1.6 Calculations Flashcards
Relative formula mass
Mr = element 1 (mass number x abundance) + element 2 (mass number x abundance)…
H2O = (1 x 2) + (16 x 1) = 18
Empirical formula
An empirical formula gives the simplest whole number ratio of atoms of each element in the compound.
Amount of (element 1) atoms (moles) = Mass in grams ÷ Ar of element 1
Amount of (element 2) atoms (moles) = Mass in grams ÷ Ar of element 2
Divide by the smaller number and express as a ratio.
Determine an empirical formula
Procedure - Measure mass of crucible with lid.
Add sample of magnesium into crucible and measure mass with lid (calculate the mass of the metal by subtracting the mass of empty crucible).
Strongly heat the crucible over a Bunsen burner for several minutes.
Lift the lid frequently to allow sufficient air into the crucible for the magnesium to fully oxidise without letting magnesium oxide smoke escape.
Continue heating until the mass of crucible remains constant (maximum mass), indicating that the reaction is complete.
Measure the mass of crucible and contents (calculate the mass of metal oxide by subtracting the mass of empty crucible).
Conservation of mass
The Law of Conservation of Mass states that no matter is lost or gained during a chemical reaction.
Mass is always conserved, therefore the total mass of the reactants is equal to the total mass of the products, which is why all chemical equations must be balanced.
Change in mass
If carried out in a closed system then the mass before and after the reaction will be the same.
If the reaction flask is open and a gaseous product is allowed to escape, then the total mass of the reaction flask will change as product mass is lost when the gas leaves the system.
If the mass of a reaction flask is found to increase then it may be due to one of the reactants being a gas found in the air and all of the products are either solids or liquids.
Percentage composition
% mass of an element = (Ar x number of atoms of the element / Mr of the compound) x 100
Concentration
Concentration (g/dm^3) = mass of solute (g) / volume of solution (dm^3)
The mole
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 10^23 per mole.
Particles = moles x Avogadro’s constant
Molar mass
Mass = moles x relative atomic mass
Limiting reagent
A chemical reaction stops when one of the reagents is used up.
The reagent that is used up first is the limiting reagent, as it limits the duration and hence the amount of product that a reaction can produce.
The amount of product is therefore directly proportional to the amount of the limiting reagent added at the beginning of a reaction.
mass / RAM : mass / RAM
smaller number x RAM of product = mass of product
Deducing stoichiometry
Stoichiometry refers to the numbers in front of the reactants and products in an equation, which must be adjusted to make sure that the equation is balanced.
These numbers are called coefficients (or multipliers) and if we know the masses of reactants and products, the balanced chemical equation for a given reaction can be found by determining the coefficients.
