Lessons

July 27 2017

How to Find Empirical and Molecular Formula Given Mass Percent

In this post, you’ll learn how to find the empirical and molecular formula of an unknown compound when given the mass percent of the compound.

First, we’ll start by going over all the steps. Then we’ll apply the step by going through an example.

Steps for Finding The Empirical Formula Given Mass Percent

Change % of each element into grams (for example, if the compound contains 40% carbon, then change it to 40 g carbon)
Convert grams of each element into moles by dividing grams by molar mass
Divide all moles by the smallest number of moles
If the moles are all whole numbers, then you’re done and that’s your empirical formula
If not, multiply all moles by number (usually 2 or 3) such that everything is a whole number

Example

Lets work through this example together and go through it step by step.

What is the empirical formula for a compound containing 38.8% carbon, 16.2 % hydrogen, and 45.1% nitrogen?

Steps

Change % in to grams: 38.8 g carbon, 16.2 g hydrogen, and 45.1 g nitrogen
Convert grams into moles: 38.8 g C / 12.011 = 3.23 mol C, 16.2 g H / 1.008 = 16.07 mol H, 45.1 g N / 14.007 = 3.22 mol N
Divide all moles by smallest moles: in this case, the smallest moles is 3.22 mol of N so we’ll divide all the moles by 3.22. This leaves us with 1 C, 5 H, 1 N
Since the moles of all the elements are whole numbers, we are done. That means the empirical formula of this compound is CH₅N

Steps for Finding The Molecular Formula from Empirical Formula

To be able to find the molecular formula, you’ll need to given the molar mass of the compound.

Divide the molar mass of the compound by the molar mass of the empirical formula. This should give you a whole number
Multiply all the subscripts of the empirical formula by the whole number. This will give you the molecular formula.

Example

Let’s continue with the example we were working through. If the molar mass of the unknown compound is 62.11 g/mol, what is the molecular formula.

Steps

First determine the molar mass of the empirical formula, which is 31.0571 g/mol. Divide 62.11 by 31.0571 and this will give you 2.
Next, multiply all the subscripts in CH₅N by 2. This gives us C₂H₁₀N_2.

And that’s it! Follow the steps, practice, and you’ll be able to solve these types of problems with no problem. Soon you’ll be conquering chemistry.

Additional Resources

How to Find the Number of Protons, Neutrons, and Electrons. Step by Step Explanation with Examples

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How to Find the Number of Protons, Neutrons, and Electrons. Step by Step Explanation with Examples

In this post, we’ll be going over how to determine the number of protons, neutrons, and electrons in an atom or ion.

We’ll first start by discussing what each of the components in the nuclide notation means. Then we’ll go through two examples together.

The Nuclide Notation

The letter(s) in the middle is the symbol of the element.

The number on the bottom left corner is the atomic number, which tells you the number of protons.

The number on the upper left corner is the mass number, which is equal to the neutrons and protons added together.

Lastly, the charge is on the upper right corner. If there isn’t any number or signs, then it means that atom has no charge and is neutral.

Rules to Finding Number of Protons, Neutrons, and Electrons

# of protons = atomic number

# of neutrons = mass number – atomic number

# of electrons = atomic number – charge

That’s it!

Examples

Great, lets apply the rules to some examples.

# of protons = 17

# of neutrons = 37 – 17 = 20

# of electrons = 17 – 0 = 17

# of protons = 16 (the atomic number is not given, but can be found on the periodic table)

# of neutrons = 32 – 16 = 16

# of electrons = 16 – (-2) = 18

Additional Practice

Try these on your own and check the answer below

⁷⁸Se^2-
³⁹K⁺

ANSWERS

34 protons, 44 neutrons, 36 electron
19 protons, 20 neutrons, 18 electron

Additional Resources

How to Calculate Molar Mass of Anything. Step by Step Explanation with Multiple Examples

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How to Calculate Molar Mass of Anything. Step by Step Explanation with Multiple Examples

What is molar mass?

Lets start by talking what the term means. Simply said, molar mass is how much one mole of a substance weighs. That substance can be an element or a compound.

How to calculate molar mass with examples

We’ll go through three examples progressing from easy to “difficult”. By the end of this post, you’ll be able to calculate the molar mass of anything.

Example #1: Single element

Sodium (Na)

Finding the molar mass of a single element is really simple. All you need to do is find the atomic mass of the element on the periodic table and report the number with the unit grams per mole or g/mol.

From this, you can see that sodium’s molar mass will be 22.99 g/mol.

Example #2: Simple compound

CH₃OH

Start by determining how many of each elements there are by looking the subscripts (small number next to the element symbol). In this compound, there are 1 C, 4 H (3+1), and 1 O. Next, multiply the number of a particular element by its molar mass. Finally, add the products together and you’ll arrive at the answer.

Example #3: “Complicated” compound

(NH₄)₂CO₃

The process is very similar to calculating the molar mass of a simple compound. The only difference is you’ll need to multiply the subscript on the outside of the parenthesis by the subscripts inside the parenthesis.

Start by determining how many of each elements there are. In this compound, there 2N, 8H, 1C, and 3O. Next, multiply the number of a particular element by its molar mass. Finally, add the products together and you’ll arrive at the answer.

After reading and working through these three examples, you should be able to calculate the molar mass of anything.

Additional Resources

This molar mass calculator allows you to double check if you’re getting the right answer.