Carbon disulfide, CS2, reacts with chlorine, Cl2, in the presence of a catalyst between 105°C and 130°C, to form carbon tetrachloride, CCl4,
Reaction 1: CS2 + Cl2 → CCl4 + S2Cl2
Carbon tetrachloride can react with antimony trifluoride, SbF3, to produce freon-12, CCl2F2, a gas that was used in air conditioning.
Reaction 2: CCl4(l) + SbF3(s)→ CCl2F2(g) + SbCl3(s)
If CCl4 is produced from CS2 (reaction 1) in 62.5% yield and CCl2F2 is produced from CCl4 (reaction 2) in 71.5% yield, what mass of CS2 must be chlorinated to obtain 125.0 grams of CCl2F2 from of CCl4 produced?
I think it is easier to work problems like this the long way. Much easier to explain and much less likely to make an error. First, what amount of CCl4 in reaction 2 must we start with to produce 125 g freon ASSUMING the reaction is 100%. This is a regular stoichiometry problem.
moles freon = grams/molar mass = 125/121 = approx 1.03 but you should redo this and all of the calculations that follow and use more accurate numbers than an approximation. Now convert moles freon to moles CCl4.
Note the equation in the problem isn't balanced and I fixed that.
3CCl4(l) + 2SbF3(s)→ 3CCl2F2(g) + 2SbCl3(s)
moles CCl4 = mols CCl2F2 x (3 moles CCl4/3 moles CCl2F2) = 1.03 x 3/3 = 1.03
But wait. That assume the reaction is 100%; it is only 71.5 so we must start with a larger quantity into order to get the 1.03 moles we want. That quantity is 1.03/0.715 = 1.44 mols CCl4. Next step. Now we want 1.44 mols CCl4. What must we start with of CS2 to produce that? That's reaction 1. I balanced that equation also.
CS2 + 3Cl2 → CCl4 + S2Cl2
Go through this just as I did above. Starting with 1.44 moles CCl4 needed and assuming 100% yield, calculate moles CS2 needed to produce 1.44 mols CCl4. Then correct for it being only 62.5 % yield. Finally, convert mols CS2 to grams CS2. Post your work if you get stuck.