In this topic of IBDP Chemistry, we will be learning about the different types of chemical formulas, and how we can calculate and experimentally determine these formulas
Types of Chemical Formulas
In IBDP Chemistry, often times you will be working with different types of chemical compounds. These compounds consist of different elements combined chemically in a fixed ratio. An example of a compound would be H2O, or more commonly known as water. In order to understand the ratio of the different elements involved in the compound, we use chemical formulas.
There are 3 different types of chemical formulas - empirical formula, molecular formula, and structural formula.
Empirical formula shows the simplest whole number ratio of atoms of each element in a single particle of the substance.
- An example of the empirical formula of a compound is CH4.
- All chemical compounds have an empirical formula, including both ionic and non-ionic compounds.
Molecular formula shows the actual number of atoms of each element in a single molecule of the substance.
- Only compounds with molecules have molecular formula. Ionic compounds do not have molecular formulas, only empirical.
- For example, NaCl (i.e. salt) is an ionic compound. The formula NaCl is its empirical formula, as it does not have a molecular formula.
- For some compounds, their empirical and molecular formulas are the same.
- This is because their molecular formula already shows the simplest whole number ratio of atoms of each element in the molecule.
- An example of this would be water (H2O).
- However, for other compounds, their empirical and molecular formulas differ.
- This is because the ratio of atoms of each element in a single molecule can be simplified, and the number of atoms of each element in the molecule share a common factor.
- An example of this would be glucose. Glucose has a molecular formula of C6H12O6, but because the ratio can be simplified by dividing all the numbers by 6, it has an empirical formula of CH2O.
Structural formula shows the arrangement of atoms and bonds within a molecule
- This is useful in organic chemistry, to understand the different possible shapes of organic molecules and how atoms move in particular reactions.
The table below shows a few examples of compounds and their empricial, molecular and structural formulas.
Calculating the Empirical Formula
In IBDP Chemistry, you are expected to be able to determine the empirical formula of an unknown compound, upon being given either the mass of each element in the compound or the percentage composition of each element in the compound.
If you are given the mass of each element in the compound, you can determine the empirical formula by doing the following:
- Convert the mass into moles by dividing the mass with the each element's molar mass.
- Divide all mole quantities by the smallest mole quantity found from step 1.
- If one or more numbers found in step 2 are not whole numbers, multiply all the numbers by a factor to make whole numbers.
For example: A hydrocarbon contains 0.857g of carbon and 0.143g of hydrogen. What is its empirical formula?
In this question, step 3 can be disregarded as all the numbers found from step 2 are whole numbers.
If you are given the percentage composition of each element in the compound, you will need to do a little more work in order to find the emprical formula.
- Find the percent composition for each element.
- Convert the percentages into masses by assuming that you have 100g of the compound. You can do this by multplying each percentage by 100.
- Convert the mass into moles by dividing the mass with the each element's molar mass.
- Divide all mole quantities by the smallest mole quantity found from step 3.
- If one or more numbers found in step 4 are not whole numbers, multiply all the numbers by a factor to make whole numbers.
For example: A compound contains 40% carbon, 6.73% hydrogen and 53.27% oxygen by mass. What is the empirical formula?
The empirical number of an unknown compound can be experimentally determined. In IBDP Chemistry, you are expected to be able to experimentally determine the empirical formula of magnesium oxide.
- First, measure the mass of a crucible and its lid (measurement 1).
- Add a piece of magnesium ribbon into the crucible, and weigh the crucible again, this time with the magnesium (measurement 2).
- Place the crucible on a clay tray, and heat with a bunsen burner.
- When magnesium ribbon starts to burn, lift the lid a little to let air in and continue to heat until all the magnesium is burned.
- After letting the cruicible cool, weigh the cruicible and all its contents (measurement 3).
In this experiment, the mass of oxygen and magnesium can be determined as follows:
- Mass of magnesium = measurement 2 - measurement 1.
- Mass of magnesium oxide = measurement 3 - measurement 1.
- Mass of oxygen = Mass of magnesium oxide - Mass of magnesium.
These values can then be used to calculate the empirical formula for magnesium oxide.
Calculating Molecular Formula
In IBDP Chemistry, you will need to know how to find the molecular formula of a compound. Typically, you will be given the mass of each element of the compound or the percentage composition of each element of the compound, as well as molar mass of the molecule. Using this information, you can deduce the molecular formula by:
- Determine the empirical formula of the compound using the information regarding the mass/percentage composition of each element in the compound. If empirical formula is already found, you can skip this step.
- Calculate the empirical molar mass (i.e. the molar mass for the empirical formula) using the molar mass of each element (given in the periodic table).
- Divide the given molar mass by the empirical molar mass.
- Multiply the empirical formula by the number found in step 3.
For example: A sample of a compound contains 80% of carbon and 20% of hydrogen by mass. It has an Mr of 30.08. What is the molecular formula of this compound?
This is the end of this topic.
