Chemistry
Molecular Formulas
CPM.5:
I can use data about a substance and its empirical formula to determine the molecular
formula of a substance.
Empirical Formulas represent the lowest, whole-number ratio of the atoms in a compound. A
molecular formula gives the actual composition of the compound. C6H12O6 is the molecular formula for
glucose. What would be the empirical formula of glucose_______________? Another example is
hydrogen peroxide, H2O2. What would be the empirical formula of hydrogen peroxide?_________
Notice that the molecular formula is always some multiple of its empirical formula.
If a compound exists whose empirical formula is CH2, how many possible molecular formulas could this
same compound have? ____________________. One way to identify the exact molecular formula of a
compound is to know the molar mass of the compound. This mass would be the mass of the molecular
formula. Consider these examples:
Empirical Formula of Compound #1: CH2
Molar mass of the molecular formula of compound #1 is 70.0 grams.
Molecular Formula of Compound #1??
Empirical Formula of Compound #2: N2O3
Molar mass of the molecular formula of compound #2 is 151.8.
Molecular Formula of Compound #2??
Now let’s try to put all the concepts together:
1)
Calculate the molecular formula of a compound that is 30.43 % nitrogen and 69.57 %
oxygen. The molar mass of the compound is 92.0 gr.
2)
Carbohydrates are organic compounds made up of carbon, hydrogen and oxygen.
Determine the molecular formula of a carbohydrate compound that is 40.0 % carbon
and 6.67 % hydrogen. The molar mass of the compound is between 85-115 gr.
Sample Problems:
1) Find the molecular formula for a caffeine if it consists of 49.48 % carbon, 5.15% hydrogen,
28.87% nitrogen and 16.49% oxygen and has a molar mass between 150-280 grams.