# Amount of Substance

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## Relative atomic and molecular mass

Relative atomic mass - average mass of one atom of an element/one twelfth mass of an atomic of carbon-12

Relative molecular mass - average mass of one molecule/one twelfth mass of an atomic of carbon-12

• the number of atoms in 12g of carbon-12
• 6.022  x10^23

Mole - the amount of substance that contains 6.022 x10^23 particles

number of moles = mass in grams/relative molecular mass

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## Ideal gas equation

Boyle's Law - PV = constant

Charles' Law - P/T = constant

Gay-Lussac's Law - PV/T = constant for fixed mass of gas

Ideal gas equation - PV = nRT

P = pressure - Pa

V = volume - m^3

n = number of moles

R = constant - 8.31 J/K/mol

T = temperature - K

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## Empirical and molecular formulae

An empirical formula is one which represents the simplest ratio of the atoms of each element present in a compound

Empirical

• find masses of each of the elements present
• work out number of moles of atoms of each element
• convert number of moles of each element into a whole number ratio by dividing each number by the smallest number

Molecular

• divide the relative molecular mass by the relative mass of the empirical formula
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## Moles in solutions

Concentrations

• tells us how much solute is present in a known volume of solution
• measured in mol/dm^3
• number of moles/volume in dm^3

Moles

• (concentration*volume)/1000 when concentration is given in mol/dm^3 and volume in cm^3
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## Ionic equations

In some reactions, the equation can be simplified by considering the ions present.

Example

HCl + NaOH ---> NaCl + H2O

Ions present:

• H+
• Cl-
• Na+
• OH-
• Na+
• Cl-

H+ + Cl- + Na+ + OH- ---> Na+ + Cl- + H20

H+ + OH- ---> H2O

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## Atom economies and percentage yields

Atom economy = (100*mass of desired product)/total mass of reactants

Percentage yield = (100*number of moles of specified product)/theoretical maximum number of moles of product

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