A student mixed a sample of borax in water at 48 degrees celcius. He then pipett

Question

A student mixed a sample of borax in water at 48 degrees celcius. He then pipetted 5.00 mL of the borax solution into a flask and added 50mL of distilled water and 4 drops of bromcresol green indicator. This solution was titrated with 0.2500M HCl. The green endpoint was reached when 15.86mL of HCl was added.

a) How many moles of H+ were added in the titration?

b) How many moles of OH- were in the borax solution?

d) What is the tetraborate concentration [B4O5(OH)42-] in the 5.00 mL sample?

Please show all work. I know there is a lot here, but I am having a hard time understanding how to answer these questions. Thank you very much.

Explanation / Answer

a) moles of HCl added in the titration = molarity x volume = 0.25 x 0.01586 = 3.965 x 10^-3 mols

b) moles of OH- in th borax solution = 2 x 3.965 x 10^-3 = 7.93 x 10^-3 mols

c) moles of tetraborate = 7.93 x 10^-3/2 = 3.965 x 10^-3 mols

d) Tetraborate concentration in 5 ml = moles/L = 3.965 x 10^-3/0.005 = 0.793 M

e) concentration of sodium ions

2 moles of sodium per mole of tetraborate

moles of sodium = 2 x 3.965 x 10^-3 = 7.93 x 10^-3

concentration of sodium ions in 5 ml solution = 7.93 x 10^-3/0.005 = 1.586 M

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