA) = 0.117 and not your answer.

For the last part, how much OH was added initially.
.............HA + OH^- ==> A^- + H2O
initial....0.135..0.........0
added.............x
change.......-x..-x.........x......x
equil....0.135-x..0..........x......x

Since the 675 mL is the same for both base and acid, I can use the ratio simply as moles.
(A^-/HA) = 0.117
(x/0.135-x) = .117
Solve for x which will give you moles OH added as well as the moles base produced. I get something close to 0.014 (but you need to do it more accurately--and I like to check it to see if the ratio calculated really is 0.117).

• Chemistry -

how do you get 0.014 ? i seem to be getting another answer

• Chemistry -

How did you get the answer .117 ?

## Respond to this Question

 First Name School Subject Your Answer

## Similar Questions

1. ### Chemistry

If an equal number of moles of the weak acid HOCN and the strong base KOH are added to water, the resulting solution will be acidic, basic or neutral?
2. ### CHEMISTRY

Strong base is dissolved in 675 ml of 0.200 m weak acid (ka=3.25x10^-5) to make a buffer with a ph of 3.95. Assume that the volume remains constant when the base is added. HA + OH ---> H2O + A^- calculate the pka value of the acid …
3. ### CHEMISTRY

Strong base is dissolved in 675 ml of 0.200 m weak acid (ka=3.25x10^-5) to make a buffer with a ph of 3.95. Assume that the volume remains constant when the base is added. HA + OH ---> H2O + A^- calculate the pka value of the acid …
4. ### Chemistry

I keep getting the last part of this question wrong. Strong base is dissolved in 765 mL of 0.200 M weak acid (Ka = 4.63 x 10^-5) to make a buffer with a pH of 4.18. Assume that the volume remains constant when the base is added. HA(aq) …
5. ### chemistry

Strong base is dissolved in 665 mL of 0.400 M weak acid (Ka = 3.69 × 10-5) to make a buffer with a pH of 3.94. Assume that the volume remains constant when the base is added. HA(aq) + OH-(aq) -> H2O(l) + A-(aq) Calculate the pKa …
6. ### Chemistry

Select the statements that correctly describe buffers.?
7. ### Chemistry

Strong base is dissolved in 535 mL of 0.200 M weak acid (Ka = 3.16 × 10-5) to make a buffer with a pH of 4.04. Assume that the volume remains constant when the base is added. HA (aq)+ OH^-(aq) -> H2O(l) + A^-(aq) Calculate the …
8. ### Chemistry

Is it possible to make a buffer with a strong acid and a strong base?
9. ### Chemistry

Is it possible to make a buffer with a strong acid and a strong base?
10. ### AP Chemistry

A solution is prepared by titrating a 100.0 mL sample of 0.10 M HF (Ka = 7.2 × 10-4) with 0.10 M NaOH a. Does the solution contain a strong acid with a strong base, a strong acid with a weak base, a weak acid with a strong base, or …

More Similar Questions