The Effect of Ionic Strength on the Solubility of an Electrolyte

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Title of Experiment 12:
The Effect of Ionic Strength on the solubility of an Electrolyte

Aim:
The purpose of this experiment was to determine the thermodynamics variable of enthalpy, ∆H for the dissolution reaction of boric acid in water. The solubility of boric acid was measured over a range of various temperatures by finding out at what temperature crystallization began for solution of different molarities. A graphical relationship between the natural logarithm of molal concentration and the inverse of the temperature gave a linear plot to determine the enthalpy changes associated with the reaction.

Methodology:

The procedure was carried out as follows in the CHEM2210 Laboratory Manual from page 58 to 59.

Results

 | Weight of conical flask (g)| Weight of Boric Acid (g)| Weight of water (g)| Total weight of conical flasks with boric acid solution (g)| Temperature of boric acid crystallise from solution (°C)| Total weight of conical flasks with boric acid solution at Room Temperature (g)| Total water mass lost (g)|

1| 133.738| 4.506| 50.021| 188.265| 37.4| 186.447| 1.818| | 114.311| 4.553| 50.127| 168.991| 36.8| 167.530| 1.461| | 115.476| 4.509| 50.069| 170.054| 37.2| 169.152| 0.902| 2| 133.558| 6.108| 50.147| 189.813| 46.5| 188.306| 1.507| | 114.115| 6.011| 50.269| 170.395| 45.3| 168.662| 1.733| | 115.387| 6.009| 50.210| 171.606| 45.7| 170.098| 1.508| 3| 74.593| 8.042| 54.712| 137.347| 58.5| 136.689| 0.658| | 86.137| 8.083| 50.367| 144.587| 62.5| 143.280| 1.307| | 63.950| 8.045| 50.940| 122.935| 59.5| 121.619| 1.316| 4| 124.694| 10.342| 50.321| 185.357| 62.8| 183.362| 1.995| | 132.160| 10.300| 50.017| 192.477| 63.3| 190.669| 1.808| | 115.665| 10.326| 50.048| 176.039| 64.7| 174.206| 1.833| 5| 114.297| 12.003| 50.019| 176.319| 75.2| 173.379| 2.940| | 168.315| 12.088| 50.132| 230.535| 73.4| 227.730| 2.805| | 111.457| 12.007| 50.111| 173.575| 76.7| 170.512| 3.063| Table 1: Raw data for 4.50g, 6.00g, 8.00g, 10.00g and 12.00g of Boric Acid dissolved in 50.00g of water.

1. Plot the results as a solubility curve of temperature versus molal concentration (mol/kg). Table 2: Mean Mass of Boric Acid, water and Number of Moles with Associated Errors

Average Mass of Boric Acid (g)| Error| Mass of water (g)| Error| No. of moles of Boric Acid| Error| 4.52| 0.061| 50.07| 0.1378| 0.0732| 0.00099|
6.04| 0.077| 50.21| 0.0938| 0.0977| 0.00125|
8.06| 0.053| 52.01| 5.649| 0.1304| 0.00086|
10.32| 0.042| 50.13| 0.3952| 0.1670| 0.00068|
12.03| 0.111| 50.09| 0.1469| 0.1946| 0.00180|
Molecular mass of Boric Acid, H3BO3 = 10.811 + 3(15.9994) + 3(1.00794) = 61.833
= 61.83g/mol (2 d.p.)

Mean (x̄) mass of Boric acid = 4.506+4.553+4.509 3
= 4.523g = 4.52g

Associated Error = x̄ ± cw
= 4.52±(1.30 x 0.047)
= 4.52±0.061g

Number of moles in 4.52g of Boric Acid = MassMolecular Mass

= 4.52±0.061g61.83

= 0.07310±0.00099mol
= 0.0731±0.00099 mol

Mean (x̄) mass of water = 50.021+50.127+50.069 3
= 50.072g = 50.07g

Associated Error = x̄ ± cw
= 50.07 ±(1.30 x 0.106)
= 50.07±0.138g

y = 15.0975x + 15.3231
R² = 0.9490

y = 15.0975x + 15.3231
R² = 0.9490

Graph 1: Solubility curve of temperature versus molal concentration (mol/kg).

Molal Conc. (mol/kg)| Error| Temperature (°C) at which
first Crystals Appears| Error|
1.4609| 0.0225| 37.1| 0.78|
1.9465| 0.0267| 45.8| 1.56|
2.5066| 0.1251| 60.2| 5.2|
3.3306| 0.0214| 63.6| 2.47|
3.8855| 0.0388| 75.1| 4.29|
Table 3: Mean Temperature of Crystallization at different Molar Concentrations

Molar concentration of 4.52g Boric Acid= number of molesVolume

= 0.0731±0.00099 mol0.05007 0.000138kg
= 1.4599±0.0225mol/kg

Mean (x̄)...
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