The following results were obtained for the \'Determination of some Thermodynami

Question

The following results were obtained for the 'Determination of some Thermodynamic data for the dissolution of Borax' lab. A 5.0 mL solution of sodium tetraborate was made upto 25.0 mL by adding hot water and the titration was done at 50 degree C. 5 drops of methyl orange indicator was used. 17.4 mL of 1.0 M HCI was required for the titration. Fill in the missing information and calculate the Ksp for sodium tetraborate using the relevant information. The relevant equations are: Na_2 B_4 O_5 (OH)_5 (s) 2Na^+ (aq) + B_4 O_5 (OH)_4^2 (aq) B_4 O_5 (OH)_4^2- (aq) + 2H^+ (aq) + 3H_2 O(l) rightarrow 4H_3 BO_3 (aq) To calculate [B_4 O_5 (OH)_4^2-], use the total volume (25 +17.4)mL

Explanation / Answer

Ksp calculation,

7. moles HCl = molarity x volume

                     = 1.0 M x 17.4 ml

                     = 17.4 mmol

8. moles B4H5(OH)4^2- = 17.4 mmol/2

                                        = 8.7 mmol

10. [B4O5(OH)4^2-] = 8.7 mmol/(25 + 17.4) ml

                                 = 0.205 M

11. [Na+] = 2 x 0.205 = 0.410 M

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

             = (0.410)^2.(0.205)

            = 0.0345

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