Question
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
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
Answers
DrBob222
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
........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
craig
thank great help! much appreciated