PRACTICE PROBLEMS:
1. The empirical formula for a common drying agent is P2O5. The molecule has a molar mass
of 283.88 g/mol. Find the molecular formula of the compound.
Answers:
- When determining Molecular Formulas, you must first determine the Empirical formula. In
this problem, the Empirical formula is given.
- Since you are given the Empirical formula, we can determine the Molecular formula by using
the following equation:
)(
)(
calculatemustyoumassFormulaEmpirical
problemtheingivenmassFormulaMolecular
n
- Therefore, we must first determine the Empirical formula mass for P2O5 :
P2O5 = ( 2 ) ( 30.97 g / mol ) + ( 5 ) ( 16.00 g/mol ) = 141.94 g / mol
- Let’s now insert the values into the equation and solve for “n”:
2
94.141
88.283
mol
g
mol
g
massFormulaEmpirical
massFormulaMolecular
n
- I guess now would be a good time to discuss the significance of “n”… “n” represents the
multiplier that we must use to determine the molecular formula. Recall, Empirical formula
represents the “smallest-whole-number ratio” of elements within a compound. Molecular
formula represents the “actual” number of elements within a compound (that is not an ionic
compound – therefore, molecular (hence the name)). So…
( P2O5 )n ; where n = 2
( P2O5 )2 = P4O10
- Since we calculated “n” to equal 2, then the Empirical formula is multiplied by a factor of two
to determine the molecular formula for the compound.
Empirical Formula = smallest whole number ratio
Molecular Formula = the actual formula for the compound
Yes, in some cases - the compounds have the same formula for empirical and molecular…
