Homework Help: Chemistry

Posted by Alex on Wednesday, November 9, 2011 at 5:12pm.

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 - DrBob222, Wednesday, November 9, 2011 at 10:21pm
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 - Alexander, Thursday, November 10, 2011 at 4:54pm
how do you get 0.014 ? i seem to be getting another answer

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