Chemistry help. Please help me understand how to do this.

The reaction of 58.9 g Hg with 20.8 g Cl
produces mercuric chloride, the sole product,
with no reactant left over. How much mercury (Hg) is needed to form 121 g mercuric
chloride?
1. 31.5 g
2. 341 g
3. 0.00293 g
4. 0.0112 g
5. 42.6 g
6. 462 g
7. 163 g
8. 0.0235 g
9. 89.1 g
10. 0.00217 g

To figure this one out you first need to determine how many grams of each reactant is in one mole. Just take a look on the periodic table to do this:

1 mole of Hg is 200.59g
1 mole of Cl is 35.45g

Next figure out how many moles you have of each reactant are needed for the product based on the information you obtained from the periodic table. You equations should look something like this:

(grams of reactant)/(grams in one mole) = no. of moles of reactant

Since the reaction goes to completion, you should be able to determine the ratio of moles of Hg to moles of Cl needed too form one mole of mercuric chloride:

x moles of Hg : x moles of Cl

From here you can figure out the mass of one mole of your product:

mass of x moles of Hg + x moles of Cl = mass of 1 mole of mercuric chloride

Once you have determined the mass in mole of mercuric chloride, you will then determine how many moles of the product 121g is equal to:

(1 mole mercuric chloride)/(mass of one mole mercuric chloride) x 121g = moles of product

After you have obtained the moles of product, all you have to do is to figure out how many moles of Hg are in it from there:

moles of product x moles of Hg needed to form one mole of mercuric chloride = moles of Hg in product

Just punch in the numbers and there you have it!