Calculating the Empirical and Molecular Formula and Percent Composition

There are two kinds of formulas that we can express in chemistry "11".   The empirical and molecular formula.

The Empirical Formula


-Gives the lowest terms of atoms OR moles.
-All formulas of ionic components are empirical formulas.

The Molecular Formula


-Gives all atoms which make up a molecule.
-This can be ionic or covalent compounds.


EG: C6H12O6

To put it into empirical formula, reduce to lowest terms.  Therefore C6H12O6 = CH2O(empirical).

Remember: 1) molecular  = empirical x whole #
2) molecular formula mass = empirical x whole #
3) mass(mole) = EFM(g) x whole #

How to determine empirical formula given mass


EX: Determine the empirical formula of Fe and O given 10.87g of Fe and 4.66g of O.

1) Convert grams ------> moles

10.87 x 1/55.8g = 0.1948 mol
 4.66 x 1/16.0g = 0.0291 mol

2) Divide each molar amount by the smallest molar amount.

Fe 0.1948/0.1948 = 1
O 0.291/0.1948 = 1.49 ~ 1.5

3) Scale ratios to whole #'s by multiplying.

1.5 x "2" = 3(whole number)
1 x "2" = 2(whole number)


Therefore, the empirical formula is Fe2O3

PERCENT COMPOSITION


- The % by mass of the elements in a compound.
                -calculate molar mass.
                -calculate each element's % of that total(ONE DECIMAL PLACE)
This tells what part of a compound each component element makes up.

Remember: %composition = mass of element/mass of compound x 100%


The ratio of moles can be determined from percent composition.  How?

1)Assume 100.0g of all material
2)Convert all %'s to grams.
3)Follow the steps to solve empirical formula.