The Mole

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Chemical amounts are measured in moles
The symbol for the unit mole is mol
One mole of a substance contains the same number of the stated particles, atoms, molecules, or ions as one mole of any other substance
The number of atoms, molecules or ions in a mole (1 mol) of a given substance is the Avogadro constant. The value of the Avogadro constant is 6.02 x 10²³per mole

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 10²³formula units of sodium chloride

The Moles & Particles Formula Triangle, downloadable IB Chemistry revision notes

The formula triangle showing the relationship between moles, particles and the Avogadro constant

Particles from Moles: How many hydrogen atoms are in 0.010 moles of CH₃CHO?

Answer:

- There are 4 H atoms in 1 molecule of CH₃CHO
- So, there are 0.040 moles of H atoms in 0.010 moles of CH₃CHO
- The number of H atoms is the amount in moles x L
- This comes to 0.040 x (6.02 x 10²³) = 2.4 x 10²² atoms

Moles from Particles: How many moles of hydrogen atoms are in 3.612 x 10²³ molecules of H₂O₂?

Answer:

- In 3.612 x 10²³ molecules of H₂O₂there are 2 x (3.612 x 10²³) atoms of H
- So, there are 7.224 x 10²³atoms of H
- The number of moles of H atoms is the number of particles ÷ L
- This comes to 7.224 x 10²³÷ (6.02 x 10²³) = 1.20 moles of H atoms

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Linking the Mole and Relative Atomic Mass

One mole of any element is equal to the relative atomic mass of that element in grams or for a compound the relative formula mass in grams
This is called the molar mass
If you had 6.02 x 10²³ atoms of carbon 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 would have a mass of 4 g (A_rof He is 4), one mole of lithium would have a mass of 7 g (A_rof Li is 7) and so on
To find the mass of one mole of a compound, we add up the relative atomic masses
- So one mole of water would have a mass of (2 x 1) + 16 = 18 g
- So one carbon atom has the same mass as 12 hydrogen atoms

Although elements and chemicals react with each other in molar ratios, in the laboratory we use digital balances and grams to measure quantities of chemicals as it is impractical to try and measure out moles
Therefore we have to be able to convert between moles and grams
We can use the following formula to convert between moles, mass in grams and the molar mass:

The Moles & Mass Formula Triangle, downloadable IB Chemistry revision notes

Formula triangle for moles, mass and molar mass

Mass from Moles: What is the mass of 0.250 moles of zinc?

Answer:

- From the periodic table the relative atomic mass of Zn is 65.38
- So, the molar mass is 65.38 g mol^-1
- The mass is calculated by moles x molar mass
- This comes to 0.250 mol x 65.38 g mol^-1 = 16.3 g

Moles from Mass: How many moles are in 2.64 g of sucrose, C₁₂H₂₂O₁₁ (M_r = 342.3)?

Answer:

- The molar mass of sucrose is 342.3 g mol^-1
- The number of moles is found by mass ÷ molar mass
- This comes to 2.64 g ÷ 342.3 g mol^-1 = 7.71 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.