Molecular Formulas

1
Molecular Formulas

• From Empirical Formulas

2
A compound containing only carbon, hydrogen, and
oxygen is 63.16 C and 8.77 H. It has a molar
mass of 114 g/mol. What is its empirical and
molecular formula?

1. Write the formula in terms of grams
2. Convert g to moles by dividing by the molar mass
   of each element.
3. Convert the small numbers by dividing each mole
   quantity by the smallest mole quantity.
4. All quantities are whole numbers.
5. Divide the known molar mass by the mass of one
   mole of the empirical formula. The result
   produces the integer by which you multiply the
   empirical formula to obtain the molecular formula.

3
Empirical Formulas

• From Combustion Analysis

4
A 3.489 g sample of a compound containing C, H,
and O yields 7.832 g of CO2, and 1.922 g of water
upon combustion. What is the empirical formula of
the compound?

1. CxHyOz O2 ? CO2 H2O
2. X moles of C or moles of CO2
3. Y moles of H which is twice moles of water.
4. Z moles of O. Obtain g of O and then convert
   to moles.
5. Obtain empirical formula.

5
Formulas of Hydrates
6
When 21.91 g of a hydrate of copper(II) sulfate
is heated to drive off the water, 14.00 g of
anhydrous copper(II) sulfate remain. What is the
formula of the hydrate?

1. CuSO4XH2O(s) ? CuSO4(s) XH2O(g)
2. Calculate the g of water by subtracting the grams
   of copper(II) sulfate from g of the hydrate.
3. Find moles of H2O/moles of CuSO4 will give you X.