Finding the Emprical Formula and Molecular Formula
Emperical Formula: represents the formula of a compound which is the simplest whole number ratio of atoms. It is not necessarily the actual molecular formula. Eg: Molecular formula of glucose = C6H12O6
Emperical formula of glucose = CH2O
Sometimes the emperical and molecular formula are the same eg. H2O
Emperical formulas can be determined if the percent composition is known:
Use this rhyme to help find the E.F. : % to mass
Mass to mole
Divide by small
Multiply till whole
Use a chart to help organize your work!
Eg: Find the emperical formula of a compound that contains 29.08% Na, 40.58% S, and 30.34% O.
The emperical formula is therefore: Na2S2O3
The actual molecular formula can only be found if the molar mass is given
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The actual molecular formula is calculated using the following formula to find the ratio of the true to emperical formula
Ratio = True molecular molar mass
Emperical formula molar mass
Once this ratio is found, multiply the emperical formula subscripts by ratio to obtain the true molacular formula
In class work
1) p. 211 # 16. First find the emperical formula then find the molecular formula if the molar mass is 412.52 g/mol. 2) p. 218 # 20
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