Results of addition of HCL and NaOH to acetic acid/acetate buffer solutions Writ
ID: 1067562 • Letter: R
Question
Results of addition of HCL and NaOH to acetic acid/acetate buffer solutions Write reaction equations to explain how your acetic acid-acetate buffer reacts with an acid and reacts with a base. Buffer capacity has a rather loose definition, yet it is an important property of buffers. A commonly seen definition of buffer capacity is: "The amount of H^+ or OH^- that can be neutralized before the pH changes to a significant degree." Use your data to determine the buffer capacity of Buffer A and Buffer B. Say, for example, that you had prepared a Buffer C, in which you mixed 8.203 g of sodium acetate, NaC_2H_3O_2, with 100.0 mL of 1.0 M acetic acid. a. What would be the initial pH of Buffer C? b. If you add 5.0 mL of 0.5 M NaOH solution to 20.0 mL each of Buffer B and Buffer C, which buffer's pH would change less? Explain.Explanation / Answer
Buffer A
molarity of NaC2H3O2 = 0.07 g/82.03 x 0.1 = 0.0085 M
initial pH = 4.75 + log(0.0085/0.1) = 3.70
to raise pH by 2 units
5.70 = 4.75 + log((0.0085 x 0.1 + x)/(0.1 x 0.1 - x))
0.0095 - 0.95x = 0.00085 + x
x = 0.00865/1.95 = 0.0044 mol
volume of NaOH to be added = 0.0044/0.5 = 8.9 ml
to lower pH by 2 units
1.70 = 4.75 + log((0.0085 x 0.1 - x)/(0.1 x 0.1 + x))
9 x 10^-6 + 0.0009x = 0.00085 - x
x = 0.00084/1.0009 = 0.00084 mol
volume of HCl to be added = 0.00084/0.5 = 1.7 ml
Similarly we may do the calculation for buffer B.
1. In acid
C2H3O2 - + H+ <==> HC2H3O2
In base,
HC2H3O2 + OH- <==> C2H3O2- + H2O
2. Buffer capacity is amount of acid or base needed to completely quench the buffer solution and make it a non-buffer solution.
3. Buffer C
NaC2H3O2 = 8.203/82.03 x 0.1 = 1 M
initial pH = 4/75 + log(1/1) = 4.75
Added NaOH
pH = 4.75 + log[((1 x 20 + 0.5 x 5)/25)/((1 x 20 - 0.5 x 5)/25)
= 4.86
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