**Analytical Chem Question**

sebmista · Junior Member Joined: 15 Aug 2006 Posts: 4

Alright guys PLEASE help me out here.. I've been going at this question for almost an hour..

Heres the exact Question

The sulfur (S) from a 5.141 g steel sample was evolved as H2S and collected in ammonical cadmium solution producing CdS. The resulting CdS precipitate was washed and suspended in water to which a few drops of acid was added. Then 25.00mL of a 0.002027 M KIO3 solution was added to the mixture, followed by 3 g of KI and 10mL of concentrated HCl. The liberated I3- oxidized the H2S to S and the excess unreated I3- was titrated with 1.085mL of .1127 M NaS2O3.

a) Calculate the % S in the steel sample.
b) What is the significance of the 3 g KI; of the 10 mL HCl?

it also gives the following balanced equations

IO3- + 6H+ + 8I- => 3I3- + 3H2O
I3- + H2S => 2H+ + S
I3- + 2S2O3 => 3I- + S4O6

sebmista · Junior Member Joined: 15 Aug 2006 Posts: 4

I have the following worked out..

.025L x .002027M = 5.0675e-5 moles IO3
now 5.0675e-5 moles IO3 x (3 moles I3 / 1 mole IO3) gives 1.52025e-4 moles of I3 TOTAL.

so.. .001085L x .1127 M S2O3 = 1.222795e-4 moles S2O3 and there are 2 moles of S2O3 for every 1 mole I3 so we devide this number by 2 to get moles of I3 in excess.. which gives 6.113975e-5 moles I3 in excess..

so now I took the total number of I3 moles (1.52025e-4) and subtracted the excess I3 moles from it (6.113975e-5) and got... 9.088525e-5 moles of I3 used total..

From the second balanced equation we can see the molar ratio of I3 and S is 1:1.. there for the moles of S should also be 9.088525e-5... so I multiplied it by Sulfurs molecular weight (32.066) and get.. .0029143264 grams of Sulfur

Now i take the sample (5.141g) and divide that into Sulfurs weight (.0029143264g)... .0029143264/5.141g x 100 = .056687....% WHICH IS MY ANSWER

the teacher actually gave us the homework answers and his answer is
"0.056%" so I hope he just rounded earlier or something...

BUT I DONT KNOW HOW TO ANSWER B!!
... is it just because 3 grams of KI and 10mL of HCl garantees an excess of I and H so that the reactions go to completion? Please help!!