Question

1.)A student prepares a potassium dichromate solution by dissolving .5263 gram of dry solid potassium dichromate in enough water to make 1000.mL of solution. What is the concentration of dichromate ions in the resulting solution?

2)A student measures 10.00 mL of an unknown solution of Fe2+ into an Erlenmeyer flask and dilutes with 90 mL of 1.0 M hydrochloric acid. She then titrates this sample with the solution prepared in question#1 .If 29.50 mL of the potassium dichromate solution is needed to reach the endpoint, what is the concentration of Fe2+ in the original unknown solution.

Answer 1

1) concentration of dichromate ions in solution in molarity

Molarity = moles/L = g/molar mass x L

1 mole of potassium dichromate has 1 mole of chromate ion.

Molarity of dichromate ion in solution = 0.5263 g / 294.18 g/mol x 1 L = 1.79 x 10^-3 M

2) The equation for the reaction of Fe2+ with K2Cr2O7 in acidic medium is,

6Fe(2+) + Cr2O7(2-) + 14H(+) -------------> 6Fe(3+) + 2Cr(3+) + 7H2O

Molarity of K2Cr2O7 solution = 1.79 x 10^-3 M

29.50 mL of this solution was used.

amount of K2Cr2O7 in 29.50 mL = amount in 0.0295 L = molarity x volume = 1.79 x 10^-3 x 0.0295 = 5.281 x 10^-5 mol

hence we now know that 5.281 x 10^-5 mol potassium dichromate was needed to reach the end point.

according to the equation, the mole ratio of Cr2O7(-) ions and Fe(2+) ions is 1:6

so, for 5.281 x 10^-5 mol potassium dichromate, amount of Fe(2+) needed = (5.281 x 10^-5 mol) x6 = 3.17 x 10^-4 mol

this was the amount present in the original 10mL(0.01 L) solution.

concentration = mol/vol = (3.17 x 10^-4 mol)/0.01 L = 0.0317 M

Thus, the concentration of Fe2+ in the original unknown solution is 0.0317 M