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# IB Stoichiometry 1.2

### the mole concept and avogadro's constant

Avogadro's constant (L) = 6.02 x 10^23 mol-1
What does avogadro's constant mean... It is the number of atoms or molecules or ions in 1 mole of a substance.
Calculating Number of moles (n) number of particles (N) / 6.02 x 10^23 (L)
Water of crystallization... when heated... is when compounds contain water molecules chemically bonded into the structure of the crystals. It is when compounds are hydrated. When heated the water is given off and compound is dehydrated.
Relative atomic mass (Ar) the mass of an average atom of the element, relative to an atom of carbon-12 isotope (mass of 12g)
Relative molecular mass (Mr) mass of a molecule
Ar and Mr do not... have units
Atomic mass of a substance is... the mass of one mole of that substance (g.mol-1)
Molecular mass of a substance is... the mass of one mole of the molecules (sum of atomic masses of component elements)
Formula mass of an IONIC substance is... the sum of the atomic masses of the atoms in the formula (including water of crystallisation if ionic substance is hydrated)
Number of moles (n) = mass in g (m) / molar mass in g mol-1 (M)
Percentage composition 1. find proportion by mass of each element 2. convert it to a percentage i.e. (mass of desired element / molar mass) x 100
Empirical formula (EXPERIMENTAL) simplest whole number ratio of the atoms of each element in the compound
How to find the empirical formula 1. find moles of each element present (n=m/M) 2. divide all moles from smallest value 3. get the simplest whole number ratio 4. can round up 5. in some cases multiplying by a small integer will be necessary to get whole number ratio
Finding Percentage composition of ORGANIC COMPOUNDS 1. Burn known mass of compound in excess oxygen 2. Moles of CO2 (C) formed (n=m/M) 3. Moles of H2O formed (x2 for moles of H) 5. Mass of C+H 6. Mass of O = initial mass - mass of C+H 7. Find mole ratio to get the formula!
Molecular Formula (THEORETICAL) 1. Find the factor ( theoretical molar mass/empirical molar mass) 2. Multiply empirical formula by this factor
Created by: 1392166876