# Chemistry

A student does an experiment to determine the percentage of copper in an alloy.
The Student. Reacts 985mg of the alloy with concentrated nitric acid to form a solution (all of the copper in the alloy reacts to form aqueous copper (II) ions). Pours the solution into a volumetric flask and makes the volume up to 250cm^3 with distilled water. Shakes the flask thoroughly. Transfers 25cm^3 of the solution into a conical flask and adds an excess of potassium iodide. Uses exactly 9.00cm^3 of 0.08mol dm^-3 sodium thiosulfate (Na2S2O3) solution to react with all the iodine produced.
2Cu^2+ + 4I^- —> 2CuI + I2.
2S2O3^2- + I2 —> 2I^- + S4O6^2-
Calculate the percentage of copper by mass in the alloy?
you gotta follow it through backward:
1. how many moles of sodium thiosulfate reacted with it? n=cv
2. what are the ratios? it seems 2 moles of thiosulfate react with __ moles of I2, we know the number of moles of thiosulfate so we can work out number of I2 there reacted using the ratio
3. this is the amount of moles of I2 in 25cm^3 so we need to scale it up to the 250cm^3 volume, so there are __ moles of I2 in 250cm^3
4. look at the ratios from the 1st equation given, how many moles of copper are needed to produce 1 mole of I2? we know the number of moles of I2 (from the scaled up volume) so we can use the ratios to work out number of moles of Cu.
5. we have number of moles of copper so we can work out its mass using number of moles= mass/MR
6. we now know mass of copper and mass of the alloy is given so we can work out percentage mass in the alloy.
Original post by burntcherrio
you gotta follow it through backward:
1. how many moles of sodium thiosulfate reacted with it? n=cv
2. what are the ratios? it seems 2 moles of thiosulfate react with __ moles of I2, we know the number of moles of thiosulfate so we can work out number of I2 there reacted using the ratio
3. this is the amount of moles of I2 in 25cm^3 so we need to scale it up to the 250cm^3 volume, so there are __ moles of I2 in 250cm^3
4. look at the ratios from the 1st equation given, how many moles of copper are needed to produce 1 mole of I2? we know the number of moles of I2 (from the scaled up volume) so we can use the ratios to work out number of moles of Cu.
5. we have number of moles of copper so we can work out its mass using number of moles= mass/MR
6. we now know mass of copper and mass of the alloy is given so we can work out percentage mass in the alloy.

Thank you 😊