Question

1. In the Thermodynamics of Borax Dissolution experiment, a student quickly transferred a 5.00 mL sample of the saturated borax solution into an Erlenmyer flasks using a pipet and pipet pump after the borax solution had cooled to 35oC. The student rinses the pipet with warm ddH2O, and add the rinsing to the flask. Then the student titrates the solution with a 0.500 M HCl solution. 12.00 mL of the HCl solution was used to reach the endpoint of the titration. Calculate the value of Ksp for borax at 35oC. You can use the following steps to do the calculation. Show your work.
a. Calculate the number of moles of HCl that were added during the titration.




b. Use following equation to relate the number of moles of HCl to the number of moles of tetraborate ion in the 5.00 mL sample.
B4O5(OH)42- (aq) + 2H+ (aq) + 3H2O (l)  4H3BO3 (aq)




c. Calculate the concentration of tetraborate ions in the 5.00 mL sample.





d. Use the concentrations of tetraborate ions to calculate the equilibrium constant (Ksp) at 35oC.
Na2B4O5(OH)4·8H2O (s) 2Na+ (aq) + B4O5(OH)42-(aq) + 8H2O (l)
Ksp = [Na+]2[B4O5(OH)42-]
[Na+] = 2[B4O5(OH)42-]
Ksp = 4[B4O5(OH)42-]3

Answer 1

a).500Mx.012L=.006mole
b) ..5*.006=.003moles
c) .003mole/.005Liter=.0M
d)4*.6^2=.90=Ksp