Question 4.15: Obtaining an Empirical Formula from Experimental Data A labo...

1. Write down (or calculate) as given the masses of each element present in a sample of the compound. If you are given mass percent composition, assume a 100-g sample and calculate the masses of each element from the given percentages.

GIVEN: In a 100-g sample: 60.00 g C, 4.48 g H, 35.52 g O
FIND: empirical formula

2. Convert each of the masses from Step 1 to moles by using the appropriate molar mass for each element as a conversion factor.

60.00 \ \cancel{g \ C }\times \frac{1 \ mol \ C}{14.01 \cancel{g \ C }}=4.996 \ mol \ C
4.48 \ \cancel{g \ H } \times \frac{1 \ mol \ H}{1.008 \cancel{g \ H }}=4.44 \ mol \ H
35.52 \ \cancel{g \ O } \times \frac{1 \ mol \ O}{16.00 \cancel{g \ O }}=2.220 \ mol \ O

3. Write down a pseudoformula for the compound using the number of moles of each element (from Step 2) as subscripts.

C_{4.996}H_{4.44}O_{2.220}

4. Divide all the subscripts in the formula by the smallest subscript.

C\frac{4.996}{4.44}H\frac{4.38}{2.220}O\frac{2.220}{2.220} \longrightarrow C_{2.25}H_2O_1

5. If the subscripts are not whole numbers, multiply all the subscripts by a small whole number (see table) to determine whole-number subscripts.

C_{2.25}H_2O_1 \times 4 \longrightarrow C_9H_8O_4

The correct empirical formula is C_9H_8O_4 .