A 1.2431 -g alloy sample was analyzed for its copper content iodometrically. The alloy was dissolved in hot, concentrated nitric acid and prepared for analysis. The addition of excess KI(aq) solution precipitated the copper as cuprous iodide and liberated elemental iodine according to the following reaction:

Cu2+(aq) + I–(aq) → CuI(s) + I2(aq)

The liberated iodine was titrated against a standard thiosulfate solution until the disappearance of the blue starch-iodine indicator complex according to

I2(aq) + 2S2O32–(aq) → 2I–(aq) + S4O62–(aq)

Calculate the percent copper in the sample if 26.41 mL of a 0.2300 M thiosulfate solution was required to react with the liberated iodine.

mols S2O3^-2 = M x L

Convert moles S2O3^-2 to moles I2, then to moles Cu.
g Cu = moles Cu x atomic mass Cu

%Cu = (mass Cu/mass sample)*100 =

S2O3^-2..... why is it to the power of -2?

how do you Convert moles S2O3^-2 to moles I2

It is not to the power. We can't write superscripts easily on these boards so we write the carat to denote that a superscript follows. If we had the time we could do it right but few of us take the time and students don't take the time either. Thiosulfate should be

S2O3-2. the -2 shows the charge on the ion.

Cu (+ HNO3) ==> Cu^+2 but not balanced.

2Cu^+2 + 4I^- ==>2CuI + I2
I2 + 2S2O3^-2 ==> 2I^- + S4O6^-2

2 moles Cu = 2 mole Cu^+2 = 1 mole I2 = 2 moles S2O3^-2

Therefore, 1 mole Cu metal = 1 mole S2O3^-

To calculate the percent copper in the sample, we need to determine the amount of copper present in the alloy sample and then calculate it as a percentage of the total sample weight.

First, we need to find the moles of iodine that reacted with the thiosulfate solution. Since the equation shows that 1 mole of iodine reacts with 2 moles of thiosulfate ion, we can use the following equation:

Moles of I2 = (Volume of thiosulfate solution) × (Molarity of thiosulfate solution) × (1 mole of I2 / 2 moles of thiosulfate)

Moles of I2 = 26.41 mL × 0.2300 M × (1 mole of I2 / 2 moles of thiosulfate)

Next, we need to find the moles of copper that reacted with the iodine. By stoichiometry, we know that 1 mole of copper reacts with 1 mole of iodine. Therefore, the moles of iodine that reacted is equal to the moles of copper in the sample.

Moles of Cu = moles of I2

Assuming the molecular weight of CuI is 190.45 g/mol, we can calculate the mass of copper (Cu) in the alloy sample using the following equation:

Mass of Cu = Moles of Cu × Molecular weight of Cu

Mass of Cu = Moles of Cu × 190.45 g/mol

Finally, we can calculate the percent copper in the sample by dividing the mass of copper by the initial sample weight and multiplying by 100:

Percent Cu = (Mass of Cu / Initial sample weight) × 100

Substituting the values into the equations, we can calculate the percent copper in the sample.