Mole Calculations Explained – Formula mass and mole calculations

by | Jun 8, 2021

Want to know how to calculate moles? Need to convert grams to moles, or moles to grams? Then welcome!

What is a mole? What is a moles equation?

A mole, in chemistry, is how chemists define an amount of substance, useful when dealing with many different molecules reacting at once, for example any chemical reaction. The official International System of Units definition is that a mole is, “the amount of a chemical substance that contains exactly 6.02214076×1023 (Avogadro’s constant) atoms, molecules, ions or electrons (constitutive particles), as of 20th May 2019. Prior to that, a mole was defined as the number of atoms in 12 grams of carbon-12 (an isotope of carbon).” That was a mouthful!


Why do Chemist use moles?

Why do Chemists use moles and why do you need to calculate it? It provides a useful metric when dealing with reactions, let’s look at any example:

Real Moles calculation example

“Let’s say you want to neutralise 10 g of hydrochloric acid (HCl in water) with some sodium hydroxide (NaOH). Now, you want the resulting solution to be perfectly neutral, so you don’t want to add too much or to little NaOH, making it too basic or acidic respectively. It is therefore useful to find out exactly how many molecules of HCl are in the solution. This is where moles come in handy. To know how to calculate moles, the equation is:

mole = mass / molecular weight"

How to calculate moles – create a moles to grams converter

Let’s do any example on how we can convert moles to grams, or grams to moles.

Real moles to grams example

“We know we have 10 g of HCl, and it has a molecular weight of 36.5 g / mol. Lets plug these numbers into the above equation:

mole = 10 / 36.5 = 0.27 moles = 1.626×10^23 molecules of HCl

We can work out the number of molecules by timesing the moles by Avogadro’s constant above. Now we know the amount of molecules of HCl we have, and, since the reaction is 1:1, we need the exact same number of molecules of NaOH to neutralise it. As we need to find the mass of NaOH to add, lets quickly rearrange the equation:

mole = molecular weight / mass (mulitply both sides by mass)

mole * mass = molecular weight (divide both sides by mole)

mass = molecular weight / mole

As 1.626×1023 molecules of NaOH is also equal to 0.27 moles, and we know that the molecular weight of NaOH is 40, we can use these numbers to get:

mass = 40 / 0.27 = 10.8 g

So we now know we need 10.8 g of NaOH to exactly neutralise our amount of hydrochloric acid.”


What is a mole?

1 mole is the amount of a substance that contains exactly 6.022×1023 of something. While this something could be anything, because it is such a large number it is usually reserved for atoms, molecules, electrons, and ions.

How to find molar mass?

  1. Find the chemical formula for the compound in question.
  2. Get a periodic table.
  3. Look up the atomic mass of each atom.
  4. Add together the atomic mass of every atom.
  5. The result is the molar mass of your substance.

How to find moles?

  1. Measure the weight of your substance.
  2. Use a periodic table to find its atomic or molecular mass.
  3. Divide the weight by the atomic or molecular mass.
  4. Check your results with Omni Calculator.

How to convert grams to moles?

  1. Find a periodic table.
  2. Calculate the atomic or molecular mass of your substance.
  3. Divide the weight by the atomic or molecular mass.
  4. Use Omni Calculator to verify your answer.

How many grams in a mole?

The amount of grams in a mole depends on the substance you have. To work it out, find the atomic or molecular mass of your substance and multiply it by the number of moles you have. For one mole, the atomic or molecular mass will be the same as the weight.

How many atoms in a mole?

There are 6.02214076×1023 atoms in a mole. It is derived from the amount of atoms in 12 g of the isotope carbon-12.

Much of the information for this blog post was found on


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