Chem – Empirical Formula from Percent Part 4

How do you get the empirical formula from the percent of each element?


VIDEO Empirical and Molecular Formula Demonstrated Example 3: What is the empirical formula if the compound has a mass that is 60% oxygen and 40% sulfur?  Use this periodic table if needed.


What is all the information we are given?



60% Oxygen = 0.6

40% Sulfur = 0.4


For the next step we have to take a leap of imagination. Let us pretend that we have a small pile of this compound in front of us and we are weighing it. What mass is it going to be? The trick is we can start with any mass we want because we were given percentages in the original question. The easiest mass to pick is 100 grams total. Why? Because it makes the first set of calculations very simple. Take a look for yourself in the next steps.


What is the mass of oxygen if our total mass is 100g?

Answer: 0.6 * 100g = 60g


What is the mass of sulfur if our total mass is 100g?

Answer: 0.4 * 100g = 40g


To create the empirical formula, we need to know the masses of each element. The most important thing is how many moles of each element we have.


How many moles of oxygen do we have?

60g 1 mol = 3.75 mol
16 g

Answer: 3.75 mol O


How many moles of sulfur do we have?

40g 1 mol = 1.25 mol
32 g

Answer: 1.25 mol S


If we take both of our answers for the moles of each element, we can now compare the two elements in a compound. The best way to compare them is to divide them. Since an empirical formula is nothing more than a mole ratio between two or more elements, we can do the same with the moles that we have from our above answers.


What is the ratio of oxygen to sulfur?

Oxygen = 3.75 mol = 3
Sulfur 1.25 mol 1

Answer: 3


What does this ratio mean?

Answer: It means for every 3 oxygen, we have 1 sulfur.


What is the empirical formula?



PRACTICE PROBLEMS:  Solve the empirical formula problems below. Use this periodic table if needed.


What is the empirical formula of a compound that is 14% hydrogen and 86% carbon?

Answer: CH2


What is the empirical formula of a compound that is 42.5% sodium, 44.5% carbon, and 13% nitrogen?

Answer: Na2C4N



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