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Daily Dose 038 | Chemistry
How to define the empirical formula for water?
So how do you define the empirical formula?
I know for me, differentiating between the Empirical Formula and the Molecular Formula was always a point of confusion that often tripped me up.
Sometimes they were the same, sometimes they were not, but what determined this?
In this video, we break it all down, showing you exactly how to define the empirical formula for any compound you end up seeing on the day of your FE Exam.
Key Definition
What is the EMPIRICAL FORMULA?
The EMPIRICAL FORMULA of a chemical compound is the simplest positive integer ratio of atoms present in a given compound.
An EMPIRICAL FORMULA does not necessarily represent the actual number of atoms present in each molecule of a compound; it represents only the ratio between those numbers.
The actual number of atoms of each element that occur in the smallest freely existing unit or molecule of the compound is expressed by the MOLECULAR FORMULA of the compound.
The MOLECULAR FORMULA of a compound may be the EMPIRICAL FORMULA, or it may be a multiple of the EMPIRICAL FORMULA.
The Process
How can you define the empirical formula for any compound?
The empirical formula of a compound represents the simplest whole number ratio of atoms present in a compound.
It provides the relative proportions of different elements in a molecule, disregarding the actual number of atoms or their arrangement.
To determine the empirical formula of a compound, you need to know the elemental composition or the percent composition by mass of the compound.
Let’s step through the process by defining the empirical formula for ethane (C2H6):
- Step 1: Determine the mass or percent composition
- Step 2: Convert mass to moles
- Step 3: Find the mole ratio
- Step 4: Write the empirical formula
The first step requires us to determine the mass or percent composition of each element present in the compound. This information can be obtained through experimental methods or given in the problem statement.
In the case of ethane, we already know the molecular formula and can directly determine the percent composition of carbon (C) and hydrogen (H) in ethane.
For Carbon, which has an atomic weight of 12.01 g/mol, we get:
Mass of carbon in ethane = 2 * 12.01 g/mol = 24.02 g/mol
For Hydrogen, which has an atomic weight of 1.01 g/mol, we get:
Mass of hydrogen in ethane = 6 * 1.01 g/mol = 6.06 g/mol
Next we convert the mass or percent composition of each element to moles by dividing the mass by its atomic weight or using the percent composition assuming a 100g sample.
Continuing with our example, we get:
Moles of carbon (C) = 24.02 g / 12.01 g/mol = 2.00 mol
Moles of hydrogen (H) = 6.06 g / 1.01 g/mol = 6.00 mol
With the moles defined, we can now determine the mole ratio by dividing the moles of each element by the smallest number of moles obtained – and round the ratios to the nearest whole number.
Doing this we get:
Moles of carbon (C) / Moles of hydrogen (H) = 2.00 mol / 2.00 mol = 1
Moles of hydrogen (H) / Moles of hydrogen (H) = 6.00 mol / 2.00 mol = 3
The mole ratio is 1:3.
Lastly, we write the empirical formula using the whole number mole ratios as subscripts. If the ratios are close to whole numbers but not exact, multiply them by a common factor to obtain the simplest whole number ratio.
Wrapping up our example, we find that the empirical formula for ethane is CH3.
This process can be used for simple problems like the one presented, or more complex problem statements such as:
A compound is composed of 40% carbon, 6.7% hydrogen, and the remainder is oxygen by mass. Determine the empirical formula of the compound.
Check out the video and see how we can go about solving this type of problem in the most efficient manner.
As always, with Love, Prepineer
