Combustion analysis of 0.284 g of a compound containing C, H, and O produces 0.623 g of CO2 and 0.340 g of H2O. Mass spectral analysis shows that the compound has a molar mass around 120 g mol-1. What is the compound's:
Empirical Formula: Molecular Formula:
Convert 0.623 g CO2 to g C then moles C.
Convert 0.340 g H2O to g H then moles H.
Add g C to g H and subtract from 0.284 to obtain g O.
Now you want to find the ratio of the MOLES of these three elements to each other in small whole numbers. The easy way to do that is to divide the smallest number by itself which assures you of getting 1.000 for that element. Then divide the other two numbers by the same small number and round to a whole numbers. This will obtain the empirical formula.
To find the molecular formula, add the atomic masses of the elements to find the empirical mass and divide that number into 120, then round to a whole number which I will call x. That is the number of units of empirical formula you have; i.e., (CyHzOw)x.
Post your work if you get stuck.
This sound complicated because there are several steps but its quite straight forward.
To find the empirical formula and molecular formula of the compound, we need to analyze the given data.
1. Calculate the number of moles of CO2 produced.
- The molar mass of CO2 is 44 g/mol.
- Therefore, moles of CO2 = mass of CO2 / molar mass of CO2
= 0.623 g / 44 g/mol
= 0.01416 mol
2. Calculate the number of moles of H2O produced.
- The molar mass of H2O is 18 g/mol.
- Therefore, moles of H2O = mass of H2O / molar mass of H2O
= 0.340 g / 18 g/mol
= 0.01889 mol
3. Calculate the number of moles of carbon (C) in the compound.
- The moles of C can be found based on the moles of CO2 produced since each CO2 molecule has one carbon atom.
- Moles of C = moles of CO2
= 0.01416 mol
4. Calculate the number of moles of hydrogen (H) in the compound.
- The moles of H can be found based on the moles of H2O produced since each H2O molecule has two hydrogen atoms.
- Moles of H = 2 * moles of H2O
= 2 * 0.01889 mol
= 0.03778 mol
5. Calculate the number of moles of oxygen (O) in the compound.
- The total moles of C, H, and O in the compound can be found by subtracting the sum of moles of C and H from the total moles of the compound.
- Moles of O = Total moles of the compound - Moles of C - Moles of H
= 0.284 g / 120 g/mol - 0.01416 mol - 0.03778 mol
= 0.002367 mol
6. Convert the moles of each element to the simplest whole number ratio.
- Divide the moles of each element by the smallest number of moles among them.
- Moles of C / 0.01416 mol = 1
- Moles of H / 0.03778 mol = 2
- Moles of O / 0.002367 mol = 1
- The empirical formula is CH2O.
7. Determine the molecular formula.
- Find the empirical formula mass by adding up the atomic masses of C, H, and O in the empirical formula.
- Empirical formula mass = (1 * atomic mass of C) + (2 * atomic mass of H) + (1 * atomic mass of O)
= (1 * 12.01 g/mol) + (2 * 1.008 g/mol) + (1 * 16.00 g/mol)
= 30.03 g/mol
- Calculate the factor by dividing the molar mass of the compound by the empirical formula mass.
- Factor = molar mass of the compound / empirical formula mass
= 120 g/mol / 30.03 g/mol
= 3.99 (approximately 4)
- Multiply the subscripts in the empirical formula by the factor to determine the molecular formula.
- Molecular formula = C4H8O4
Therefore, the empirical formula of the compound is CH2O, and the molecular formula is C4H8O4.