Experiment 12
THE EFFECT OF IONIC STRENGTH
ON THE SOLUBILITY OF AN ELECTROLYTE

Name: Student # Date

Determination of the Ca2+ concentration by AA spectrophotometry.
Complete the entries in the following Table:
Concentration
AAS reading mg/L M with error
Trial 1
Trial 2
Trial 3
Mean with 95 % CI
0

0
0.002
0.002
0.003
0.00716316
0.5

0.09990.0004
0.030
0.035
0.039
0.00716316
1.0

0.19980.0008
0.053
0.056
0.068
0.00716316
2.0

0.39950.0016
0.105
0.098
0.095
0.00716316
3.0

0.59930.0024
0.137
0.147
0.139
0.00716316
Calculate the regression line with Linplot and attach the graph.

The regression line is:

Solubility of Ca2+ observed.

[Ca2+]
Mean with 95 % CI

Trial 1
Trial 2
Trial 3

DI Water
0.235
0.246
0.190
0.15736
0.25 M NaCl
0.237
0.240
0.240
0.23430
Perform the following calculations and answer the accompanying questions for the laboratory report. All equations can be found in Table 2. Make sure you include errors.
From these data calculate the solubility (in mol L-1) ,with associated error of CaSO4 ,in distilled water.

From these data calculate the solubility (in mol L-1) ,with associated error, of CaSO4 in 0.25 M NaCl solution.

Determination of the Ca2+ concentration byEDTA titration.

Solution
Trial
Initial Reading
Final Reading
Total volume
Blanks
1
0.40.1
1.80.1
1.40.2

2
1.80.1
3.20.1
1.40.2

3
3.20.1
4.50.1
1.30.2

Mean
1.40.2
Di Water
1
19.60.1
33.60.1
14.00.1

2
34.60.1
48.50.1
13.90.1

3
10.60.1
24.60.1
14.00.1

Mean
14.00.2
0.25 M NaCl
1
24.60.1
34.50.1
9.90.1

2
0.50.1
10.80.1
10.30.1

3
10.80.1
20.90.1
9.90.1

Mean
10.0
From these data calculate the solubility (in mol L-1) ,with associated error of CaSO4 ,in distilled water.

From these data calculate the solubility (in mol L-1) ,with associated error, of CaSO4 in 0.25 M NaCl solution.

Calculate the activity coefficient for calcium Ca2+ in each solution using your measured solubilities S with eq 12 assuming that the mean activity coefficient  = Ca2+ = SO42-. It is not possible to measure the value of the activity coefficient for an individual ion like Ca2+ directly; only mean activity coefficients can be measured. Notice that the solubility depends only on this activity coefficient; all other parameters in eq 12 are constants.

1. Calculate the ionic strength (eq 2) of each solution assuming that both are saturated with respect to the solubility of gypsum and including the ions Ca2+, SO42-, Na+, and Cl- as appropriate. Calculate Ca2+ in each solution using the Davies Equation (eq 3), an extension of the Debye-Hückel Equation that can be used in solutions with ionic strength up to 0.5 mol/L. Compare these values with those obtained in the previous step.

2. Using the solubility product for gypsum (the mineral name for CaSO4.2H2O), calculate the activity of calcium, (Ca2+(aq)), in each solution (equation 6).