Help Please! A) preparation of buffer solution: 6.606g of sodium acecate trihydr
ID: 695693 • Letter: H
Question
Help Please!
A) preparation of buffer solution:
6.606g of sodium acecate trihydrate was weighed out (MM=136 g/mole) in a small beaker. 25 ml of water was added to 100 ml volumetric flask. 8.00 ml of a 6 M acetic solution was added.
B) pH of water with added NaOH and HCl
Prepare two beakers (Beaker 1 and Beaker 2) and pipet 25 mL of deionized water into each beaker. Measure the pH of the water in the beaker. In Beaker 1, add 1 drop of 1 M NaOH into the beaker. Stir and read the pH of the resulting mixture. Add an additional drop of the 1 M NaOH solution to the beaker, stir and measure the pH. Add an addition 4 drops for a total of 6 mL and then 4 more drops for a total of 10 drops, add 10 more drops and finally using a volumetric or graduated pipet, add 1 mL of 1M NaOH and read the pH after each addition. To Beaker 2, add 1 drop of 1 M HCl into the beaker. Stir and read the pH of the resulting mixture. Add an addition 1 drop of the 1 M HCl solution to the beaker, stir and measure the pH. Add an addition 4 drops for a total of 6 drops and then an additional 4 drops for a total of 10 drops, add 10 more drops and finally add 1 mL of 1 M HCl, read the pH after each addition.
C) pH of buffer solution with added NaOH and HCl
Prepare two additional beakers (Beaker 3 and Beaker 4). Pipet 25 mL of the prepared buffer solution from part A into each beaker. Measure the pH of the buffer solution. To Beaker 3, using a graduated pipet, add 1 mL of 1 M NaOH. Stir and read the pH of the resulting mixture. Add an addition 1 mL of the 1 M NaOH solution to the beaker, stir and measure the pH. Add 4 mL more for a total of 6 mL and then an additional 4 mL for a total of 10 mL, finally add two 10 mL additions, read the pH after each addition. To Beaker 4 containing 25 mL of buffer, repeat the steps of beaker 3 with the addition of 1 M HCl into the beaker. Stir and read the pH after each addition.
Questions: only need 4-6
4)Calculate the pH of the buffer solution after 2 mL of NaOH was added to Beaker 3 and after 2 mL of HCl was added to Beaker 4. Compare this to your measured pH
5)Compare the measured pH for the addition of 1M NaOH and 1 M HCl to beaker 3 and 4 with the buffer solutions to the measured pH in Beaker 1 and 2 with the water.
6)From the measured pH of the acetic acid buffer solution above before the addition of any NaOH or HCl solution, calculate the K?a for acetic acid. Compare to CH3COOH Ka value which is 9.1x10^-6?
Water
Buffer solution
Beaker 1
Beaker 2
Beaker 3
Beaker 4
Total Volume of
1 M NaOH
added
pH
Total Volume of
1 M HCl
added
pH
Total Volume of
1 M NaOH
added
pH
Total Volume of
1 M HCl
added
pH
0 mL
7.08
0 mL
7.01
0 mL
6.02
0 mL
6.05
1 drop
9.86
1 drop
3.13
1 mL
11.87
1 mL
5.52
2 drops
10.38
2 drops
2.69
2 mL
12.38
2 mL
5.22
6 drops
10.86
6 drops
2.25
6 mL
12.74
6 mL
4.46
10 drops
11.08
10 drops
2.02
10 mL
12.87
10 mL
3.53
20 drops
11.35
20 drops
1.76
20 mL
12.99
20 mL
1.04
2 mL =
(40 drops)
11.68
2 mL
1.47
30 mL
13.06
30 mL
0.89
Water
Buffer solution
Beaker 1
Beaker 2
Beaker 3
Beaker 4
Total Volume of
1 M NaOH
added
pH
Total Volume of
1 M HCl
added
pH
Total Volume of
1 M NaOH
added
pH
Total Volume of
1 M HCl
added
pH
0 mL
7.08
0 mL
7.01
0 mL
6.02
0 mL
6.05
1 drop
9.86
1 drop
3.13
1 mL
11.87
1 mL
5.52
2 drops
10.38
2 drops
2.69
2 mL
12.38
2 mL
5.22
6 drops
10.86
6 drops
2.25
6 mL
12.74
6 mL
4.46
10 drops
11.08
10 drops
2.02
10 mL
12.87
10 mL
3.53
20 drops
11.35
20 drops
1.76
20 mL
12.99
20 mL
1.04
2 mL =
(40 drops)
11.68
2 mL
1.47
30 mL
13.06
30 mL
0.89
Explanation / Answer
A) preparation of buffer solution:
6.606g of sodium acecate trihydrate was weighed out (MM=136 g/mole) in a small beaker. 25 ml of water was added to 100 ml volumetric flask. 8.00 ml of a 6 M acetic solution was added.
mol of sodium acetate trihydrate=6.606g/136g/mol=0.0486 mol
mol of acetic acid =0.008L*6mol/L=0.048mol
concentration of acetate=0.0486mol/108ml=0.0486mol/0.108L=0.45M
concentration of acetic acid=0.048mol/0.108L=0.44M
pka acetate/acetic acid buffer=4.75
using henderson-hasselbach equation,
pH=pka+log[acetate]/[acetic acid]=4.75+ log 0.45/0.44=4.75+0.0097=4.76
pH=4.76
4) After 2ml of 1M NaOH is added,mol of NaOH added=0.002L*1mol/L=0.002mol
0.002mol NaOH is added to neutralize 0.002 mol acetic acid in the buffer ,leading to formation of 0.002mol acetate
mol of acetic acid in beaker 3 =0.025L*0.44 mol/L=0.011mol
mol of acetic acid after neutralization=0.011mol-0.002=0.009mol
mol of acetate in beaker 3=0.025L*0.45mol/L=0.01125mol
mol of acetate remaining in the beaker=0.01125+0.002=0.0132mol
pH=4.75+log 0.0132/0.01125=4.82
pH=4.82
measured pH=12.38 very high comparatively
After 2ml of HCl was added to beaker 4,
0.002mol HCl is added to neutralize 0.002 mol acetate in the buffer ,leading to formation of 0.002mol acetic acid
mol of acetate in beaker 4 =0.025L*0.44 mol/L=0.011mol
mol of acetic acid after neutralization=0.011mol+0.002=0.013mol
mol of acetate in beaker 3=0.025L*0.45mol/L=0.01125mol
mol of acetate remaining in the beaker=0.01125-0.002=0.00925mol
pH=4.75+log 0.00925/0.013=4.602
pH=4.602
measured PH=5.22
pH=4.82
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