Chemical Formulae, Equations and Calculations Flashcards
What is relative formula mass (Mr)?
Also known as relative molecular mass (but only if it is just a molecule, not a compound + molecule)
The total mass of a substance
How do you work out relative formula mass (Mr)?
Find the relative atomic mass (Ar) of each atom and times it by the number of atoms of that element in the compound/molecule
Relative atomic mass is the mass number taking into account abundances
What is a mole?
- An exact number of atoms, molecules or compounds (avogadros constant: 6 x 10²³) of a given substance
- For example, one mole of sodium will have exactly 6 x 10²³ atoms of sodium, one mole of H₂ would have exactly 6 x 10²³ molecules of hydrogen, one mole of Sodium Chloride has exactly 6 x 10²³ units (compounds) of Sodium Chloride
How do you find molar mass?
- The Ar, or the Mr for a compound in grams
- For example, one mole of water would be 18g ((2x1) + 16)
How do you find the number of moles when given the mass and the Ar/Mr
Mass divided by Ar or Mr
How do you find the mass of a substance when given the number of moles and the Ar/Mr
Number of moles x Ar or Mr
How do you find reacting masses?
- Find the number of moles of the given substance (worked out from the mass and Mr)
- Work out how many moles there are in the other substance using ratios from the balanced equations
- Times the number of moles with the Mr to get the mass wanted
How do you work out percentage yield?
- Work out theoretical yield using reacting masses (it may be given)
- Divide actual yield by theoretical yield, then x by 100
- Actual yield is how much product was actually formed
- Theoretical yield is the amount of product that could potentially be formed under perfect conditions, which is calculated by reacting masses
What is molecular formula?
- The formula that shows the actual number of atoms in a formula
- For example, the molecular formula for ethanoic acid is C₂H₄O₂
What is empirical formula?
- The simplest whole number ratio of each element present in one formula unit of a compound
- For example, the empirical formula for ethanoic acid is CH₂O
How do you work out empirical formula when given the masses or percentages of two elements in a compound?
- Find the number of moles of each element by dividing mass/percentage by the Ar of that element
- Find the ratio between the two values of moles
- Simplify it to find empirical formula
How do you work out molecular formula when given the Mr of a compound and the empirical formula?
- Divide the Mr given by the Mr of the empirical formula
- Times each subscript in the empirical formula by this number to find the molecular formula
How do you find the formula of a metal oxide by combustion experimentally?
With the example of magnesium
- Weigh some pure magnesium
- Burn it in the air until the weight stops increasing, indicating the reaction in fully complete
- Weigh the magnesium oxide
- Substract the mass of magnesium oxide from the mass of the pure magnesium to find the mass of the oxygen
- Work out the moles of the oxygen and the magnesium
- Put it into a ratio, then simplify it to find the empirical formula
When burning it, cover it with a lid. Lift this lid frequently to let sufficient oxygen in but not to let any magnesium smoke escape
How do you find the formula of a metal oxide by reduction experimentally?
Example using copper oxide
- Find the mass of some copper oxide by measuring the mass of the reduction tube with and without the copper oxide and subtracting one from the other
- Pass a steady stream of methane or hydrogen over the copper oxide
- Heat the copper oxide until it changes colour, indicating all oxygen has been removed
- Measure the mass of the remaining copper powder, and subtract it from the mass of the copper oxide to find the mass of the oxygen
- Work out the moles of copper and the oxygen
- Put it into a ratio, then simplify it to find the empirical formula
Be sure to pass a constant stream of the gas through to ensure it is fully oxidised but ensure that you burn it as it comes out as hydrogen/methane is very flammable
How do you find the formula of a hydrated salt experimentally?
Using hydrated copper sulfate as an example
- Measure a mass of hydrated copper sulfate
- Add it to an evaporating dish, heat and stir until the blue salt turns completely white, indicating all water has been lost
- Record the mass of the dry copper sulfate
- Subtract the mass of the anhydrous (dry) copper sulfate from the mass of the hydrated copper sulfate to find the mass of the water
- Work out the moles of the anyhydrous copper sulfate and the water
- Put it into a ratio, then simplify it to find the empirical formula
Don’t overheat the salt as it could decompose
