Unit 9 - Calculations Involving Masses Flashcards
What is an empirical formula?
The simplest whole number ratio of atoms or ions of each element in a substance
What is a molecular formula?
The actual number of atoms of each element in one molecule
Give an example where the empirical formula is different to the molecular formula
Ethene - molecular formula = C2H4, empirical formula = CH2
Give an example where the empirical formula is the same as the molecular formula
Water - molecular formula = H20, empirical formula = H20
What is the relative formula mass? (Mr)
The sum of the relative atomic masses of all he atoms or ions in the formula
How do you work out the empirical formula using the amount of each substance?
Write each element in its own column
Divide the % or g by the RAM
Divide each of these answers by the smallest answer.
This is the atomic ratio for the empirical formula
How do you find the molecular formula from the empirical formula?
Find the empirical formula mass (total mass)
Divide the relative formula mass by the empirical mass (should be given)
The answer is multiply the empirical formula by to get the molecular formula.
What is the law of conservation of mass
The mass of the solution is equal to the mass of the solvent and the mass of the solute.
What is the concentration
The amount of solute dissolved in a stated volume of solution
What is 1 dm^3 equal to
1 litre or 1000cm^3
How can you calculate the the concentration of a solution
Concentration = mass of solute in g / volume of solution in dm^3
What are the units for concentration
gdm^-3
How do you convert from cm^3 to dm^3
Divide by 1000
What is a closed system?
When no new substances are added or removed
Give an example of a closed system
Lead nitrate + potassium iodide -> Lead iodide + potassium nitrate
What is a non-enclosed system?
When the mass of the products appear to change from the reactants. E.g from a gas
Give an example of a non-enclosed reaction
Copper carbonate -> copper oxide + carbon dioxide
How do you calculate the mass of a substance needed to make a stated amount of product
Write the balanced equation
Find the Mr of the substances needed (excluding the balanced numbers)
Multiple each of the Mr by their balancing numbers
Since we know the product mass, divide both answers by the previous answer for the product (get one g of product but must do the same for reactant)
Multiply both answers by the stated mass given in the question
What is the Avogadro number?
6.02 x 10^23
What does the Avogadro number mean?
The amount of particles in 1 mole of a substance
What is a mole?
An amount of a substance which contains the Avogadro number. The relative atomic mass is the amount of g in 1 mole of a substance
Give an example of an element, the mass of 1 mole, and the amount of atoms it contains
Magnesium, 1 mole = 24g, it contains 6.02 x 10^23 atoms.
What is the equation for calculating the smith of moles in a substance?
Moles = g/Ar or Mr
What is a limiting reactant?
Usually one of the reactants is added in excess and not completely used up, the limiting reactant is the reactant that is not in excess
How do you work out the limiting reactant?
Work out how many moles of 1 substance there is in the reaction. (g/Mr or Ar)
Multiply this by any balancing numbers for both substances. This will give the amount of moles for the other substance needed to react with it.
Calculate how many moles there are in the other substance
This will show if there is less or more than needed to react with the first substance.
How do you work out the mass of a product formed using the masses of the reactants
The equation should show that the amount of moles in the product will equal an amount of moles in one of the reactants.
Calculate the moles of the reactant using it’s mass and Ar.
Multiply this by the Mr of the product formed.
What is the stoichiometry of a reaction?
The ratio of the moles of each substance.
How do you work out the balanced equation using the masses of the substances?
Calculate the moles of each substance
Divide each by the smaller answer
Multiply both by a factor so that they are both whole numbers.
Add the formula and balance
