### GCSE Chemistry

Quantitative Chemistry
Chemical Changes
Energy Changes
Organic Chemistry
Chemical Analysis
Earth and Atmospheric Science
Using Resources

# Moles and Masses

The triangle below links 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.

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