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the empirical formula of the sugar glucose

Determining Molecular Formulas

To determine a molecular formula, first determine the empirical formula for the compound as shown in the section above and then determine the molecular mass experimentally. Next, divide the molecular mass by the molar mass of the empirical formula (calculated by finding the sum the total atomic masses of all the elements in the empirical formula). Multiply the subscripts of the molecular formula by this answer to get the molecular formula.

EXAMPLE 9

In the example above, it was determined that the unknown molecule had an empirical formula of CH2O.

1. Find the molar mass of the empircal formula CH2O.

12.011g C + (1.008 g H) * (2 H) + 15.999g O = 30.026 g/mol CH2O

2. Determine the molecular mass experimentally. For our compound, it is 120.056 g/mol.

3. Divide the experimentally determined molecular mass by the mass of the empirical formula.

(120.056 g/mol) / (30.026 g/mol) = 3.9984

4. Since 3.9984 is very close to four, it is possible to safely round up and assume that there was a slight error in the experimentally determined molecular mass. If the answer is not close to a whole number, there was either an error in the calculation of the empirical formula or a large error in the determination of the molecular mass.

5. Multiply the ratio from step 4 by the subscripts of the empirical formula to get the molecular formula.

CH2O * 4 = ?

C: 1 * 4 = 4

H: 2 * 4 = 8

O 1 * 4 = 4

CH2O * 4 = C4H8O4

6. Check your result by calculating the molar mass of the molecular formula and comparing it to the experimentally determined mass.

molar mass of C4H8O4= 120.104 g/mol

experimentally determined mass = 120.056 g/mol

% error = | theoretical – experimental | / theoretical  * 100%

% error =  | 120.104 g/mol – 120.056 g/mol | / 120.104 g/mol  * 100%

% error = 0.040 %