chemistry Question: 0.300g of a chromium compound was oxidized.

0.300g of a chromium compound was oxidized to convert chromium to dichromate (Cr2O7 2-). An excess of iodide was added to the acidified solution and the liberated iodine was titrated with sodium thiosulfate (0.1000 moldm-3); 27.02 cm3 being required. Calculate the % (w/w) Cr in the compound.

Cr2O7^2- + 6 I- + 14 H+ > 2 Cr^3+ + 7 H2O + 3I2 (1)

I2 + 2 S2O3^2- > 2 I- + 2 S4O6^2- (2)

Hi There, I've tried this question and im not sure how to get around it. Anyone know the solution?

(edited 1 year ago)

Reply 1

TypicalNerd

Original post by thegoldenkimpa

Start by calculating the moles of thiosulphate you have used, since you have the titre and concentration of Na2S2O3 given.

Using the ratio in the second equation and the moles of Na2S2O3, calculate how many moles of I2 have reacted. This is equal to the number of moles of iodine formed in the reaction of the dichromate with the iodide.

Again, using ratios, calculate the moles of Cr^3+ ions produced. This is the number of moles of chromium in the sample.

You should now know how to calculate the mass of chromium in the sample and hopefully you should be able to calculate the percentage by mass of chromium in the compound.