What is the Avagadro Constant?
Moles are the SI unit for the amount of a substance.
The symbol for moles is mol.
There are the Avagadro’s Constant number of particles in a mole of any substance. This can be atoms, ions or molecules. The Avagadro’s Constant is 6.02 x 10²³. So there are that many atoms in a mole of carbon or the same number of molecules in a mole of water.
You can calculate the number of moles using the following equation:
number of moles = mass of substance (g) / Aᵣ or Mᵣ
Rearrange this equation to get the mass or A/Mᵣ given the other two numbers.
To do this you should –
- Find out the Mᵣ or Aᵣ of each reactant and product.
- Determine the number of moles of each reactant and product, do this using the equation in the section above “What are moles?”.
- Divide the number of moles of each reactant and product by the smallest number of moles.
- Multiply each number to make sure you have a whole number – the simplest whole number ratio.
- Write the equation as normal
- Balance the equation
Example: Determine the balanced symbol equation for a reaction between 80g of sodium hydroxide (NaOH) and 73g of hydrochloric acid (HCl) to produce 117g of sodium chloride (NaCl) and water (H₂O).
- Mᵣ of NaOH = 40, Mᵣ of HCl = 36.5, Mᵣ of NaCl = 58.5
- NaOH = 2 mol, HCl = 2 mol, NaCl = 2 mol
- The smallest number of moles = 2, so divide each by 2
- 1 mol of NaOH and 1 mol of NaCl makes 1 mol of NaCl
- NaOH + HCl → NaCl + H₂O
Since we don’t usually put 1 before chemical symbols in an equation, it is already balanced.
This is a relatively simple example where all of the substances were in the same number of moles and no work is needed to balance the equation. Follow the same procedure for more complex examples and you will be able to do these problems.
You can always search on YouTube for more examples by entering:
“Using moles to balance equations”
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Which number tells us the number of particles in one mole of a substance?CorrectIncorrect
How many particles are there in a mole of a substance?CorrectIncorrect
Which of the following statements are accurate about moles?CorrectIncorrect
Calculate the number of moles in 3.6g of carbon. Give your answer to 1 decimal placeCorrectIncorrect
You can get the relative atomic mass from the periodic table at the top of the quiz
Calculate the number of moles in 54g of water.CorrectIncorrect
You can get the relative atomic mass from the periodic table at the top of the quiz. You should know the formula for water.
Calculate the mass of 16 moles of hydrochloric acid.
Click the hint to see the formula for hydrochloric acid, you need to remember this as well as other common acids.CorrectIncorrect
HCl is the formula for hydrochloric acid.
Whilst you’re here, here’s the formulae for other important acids you need to remember…
Nitric acid = HNO3
Sulfuric acid = H₂SO₄
Calculate the mass of 25 moles of sulfurCorrectIncorrect
You have a sample of 9.1kg of nitrogen gas (N₂) which contains 650 moles of particles. Calculate the Mᵣ of nitrogen (N₂)CorrectIncorrect
You have a sample of 788g of gold which contains 4 moles of particles. Calculate the Aᵣ of gold.CorrectIncorrect
Calculate the number of molecules in 16g of oxygen O₂.CorrectIncorrect
Determine the balanced symbol equation for a reaction between 192g of magnesium (Mg) and 128g of Oxygen (O₂) to produce magnesium oxide (MgO).
Use stoichiometry to solve this question – e.g. the simplest whole number ratio.
Mg + O₂ → MgO
Determine the balanced symbol equation for a reaction between 662g of lead nitrate (Pb(NO₃)₂) and 234g of sodium chloride (NaCl) to produce 556g of lead chloride (PbCl₂) and 340g of sodium nitrate (NaNO₃).
Pb(NO₃)₂ + NaCl → PbCl₂ + NaNO₃
Mᵣ of lead nitrate = 331
Calculate the others. You’ll need them.
Use some of the following words to fill in the blanks.
Excess, limiting, product, reactant
Complete the sentence:
During a reaction, the mass of product formed is controlled by the mass of the reactant which is not in excess.
This is because the reactant is all used up and no more can be formed.