#2 (permalink)

27.49% Pb (lead). First, what you want to do is assume that the 27.49% is actually 27.49 grams of a total 100 grams. From there, it's just a simple conversion using the 27.49 grams of Pb, and dividing by it's molar mass. That will tell you how many moles of Pb you have:

27.49g/207.2g = .133 moles of Pb.

Do the same for Bismuth (Bi):
55.51g/208.98g = .266 moles of Bi.

Do the same for Sulfur (S):
17.00g/32.065g = .531 moles of Sulfur.

Now to get the mole ratio for all the compounds, find the element with the smallest mole ratio, and divide the number of moles into the number of moles you got for the other two compounds, ie, divide the .133 for Pb by the .266 for Bi, and divide the .531 you got for Sulfur by .133. You should get:

S4PbBi2 as your final mole ratio. Neat, no?

For this, you must find out the molar mass of boulangerite, which is the molar mass of Pb times 5, plus the molar mass of antimony times 4, plus the molar mass of sulfur times 11. That will give you: 1875.75 as a molar mass for the entire compound.

Now, find out how many moles of the compound you have. To do that, divide the mass of the compound (2400 grams) by the molar mass (1875.75 grams). That will give you: 1.28 moles.

You know you have 1.28 moles of boulangerite, and for every one mole of boulangerite, there exist four moles of Sb (antimony), right? Multiply 1.28 by 4 to get the number of moles, which is: 5.12 moles.

Finally, multiply that number by the molar mass of antimony, which is 121.76. You should get 623 grams of antimony.

Follow the exact same steps I did for problem 2.

Do the exact same thing I did for problem 1.

Hope that helped!