Moles 1 - 4.4 Flashcards
What is a mole?
A mole is the unit of amount of substance in chemistry.
It represents how many particles of a substance there are, taking into account that particles of different substances have different masses.
What is the formula for moles?
moles = mass/mr
What is the empirical formula?
The simplest whole number ratio of elements in a compound.
How to calculate the empirical formula?
Empirical formula can be calculated from the masses (or percentages) of each element:
1. Convert the masses (or percentages) into moles.
2. Cancel down the mole ratio.
3. Write the simplest mole ratio into a formula.
What is the molecular formula?
The molecular formula of a substance contains the actual numbers of atoms in a molecule.
How to calculate the molecular formula?
Molecular formula can be calculated from empirical formula and RFM.
This works by finding the uncancelled multiple of the empirical formula with the correct RFM
e.g. Calculate how many times the empirical formula “fits” into the RFM by division
What is water of crystallisation?
When a salt forms crystals, a particular amount of water gets trapped inside the crystals. The trapped water is called water of crystallisation and the amount of it is indicated in the formula of the crystals.
How is water of crystallisation calculated?
Water of crystallisation is calculated from the masses of salt and water produced on heating.
e.g. calculating ‘x’ if 16.7 g of MgCl2*xH2O crystals decreased in mass to 9.5 g on heating
1. Calculate the mass of water lost.
Mass of water = 16.7 g — 9.5 g = 7.2 g
2. Draw a “mass, moles, ratio” table and stick the numbers in.
Be careful to use the final mass of solid for the salt, not the original mass of crystals
How do you calculate the percentage yield?
% yield = actual/theoretical x 100
What is theoretical yield?
theoretical yield is the max mass possible using the masses/volumes reacted
What might the reasons be for not making the theoretical yield (100% yield)?
- incomplete reaction
- impure reactant
- loss of product
What is actual yield?
The actual mass collected
What is the easiest way to calculate which substance is limiting/excess?
The easiest way is to pick one, then calculate how much of the other would be needed to react with it, and then compare that to how much of the other there actually is.
