Linking Moles, Mass & Mr

Extended tier only

One mole of any element is equal to the relative atomic mass of that element in grams
If you had 1 mole of carbon atoms in your hand, that number of carbon atoms would have a mass of 12 g (because the A_rof carbon is 12)
- So, one mole of helium atoms has a mass of 4 g (A_rof He is 4)
- One mole of lithium has a mass of 7 g (A_rof Li is 7) and so on
One mole of any compound is the relative molecular mass or relative formula mass in grams
To find the mass of one mole of a compound, add up the relative atomic masses
- For example, carbon dioxide has an Mr of:
  (1 x C) + (2 x O)
  (1 x 12) + (2 x 16) = 44

Moles = $\frac{mass}{M_{r}}$

We can use the following formula triangle to convert between moles, mass in grams and the molar mass:

Formula triangle for moles, mass and molar mass

Calculating the number of moles of an element uses the same equation, but with relative atomic mass replacing M

What is the mass of 0.250 moles of zinc?

Answer:

From the Periodic Table, the relative atomic mass of Zn is 65
- So, the molar mass is 65 g / mol
The mass is calculated by moles x molar mass:
- 0.250 mol x 65 g / mol = 16.25 g

How many moles are in 2.64 g of sucrose, C₁₂H₂₂O₁₁ (M_r = 342)?

Answer:

The molar mass of sucrose is 342 g / mol
The number of moles is found by mass ÷ molar mass:
- $\frac{2.64}{342}$ = 7.72 x 10^-3 mol

Always show your workings in calculations as its easier to check for errors and you may pick up credit if you get the final answer wrong.

Calculating moles & masses

Chemical amounts are measured in moles
The mole, symbol mol, is the SI unit of amount of substance
One mole of any substance contains the same number of the stated particles
- This can be atoms, molecules or ions
One mole contains 6.02 x 10²³particles
- This number is known as the Avogadro constant
For example:
- One mole of sodium (Na) contains 6.02 x 10²³ atoms of sodium
- One mole of hydrogen (H₂) contains 6.02 x 10²³ molecules of hydrogen
- One mole of sodium chloride (NaCl) contains 6.02 x 10²³ formula units of sodium chloride

For magnesium chloride, MgCl₂, calculate the number of:

Answers:

The formula is MgCl₂, so 1 mole of MgCl₂ is:
- 1 x 6.02 x 10²³ = 6.02 x 10²³ molecules
There are 3 atoms in MgCl₂, so 1 mole of MgCl₂ contains:
- 3 x 6.02 x 10²³ = 18.06 x 10²³ atoms
There are 2 chloride ions in MgCl₂, so 1 mole of MgCl₂ contains:
- 2 x 6.02 x 10²³ = 12.04 x 10²³ chloride ions
There is 1 magnesium ion in MgCl₂, so 2 mole of MgCl₂ contains:
- 2 x (1 x 6.02 x 10²³) = 12.04 x 10²³ magnesium ions

In 15.7 g of water (M_r = 18):

Answers:

The number of molecules:
- The molar mass of water is 18 g / mol
- The number of moles is found by mass ÷ molar mass
  - 15.7 g ÷ 18 g / mol = 0.872 mol
- There are 6.02 x 10²³ molecules of water in 1 mole of water
- So, in 0.872 moles of water, there are:
  - 6.02 x 10²³ x 0.872 = 5.25 x 10²³ molecules
The number of atoms:
- In each molecule of water, there are 3 atoms (2 hydrogen atoms, one oxygen atom)
- So, the number of atoms in 15.7 g = 3 x 5.25 x 10²³ = 1.58 x 10²⁴ atoms

A solute is a solid substance that dissolves into a liquid
- The amount of solute can be expressed in grams (g) or moles (mol)
A solvent is the liquid that a solute dissolves in
- The amount / volume of a solvent is measured in cm³ or dm³
Most chemical reactions occur between solutes which are dissolved in solvents, such as water or an organic solvent
A solution is the mixture formed when a solute dissolves in a solvent
- The amount / volume of a solution measured in cm³ or dm³
Concentration refers to the amount of solute there is in a specific volume of the solvent
- The greater the amount of solute in a given volume, the greater the concentration
- Concentration is sometimes commonly referred to as strength
  - For example, dissolving more coffee in hot water results in a stronger coffee

Typically, concentration is expressed in terms of the amount of substance per dm³
- Therefore, the units of concentration are:
  - g / dm³
  - mol / dm³