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The molecular formula is an expression of the number and type of atoms that are present in a single molecule of a substance. It represents the actual formula of a molecule. Subscripts after element symbols represent the number of atoms. If there is no subscript, it means one atom is present in the compound.
The empirical formula is also known as the simplest formula. The empirical formula is the ratio of elements present in the compound. The subscripts in the formula are the numbers of atoms, leading to a whole number ratio between them.
Examples of Molecular and Empirical Formulas
The molecular formula of glucose is C6H12O6. One molecule of glucose contains 6 atoms of carbon, 12 atoms of hydrogen and 6 atoms of oxygen.
If you can divide all of the numbers in a molecular formula by some value to simplify them further, then the empirical or simple formula will be different from the molecular formula. The empirical formula for glucose is CH2O. Glucose has 2 moles of hydrogen for every mole of carbon and oxygen. The formulas for water and hydrogen peroxide are:
- Water Molecular Formula: H2O
- Water Empirical Formula: H2O
- Hydrogen Peroxide Molecular Formula: H2O2
- Hydrogen Peroxide Empirical Formula: HO
In the case of water, the molecular formula and empirical formula are the same.
Finding Empirical and Molecular Formula from Percent Composition
Percent (%) composition = (element mass/compound mass) X 100
If you are given the percent composition of a compound, here are the steps for finding the empirical formula:
- Assume you have a 100 grams sample. This makes the calculation simple because the percentages will be the same as the number of grams. For example, if 40% of the mass of a compound is oxygen then you calculate you have 40 grams of oxygen.
- Convert grams to moles. Empirical formula is a comparison of the number of moles of a compound so you need your values in moles. Using the oxygen example again, there are 16.0 grams per mole of oxygen so 40 grams of oxygen would be 40/16 = 2.5 moles of oxygen.
- Compare the number of moles of each element to the smallest number of moles you got and divide by the smallest number.
- Round your ratio of moles to the nearest whole number as long as it is close to a whole number. In other words, you can round 1.992 up to 2, but you can't round 1.33 to 1. You'll need to recognize common ratios, such as 1.333 being 4/3. For some compounds, the lowest number of atoms of an element might not be 1! If the lowest number of moles is four-thirds, you will need to multiply all ratios by 3 to get rid of the fraction.
- Write the empirical formula of the compound. The ratio numbers are subscripts for the elements.
Finding the molecular formula is only possible if you are given the molar mass of the compound. When you have the molar mass you can find the ratio of the actual mass of the compound to the empirical mass. If the ratio is one (as with water, H2O), then the empirical formula and molecular formula are the same. If the ratio is 2 (as with hydrogen peroxide, H2O2), then multiply the subscripts of the empirical formula by 2 to get the correct molecular formula. two.