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Empirical FormulasHonors Chemistry

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Formulas

• The empirical formula for CThe empirical formula for C33HH1515NN33 is is

CHCH55N. N.

• you are basically dividing by the greatest you are basically dividing by the greatest common factor , which in this case is 3, common factor , which in this case is 3, for each element subscript.for each element subscript.

Empirical formula: the lowest whole number ratio of atoms in a compound.

Molecular formula: the true number of atoms of each element in the formula of a compound.

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Formulas (continued)

Formulas for ionic compounds are ALWAYS empirical (the lowest whole number ratio = cannot be reduced).

Examples:

NaCl MgCl2 Al2(SO4)3 K2CO3

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Formulas (continued)

Formulas for molecular compounds

MIGHT be empirical (lowest whole number ratio).

Molecular:

H2O

C6H12O6 C12H22O11

Empirical:

H2O

CH2O C12H22O11

(Correct formula)

(Lowest whole number ratio)

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Calculating Empirical Formula We can get a ratio from the percent

composition.

1) Assume you have a 100 g sample- the percentage become grams (75.1% = 75.1 grams)

2) Convert grams to moles.

3) Find lowest whole number ratio by dividing each number of moles by the smallest value.

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Example Calculate the empirical formula of a

compound composed of 38.67 % C, 16.22 % H, and 45.11 %N.

Step 1 : Assume 100gram sample Step 2 : Convert grams to moles for each element

38.67 g C 1mol C = 3.22 mole C 12.01 g C

16.22 g H 1mol H = 16.06 mole H 1.01 g H

45.11 g N 1mol N = 3.22 mole N 14.01 g N

Now divide each value by the smallest value

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Example

The ratio is 3.22 mol C = 1 mol C = 1 3.22 mol N 1 mol N

The ratio is 16.06 mol H = 4.988 mol H = 5 3.22 mol N 1 mol N

Empirical Formula is = C1H5N1

which is = CH5N

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Empirical to molecular Since the empirical formula is the

lowest ratio, the actual molecule would weigh more.By a whole number multiple.

Divide the actual molar mass by the empirical formula mass – you get a whole number to increase each coefficient in the empirical formula

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Empirical to molecular

True Formula Mass = Whole #

Empirical Formula Mass ratio

The whole number ratio is what you multiply each subscript in the empirical formula by to get the true molecular formula.

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Caffeine has a molar mass of 194 g. and its and its empirical formula is Cempirical formula is C44HH55NN22O, O, what is its

molecular formula? Find the empirical formula molar mass.Find the empirical formula molar mass. 97 g/mole97 g/mole Set up ratio 194g = 2Set up ratio 194g = 2 97g 97g Multiply each subscript in the empirical Multiply each subscript in the empirical

formula by 2.formula by 2. CC88HH1010NN44OO2 2 is the molecular formula. is the molecular formula.