Prepare in your lab notebook BEFORE you attend class for the pre-lab An original
ID: 1063175 • Letter: P
Question
Prepare in your lab notebook BEFORE you attend class for the pre-lab An original title, date, procedure reference, and the following calculation: The borate ion, [B_4O_5(OH)_4]^2-, present in 4.0 mL of a saturated Na_2[B_4O_5(OH)_4]middot 8H_2O solution at 48.0 degree C is diluted to 30.0 mL with warm deionized water. The resulting solution was titrated to the Bromo cresol green endpoint with 20.00mL of 0.250 MHCl. How many moles of [B_4O_5(OH)_4]^2- are present in the sample? Calculate the molar solubility of [B^_4O_5(OH)_4]^2- in the sample? Calculate the K_sp of Na_2[B_4O_5(OH)_4] from the information above? Calculate the free energy change (delta G degree) for the dissolution of Na_2[B_4O_5(OH)_4] middot 8H_2O at the given temperature. What does the sign indicate about the solution process?Explanation / Answer
ANSWER
(a) Moles of HCl consumed by [B4O5(OH)4]2- = Molarity X Volume = 0.25M X 20mL = 5.0mmol
Two moles of H+ are consumed by one mole of [B4O5(OH)4]2-, hence 5mmol of H+ will be consumed by 2.5mmol of [B4O5(OH)4]2- . Hence 2.5mmol of [B4O5(OH)4]2- are present in solution.
(b) Total volume = 4mL + 30mL = 34mL
34mL of solutin contain 2.5mmol or 2.5 X 10-3 mol of [B4O5(OH)4]2-
1000mL (1L) will contain (2.5 X 10-3 / 34.0) X 1000 = 0.07 mol
Hence molar solubility = 0.07 mol/L
(c) Na[B4O5(OH)4] -------------> 2Na+ + [B4O5(OH)4]2-
Ksp = [Na+ ]2 [B4O5(OH)4]2-
Ksp = [0.07]2 [0.07] = 3.4 X 10-4
(d) G = -RTlnKsp
G = - 8.314 X 321 ln(3.4 X 10-4)
G = + 21.3KJ/mol
Positive sign indicaates dissolution is anisotropic.