When 1.24g of an organic compound with the formula CxHyOz is burned in excess oxygen, 1.76g of carbon dioxide and 1.08g of water vapor are obtained. What is the empirical formula of the compound?
Determine the moles of CO2 in 1.76grams, that will give you the moles of C in the original compound.
Determine the moles of H2O in 1.08 grams, that will give you the moles of H2, double it, and you have the number of moles in the original compound.
Add the weight of the moles of C in the original compouond to the weight of moles of H in the orig compound, then subtract that weight from 1.24g. The remainder is the weight of O in the compound, now calculate the moles of O in that.
You have the moles of C, H, and O in the original compound.
Take the smaller of those three numbers, and divide into all three. The numbers you get will be the mole ratio.
For instance, assume you had these numbers....C .0024 moles, H .0050 moles, and O .0025 moles.
C is the lower number, dividing, we get
CH2O
To find the empirical formula of the compound, follow these steps:
1. Determine the moles of carbon dioxide (CO2) in 1.76 grams.
- The molar mass of CO2 is 44.01 g/mol (12.01 g/mol for carbon and 16.00 g/mol for oxygen).
- Divide the mass of CO2 (1.76 g) by its molar mass (44.01 g/mol) to find the number of moles of CO2.
2. Determine the moles of water (H2O) in 1.08 grams.
- The molar mass of H2O is 18.02 g/mol (2.02 g/mol for hydrogen and 16.00 g/mol for oxygen).
- Divide the mass of H2O (1.08 g) by its molar mass (18.02 g/mol) to find the number of moles of H2O.
3. Convert the moles of CO2 to moles of carbon (C).
- Since each CO2 molecule contains one carbon atom, the number of moles of CO2 is also the number of moles of carbon.
4. Convert the moles of H2O to moles of hydrogen (H).
- Since each H2O molecule contains two hydrogen atoms, multiply the number of moles of H2O by 2 to get the number of moles of hydrogen.
5. Calculate the weight of oxygen (O) in the compound.
- Subtract the sum of the weights of carbon (C) and hydrogen (H) from the total initial weight of the compound (1.24 g) to get the weight of oxygen.
6. Convert the weight of oxygen to moles of oxygen.
- Divide the weight of oxygen by its molar mass (16.00 g/mol) to find the number of moles of oxygen.
7. Compare the mole ratios of carbon, hydrogen, and oxygen.
- Take the smallest of the three mole values (either C, H, or O) and divide all three mole values by this small value.
- Round the resulting numbers to the nearest whole number to obtain the mole ratio.
8. Write the empirical formula.
- Use the mole ratios obtained in the previous step to write the empirical formula of the compound. The subscripts in the formula represent the number of atoms in each element, based on the mole ratios.
For example, if the calculated mole ratios are C: 1, H: 2, and O: 1, the empirical formula would be CH2O.