Moles and Masses

The triangle below links mass, number of moles and relative formula mass.

Equation triangle for mass, number of moles and relative formula mass.

You can use the triangle to learn these three equations:

m = n × Mr

n = m ÷ Mr

Mr = m ÷ n

  • m = Mass, which is usually measured in grams (g)
  • n = Number of moles, which is a unit used to express the amount of a substance
  • Mr = Relative formula mass, which is the sum of the atomic masses of all the atoms in a chemical formula

By using these equations, we can convert between mass and the number of moles. Also, we can determine the relative formula mass of a substance based on its mass and the number of moles present.

Example

Calculate the mass of 2 moles of sodium hydroxide (NaOH)

To calculate the mass, we need to rearrange the equation, which gives us:

Mass (g) = Number of moles × Relative formula mass (Mr)

The relative formula mass of NaOH can be calculated by adding up the atomic masses of each of its atoms, which are:

• Na (sodium) with an atomic mass of 23

• O (oxygen) with an atomic mass of 16

• H (hydrogen) with an atomic mass of 1

So the relative formula mass (Mr) of NaOH is:

Mr = 23 + 16 + 1 = 40

Now, we can plug in the values we have to find the mass:

Mass (g) = 2 × 40

Mass (g) = 80 g

Therefore, the mass of 2 moles of NaOH is 80 grams.

Example

Calculate the number of moles in 13g of sulfur (S)

To calculate the number of moles, we use the formula:

Number of moles = Mass ÷ Relative atomic mass

The relative atomic mass of sulfur (S) is 32.

Now we can plug in the given mass of sulfur and its relative atomic mass:

Number of moles = 13 g ÷ 32

Number of moles = 0.40625

Therefore, the number of moles in 13 grams of sulfur is approximately 0.406 moles to three significant figures.