The borate ion, [B_4O_5(OH)_4]^2\'-, present in 4.0 mL of a saturated Na_2[B_4O_

Question

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 20.00 mL of 0.250 M HC1. 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

a)

assume:

1 mol of H+ reacts with 1 mol of OH-

so

1 mol of HCl = 1 mol of OH-

4 mol of HCl = 1 mol of [B4O5](OH)4]-2

so

for

mmol of acid = MV = 20*0.25 = 5

so..

ratio is 1:4 = 5*4 = 20 mmol of [B4O5](OH)4]-2

so..

mol = 20*10^-3 = 0.02 mol of [B4O5](OH)4]-2

b)

for Molar solubility

S = mol/V = 0.02 /(4*10^-3) = 5 M

c)

Ksp = [Na+]^2[[B4O5](OH)4]-2]

Ksp = (2*5)^2(5) = 500

d)

dG = -RT*ln(Ksp)

dG = -8.314*298*ln(500) = -15397.1 J/mol

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