The literature value for the solubility of borax at 25oC is 1.2 X 10-2. Predict the Ksp at 25oC using your values for ΔH and ΔS, and calculate the percent error for your experiment.
the free energy for a reaction can be related to the equilibrium
constant through the formula below.
K = e (-ΔG° / RT)
Therefore if Kc for a reaction is known, Go can be determined, or vice versa. Furthermore, if
you have the value for Go at two different temperatures, you can calculate H and S through
the familiar equation for Gibbs energy below, since you have two unknowns but also two
equations.
G = H – T S
In this lab you will be studying the solubility of borax (Na2B4O5(OH)4*8H2O), a slightly soluble
sodium salt, at two different temperatures. When solid borax is added to water, the
equilibrium below is established.
Na2B4O5(OH)4*8H2O (s) 2 Na+ (aq) + B4O5(OH)42- (aq) + 8 H2O(l)
If you measure the concentrations for those substances that show up in the reaction quotient,
then the Kc for the reaction at that temperature can be calculated. In this lab, the
concentration of borate ion (B4O5(OH)42-) in solution will be measured by titration with standard
hydrochloric acid according to the equation below.
B4O5(OH)42- (aq) + 2 HCl (aq) + 3 H2O (l) 4 H3BO3 (aq) + 2 Cl- (aq)
The concentrations of the other substances that appear in the reaction quotient can be
calculated from the borate concentration using stoichiometry.
∆S°=590 J/mol.K
∆H°=188.69 kJ/mol
The literature value for the solubility of borax at 25oC is 1.2 X 10-2. Predict the Ksp at 25oC using your values for ΔH and ΔS, and calculate the percent error for your experiment.
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