Bonus Empirical Formula Problem Answers 9 & 10

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

Fex(CO)y + O2 --> Fe2O3 + CO2

If you burn 1.959 g of Fex(CO)y and obtain 0.799 g of Fe2O3 and 2.200 g of CO2, what is the empirical formula of Fex(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 CH4; 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 C2H8, 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 C2H4D2 and the correct empirical formula is CH2D