Question 4.14: Obtaining an Empirical Formula from Experimental Data A com...
Obtaining an Empirical Formula from Experimental Data
A compound containing nitrogen and oxygen is decomposed in the laboratory and produces 24.5 g nitrogen and 70.0 g oxygen. Find the empirical formula of the compound.
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: 24.5 g N, 70.0 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.
24.5 \cancel{g \ N }\times \frac{1 \ mol \ N}{14.01 \cancel{g \ N }}=1.75 \ mol \ N
70.0 \ \cancel{g \ O } \times \frac{1 \ mol \ O}{16.00 \cancel{g \ O }}=4.38 \ mol \ O
3. Write down a pseudoformula for the compound using the number of moles of each element (from Step 2) as subscripts.
N_{1.75}O_{4.38}
4. Divide all the subscripts in the formula by the smallest subscript.
N\frac{1.75}{1.75}O\frac{4.38}{1.75}\longrightarrow N_1O_{2.5}
5. If the subscripts are not whole numbers, multiply all the subscripts by a small whole number (see table) to determine whole-number subscripts.
N_1O_{2.5} \times 2 \longrightarrow N_2O_5
The correct empirical formula is N_2O_5 .
| Fractional Subscript | Multiply by This |
| 0.2 | 5 |
| 0.25 | 4 |
| 0.33 | 3 |
| 0.4 | 5 |
| 0.5 | 2 |
| 0.66 | 3 |
| 0.75 | 4 |
| 0.8 | 5 |