A student transferred a 5.0 ml sample of the saturated borax solution at 44.50C
ID: 507072 • Letter: A
Question
A student transferred a 5.0 ml sample of the saturated borax solution at 44.50C to a titration flask. After adding some distilled water, she found that the endpoint color change came a 3.15 mL using 1.00M HCl. Show how many moles HCl were used at the endpoint? How many moles of B(OH)_4^- reacted? Show how to calculate the molarity, in mole/L, of B(OH)_4^- in this 5.0 mL sample Considering the equilibrium and the stoichiometry for the saturated solution, equation (4), fill in the relationship: Overall, when we dissolve one mole of Borax, we will produce ___mole B(OH)_4^- For the sample calculation, we found ___mole B(OH)_4^- reacted with ___mole HCl. There should also be ____mol Na^+ and _____mol B(OH)_3 in this sample from the borax dissolving equilibrium. Use equation (5) to show how to calculate the equilibrium constant using concentrations (mol/L) in the 5.0 mL sample of saturated solution.Explanation / Answer
Na2B4O7·10H2O + 2 HCl 4 H3BO3 + 2 NaCl + 5 H2O
1. mole of HCl = Molarity x volume (in L) = 1.0 x 3.15 x 10-3 = 3.15 x 10-3 mol
2. 1 moles of borax will reac with 2 moles oh HCl.
Moles of B(OH)4- reacted =3.15 x 10-3 / 2 mol = 1.57 x 10-3 mol
3. Molarity = number of moles/ volume in L
number of moles = 1.57 x 10-3 mol
Volume of the solution = 5.0 mL = 5 x 10-3 L
Molarity = (1.57 x 10-3 ) mol / (5 x 10-3) L = 0.314 M
4.
Na2B4O7 (borax) + 7H2O -------> [B4(OH)4]- + 2 Na+ + OH-
1. 1 mole of B(OH)4-
2. 1 mole of B(OH)4- react with 2 moles of HCl.
3. 2 moles of Na+ and 4 moles H3BO3 [Na2B4O7 (borax) + 7H2O -------> 4H3BO3 + 2 Na+ + 2OH- ]
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