**Problem #9:** To find the formula of a compound composed of iron and carbon monoxide, Fe_{x}(CO)_{y}, the compound is burned in pure oxygen, an reaction that proceeds according to the following unbalanced equation.

Fe_{x}(CO)_{y} + O_{2} --> Fe_{2}O_{3} + CO_{2}

If you burn 1.959 g of Fe_{x}(CO)_{y} and obtain 0.799 g of Fe_{2}O_{3} and 2.200 g of CO_{2}, what is the empirical formula of Fe_{x}(CO)_{y}?

**Solution**
Soon.

**Problem #10:** A substance is 74.34% carbon and 25.66% hydrogen. 250.0 mL of the gaseous substance weighs 0.358 g at STP. Calculate the molecular formula.

**Solution**

1) Assume 100 g of compound.

2) Convert grams to moles:

carbon: 74.34 / 12.011 = 6.189

hydrogen: 25.66 / 1.008 = 25.456

3) Convert to smallest whole-number ratio:

carbon: 6.189 / 6.189 = 1 hydrogen: 25.456 / 6.189 = 4.11

4) The proposed empirical formula is CH_{4}; the empirical formula weight is 16.

5) Calculate total moles using PV = nRT:

n = [ (1.000 atm) (0.2500 L) ] / [ (0.08206 L atm / mol K) (273 K) ] = 0.01116 mol

6) Calculate molecular weight of the gas:

0.358 g / 0.01116 mol = 32.1 g/mol

7) Divide molecular weight by empirical formula weight:

32 / 16 = 2

8) Therefore the molecular formula is C_{2}H_{8}, which is impossible. The solution is to use six hydrogens, but make some of them deuterium, with an atomic weight of 2.

9) The molecular formula is C_{2}H_{4}D_{2} and the correct empirical formula is CH_{2}D