Question

Hi! i need to calculate the Ka for acetic acid from an experiment using an ice diagram but my Ka value keeps coming up negative. I'm using the pH of the acetic acid before titration with NaOH (pH = 2.96) to get [H3o+] and [OAc] to equal 10^(-2.96) which is 1.1x10-3. And the moles of acid in my original flask was 0.004 HOAc, so can someone show me the calculations if I am doing them wrong or help me figure out what I may be doing wrong? Thank you!

Answer #1

First, write the Ka expression

HA = H+ + A-

where HA is acetic acid, A- acetate, and H+ the acidic proton

then

Ka = [H+][A-]/[HA]

we need, in equilbirium

[H+]= x

[A-] = x

[HA] = M-x

we already have M = 0.004 M

[H+]= x

[A-] = x

[HA] = 0.004 -x

and we can get x via pH

[H+] = x= 10^-ph = 10^-2.96 = 0.0010964

now,... x = 0.0010964

[H+]= x = 0.0010964

[A-] = x = 0.0010964

[HA] = 0.004 -x = 0.004-0.0010964 = 0.0029036

substitute in Ka

Ka = [H+][A-]/[HA]

Ka = (0.0010964)(0.0010964)/0.0029036

Ka = 0.00041400

Ka = 4.14*10^-4

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