Question

Question 1

Calculate the concentrations of acetic acid and sodium acetate in the buffer solution you will prepare in this experiment:

Acetic acid concentration is Blank .01 M (2 dec places)

Sodium acetate concentration is Blank .01 M (2 dec places)

The theoretical pH of this buffer solution is (hint: use Henderson-Hasselbach) is 4.74 (2 dec places).

Question 2

Which reaction occurs when you add NaOH to the buffer solution? (Ac = acetate)

	a.	OH-   +   H3O+ ↔ 2 H2O
	b.	Ac- + H3O+ ↔ HAc + H2O
	c.	HAc   + H3O+ ↔  Ac- + H2O
	d.	HAc + Ac- ↔ Ac- + HAc
	e.	Ac- + OH- ↔ AcOH
	f.	HAc + OH- ↔ Ac- + H2O
	g.	HAc + H3O+ ↔ H2 + H2O + Ac-

Question 3

What is the pH of a solution formed by adding 2 X 10^-3 mol of NaOH to 100 ml of the buffer described in question (1)? The calculated pH value is Blank 1 (2 decimal places).

Question 4

Which reaction occurs when you add HCl to the buffer solution? (Ac = acetate) (H₃0⁺ = acid)

	a.	Ac- + OH- ↔ AcOH
	b.	HAc   + H3O+ ↔  Ac- + H2O
	c.	HAc + OH- ↔ Ac- + H2O
	d.	HAc + H3O+ ↔ H2 + H2O + Ac-
	e.	OH-   +   H3O+ ↔ 2 H2O
	f.	HAc + Ac- ↔ Ac- + HAc
	g.	Ac- + H3O+ ↔ HAc + H2O

Question 5

If I put a drop of thymol blue into the above buffer solution, the solution color would be Blank 1. If I put a drop of bromothymol blue in the above buffer solution, the solution color would be Blank 2. If I put a drop of phenolphthalein into the above buffer solution, the solution color would be Blank 3.

Answer 1

pKa of acetic acid/acetate buffer = 4.74

pH = pKa + log [acetate/acetic acid]

If the concentration of acetic acid and acetate is same, pH = pKa. So, the concentration of acetic acid = 0.01M and concentration of sodium acetate is 0.01M

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2. (f) : HAc + OH^- ↔ Ac^- + H₂O

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NaOH will react with acetate to convert it to acetate.

Moles of acetic acid intially present = 0.01 moles (If you have a 1L solution of buffer)

moles of acetic acid present after adding NaOH = 0.01-0.002 = 0.008 moles

Moles of acetate present = 0.01 moles + 0.002 moles = 0.012 moles

pH = pKa + log [acetate/acetic acid]

     = 4.74 + log [0.012/0.008] = 4.92

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4. g.

Ac^- + H₃O⁺ ↔ HAc + H₂O

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If I put a drop of thymol blue into the above buffer solution, the solution color would be yellow. If I put a drop of bromothymol blue in the above buffer solution, the solution color would be yellow. If I put a drop of phenolphthalein into the above buffer solution, the solution color would be colorless.