posted by craig on .
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
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
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
thank great help! much appreciated