Delta H = 138569 J Delta S = 434 J Calculate K_sp for: Na_2B_4O_5(OH)_4 middot 8

Question

Delta H = 138569 J

Delta S = 434 J

Calculate K_sp for: Na_2B_4O_5(OH)_4 middot 8 H_2O_(s) rightarrow 2 Na^+_(aq) + B_4O_5(OH)^2-_4_(aq) + 8 H_2O_(l) at 65.0 degree C and at 20.0 degree C, using your laboratory results for delta H degree and delta S degree. Calculate the solubility of borax in grams per liter at 65.0 degree C and at 20.0 degree C, using the value of K_sp you calculated for each temperature from Question 2. Some solid borax may have been transferred along with the saturated solutions. How would/could did that affect your plot?

Explanation / Answer

2. dG = dH - TdS

with given dH and dS values,

at T = 65 + 273 = 338 K

dG = 138569 - 338 x 434 = -8123 J

Using,

dG = -RTlnKsp

-8123 = -8.314 x 338 lnKsp

Ksp = 18.00

at T = 20 + 273 = 293 K

dG = 138569 - 293 x 434 = 11407 J

Using,

dG = -RTlnKsp

11407 = -8.314 x 338 lnKsp

Ksp = 9.2 10^-3

3. let x amount of salt is in solution at T = 65 oC,

Ksp = (2x)^2.(x)

18 = 4x^3

solubility of borax = x = 1.65 M at 65 oC

                             = 1.65 x 381.37 g/mol

                             = 629.29 g/L  

let x amount of salt is in solution at T = 20 oC,

Ksp = (2x)^2.(x)

9.25 x 10^-3= 4x^3

solubility of borax = x = 0.132 M at 20 oC

                             = 0.132 x 381.37 g/mol

                             = 5.03 g/L  

4. the solubility of borax would reduce If some solid borax along with saturated solution has been transferred.

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