7 — mole concept and stoichiometry Flashcards
Relative atomic mass
Relative atomic mass (Ar) of an elemen is the average mass of one atom of that element relative to 1/12 the mass of an atom of carbon-12
Relative molecular mass
Relative molecular mass (Mr) of a molecular substance is the mass of one molecule of that substance relative to 1/12 the mass of an atom of carbon-12
Percentage mass
Percentage mass = (no. Of atoms of the element in the formula x Ar of the element/ Mr of the compound) x 100%
The avogadro constant
One mole of any substance will always contain 6.02 x 10^23 particles of that substance
Moles
No. Of moles = no. Of particles/6.02 x 10^23
No. Of moles = Mass of substance (in g)/ Ar of element or Mr of
Substance
No. Of moles = Vol of gas at rtp(in cm^3 or dm^3) / 24dm^3 or 24000cm^3
No. Of moles = Concentration in mol/dm^3 x vol of solution in dm^3
(Concentration in mol/dm^3 x Mr = concentration in g/dm^3)
Molar mass
Molar mass is the mass of one mole of a substance
Mr = Ar of the element or Mr of the compound
Empirical formula
The simplest ratio of elements in a compound
Molecular formula is a multiple of the empirical formula
Do the working as table below
Element:
% by mass or mass:
Ar:
No. Of moles/mol (% by mass divide by Ar):
Mole ratio (divide by smallest of mole value):
Molecular formula
n = Mr of compound/Mr of empirical formula
Stoichiometry
Stoichiometry is the ratio between the quantities of substances (in moles) involved in a CR
Finding volume of gas
Since mole ratio = volume ratio, …
Volumetric analysis
Titration
1. Fill a clean 50.00cm³ burette with the titrant.
2. Add a fixed volume of analyte, typically 25.0cm³, to a clean conical flask using a pipette
3. Add a few drops of indicator to the conical flask
4. Slowly add the titrant from the burette into the conical flask. Swirl the flask constantly as the titrant is added. Stop adding titrant when contents of the flask has changed colour permanently, which is the end point of the reaction. Record the volume of titrant used.
Rs between concentrations and volumes
(Ca x Va/Cb x Vb) = (no. Of moles of a/no. Of moles of b)
Limiting reactant
The reactant that is completely used up in a reaction
Common processes in chem
Combustion: water vapour formed
CH4 (g) + 2O2 (g) -(heat)-> CO2 (g) + 2 H2O (g) (becomes l when cooled)
Bubble gases into NaOH -> removes acidic gases
Percentage yield
Percentage yield = (actual yield/theoretical maximum yield) x 100%
