An unknown compound was subjected to combustion.6.608 g of water together with 12.923 g of carbon dioxide and 2.501 g NH3 were produced. What is the compound’s empirical formula?
(Molar mass: N=14 g/mol; C=12 g/mol; O= 16 g/mol)
To find the empirical formula, we need to first find the moles of water, carbon dioxide, and ammonia produced in the combustion reaction:
Moles of H2O = 6.608 g / 18.015 g/mol = 0.367 moles
Moles of CO2 = 12.923 g / 44.01 g/mol = 0.294 moles
Moles of NH3 = 2.501 g / 17.031 g/mol = 0.147 moles
Next, we need to determine the simplest whole number ratio of these moles. We can divide each of the mole amounts by the smallest of the three, which is 0.147:
Moles of H2O / 0.147 = 2.50
Moles of CO2 / 0.147 = 2.00
Moles of NH3 / 0.147 = 1.00
Since these ratios are all close to whole numbers, we can round them to the nearest whole number to determine the empirical formula:
H5C2O5N (empirical formula)
To check our answer, we can calculate the molar mass of this empirical formula:
(5x1.01 g/mol) + (2x12.01 g/mol) + (5x16.00 g/mol) + (1x14.01 g/mol) = 117.18 g/mol
The experimental molar mass of the unknown compound can be determined by adding the molar masses of the products produced in the combustion reaction:
(2x1.01 g/mol) + (2x12.01 g/mol) + (5x16.00 g/mol) + (1x14.01 g/mol) = 117.19 g/mol
This is close to the molar mass of the empirical formula, indicating that our answer is reasonable. However, the unknown compound may have a molecular formula that is a multiple of the empirical formula (e.g. C10H10N5O10), so further analysis would be needed to determine the molecular formula.
To determine the empirical formula of the compound, we need to find the ratio of the elements present in the given masses.
1. Start by calculating the moles of each element:
- Moles of water (H2O) = mass / molar mass = 6.608 g / (2 * 1 g/mol + 16 g/mol) = 0.367 moles of H2O
- Moles of carbon dioxide (CO2) = mass / molar mass = 12.923 g / (12 g/mol + 2 * 16 g/mol) = 0.258 moles of CO2
- Moles of ammonia (NH3) = mass / molar mass = 2.501 g / (14 g/mol + 3 * 1 g/mol) = 0.161 moles of NH3
2. Determine the mole ratio between the elements by dividing each element's moles by the smallest number of moles (0.161 mol in this case):
- H = 0.367 mol / 0.161 mol ≈ 2.28
- C = 0.258 mol / 0.161 mol ≈ 1.60
- N = 0.161 mol / 0.161 mol = 1
3. Round off the ratios to the nearest whole number:
- H ≈ 2
- C ≈ 2
- N ≈ 1
The empirical formula of the unknown compound is C2H2N.