Chemistry

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IN the polystyrene beaker, mix 20mL of 0.1 M Acetic acid and 25mL of 0.1M Sodium Acetate and immediately measure the pH. Remove the electrode and add 5ml of 0.1 M HCl to this buffer. Stir the solution and measure the pH.

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INFO:

concentration of acetic acid= 0.1M- 20ml
concentration sodium acetate= 0.1M- 25ml
concentration of hydrochloric acid- 0.1M - 5ml

pH measured:

Buffer solution- 4.69
Buffer + 5ml 0.1M HCL- 4.43

pH calculated????????????
Buffer solution- ?????
Buffer + 5ml 0.1M HCL- ?????
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PART A:

1.So i have to calculate pH before the addition of HCL

2. show CALCULATIONS for calculated pH after addition of HCl

I think i use the henderson hasselbalch equation:

pH= pka+log [A-]/[HA]
pH= pka+log [0.1][0.1]

I dunt know if i plugged in the right stuff and how do i calculate pka?????

• Chemistry - ,

yes and no.
pH = pKa + log (base/acid)
You are right that base is acetate and acid is acetic acid.
You know pH = -log (H^+)
pKa is done the same way.
pKa = -log Ka. If you use 1.8 x 10^-5 for Ka of acetic acid, pKa = 4.74.

Let's call acetic acid HAc, sodium acetate is NaAc and acetate ion is Ac^-.
(HAc) = 0.1 M x (20 mL/45 mL) = ?? in the original solution.
(Ac^-) = 0.1 M x (25 mL/45 mL) = ?? in the original solution.

When adding HCl, the Ac^- ties it up as HAc (which ionizes only partially and that's why this is a good buffer--it won't let the HCl loose to ionize completely) so HAc increases by the amount of HCl added and the Ac^- decreases by the amount of HCl added. My advice is to work in moles or millimoles instead of concn It makes it simpler.