Homework Help: chemistry

Posted by craig on Sunday, July 8, 2012 at 10:22pm.

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 value of the acid and determine the number of moles of acid initially present.

I got 4.43 for pKa. And 0.266 for moles HA.

When the reaction is complete, what is the concentration ratio of conjugate base to acid?

I got 0.32 for the concentration.

How many moles of strong base were initially added?

This part I keep getting 0.128 but it keeps telling me it is wrong. Please help

chemistry - DrBob222, Sunday, July 8, 2012 at 10:48pm
4.43 is right for pKa and 0.266 (266 millimols) is correct for mols HA initially. The base/acid ratio is 0.3235 (I would carry it to at least one more place so let's make it 0.324). The last part I would do this way--I think it is easier than using the ratio.
........HA + OH^- ==> A^- + H2O
initial 266,0..0......0.......0
add ...........x..................
change....-x...-x.....x.......x
equil...266-x...0.....x.......x

Then substitute into the HH equation as
3.94 = 4.43 + log(x)/(266-x)
and solve for x. I get 65 millimoles base added. The I check these things to see if that gives me the right pH.
pH = 4.43 + log (65/266-65)
pH = 4.43 + log(65/201) = 3.94
chemistry - craig, Sunday, July 8, 2012 at 11:21pm
thank great help! much appreciated

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