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3.1.2 Amount of Substance (The Mole and the Avogadro Constant (n m mr (n =…
3.1.2 Amount of Substance
Relative Atomic Mass and Relative Formula Mass
relative atomic mass: the weighted average mass of an atom of an element taking into account its naturally occurring isotopes, relative to 1/12th the RAM of an atom of carbon-12
relative formula mass: the mass of that molecule compared to the RAM of an atom of carbon-12
The Mole and the Avogadro Constant
Avogadro constant: number of particles in a mole
n = moles (mol)
m = mass (g)
Mr = relative formula mass
number of particles = amount of moles x Avogadro constant
n = moles (mol)
c = concentration (mol/dm^3)
v = volume (dm^3)
The Ideal Gas Equation
pV = nRT
p = pressure (Pa)
V = volume (dm^3)
n = moles (n)
R = ideal gas constant (JK^-1mol^-1)
T = temperature (K)
Empirical and Molecular Formula
empirical formula: simplest whole number ratio of atoms of each element in a compound
molecular formula: actual number of atoms of each element in a compound
calculate empirical formula
1) convert percentage compositions to mass of each element
2) work out Mr of each element
3) work out moles of each element (n = m / Mr)
4) find simplest mole ratio between elements
5) write formula of compound
calculate molecular formula
1) work out Mr of molecule (from periodic table)
2) work out Mr of compound from empirical formula
3) Mr of compound from periodic table x Mr of compound from empirical formula = units of empirical formula in molecule
4) units of empirical formula in molecule x each element in empirical formula = molecular formula
Balanced Equations and Associated Calculations
percentage atom economy
(Mr of desired product / Mr of all products or all reactants) x 100
percentage yield
percentage yield never 100%
reversible reaction
some of product may be lost during seperation
reactants may react in other reactions
1) balance equation
2) work out moles of product with known mass (n = m / Mr)
3) use mole ratio to find max theoretical mass of compound being worked out
4) percentage yield = (actual mass / max theoretical yield) x 100
AMOUNT OF SUBSTANCE
record answer to least number of significant figures in question
Conversions:
1000cm^3 = 1dm^3 = 1 litre
1kPa = 1000Pa
°C + 273 = K
Sammer Sheikh
