WJEC AS Chemistry Unit 1.3 - Chemical Calculations

Relative atomic mass

The average mass of one atom of the element relative to one-twelfth the mass of one atom of Carbon-12

Relative isotopic mass

The mass of an atom of an isotope relative to one-twelfth the mass of an atom of Carbon-12

Relative formula mass

The sum of the relative atomic masses of all atoms present in its formula

Relative atomic mass equation

Sum of; mass number of isotope x % abundance

Uses of a mass spectrometer

• Determining the relative atomic mass of each different isotope of the element

• Determining the relative abundance of each isotope of the element

VIADD

• Vaporisation

• Ionisation

• Acceleration

• Deflection

• Detection

Vaporisation in mass spectrometry

• the sample of heated, vaporising into a gaseous state before it is injected into the mass spectrometer

• Heater retains the sample as a vapour

Ionisation in mass spectrometry

Acceleration in mass spectrometry

An electric field accelerates the positive ions to high speed

Deflection in mass spectrometry

Detection in mass spectrometry

Molecular ions

• positive ions formed in the mass spectrometer from the whole molecule

• When a vaporised compound passes through a mass spectrometer, an electron is knocked off a molecule to form a positive ion

• Mass gives the relative formula mass of the compound

• Energetically unstable and some will break up into fragments

Fragmentation

• Splitting of molecules, in a mass spectrometer, into smaller parts

• When the element is passed into the ionisation chamber, an electron is knocked off the molecule to give a molecular ion. These ions will be unstable, and some will break apart into fragments

• 2 fragment ions at different masses = exist as isotopes

Ionisation mass spec equation

Fragmentation equation mass spec

Empirical formula

The simplest formulae showing the simplest whole number ratio of the number of atoms of each element present

Molecular formula

The actual number of atoms of each element present in the molecule. Simple multiple of the empirical formula. Usually the relative formula mass is needed to determine molecular formula

Determining empirical formulae

Determining molecular formulae

1. Determine mass of empirical formula

2. Determine number of the units of the compound by dividing the relative molecular mass by the calculated mass

3. Multiply empirical formulae by the number of units in a molecule

Moles of a gas equation

Ideal gas equation

• pV=nRT

• p in Pa

• V in metres cubed

• T in K

Atom economy

Mass of required product/total mass of reactants x100

Percentage yield

Mass (or moles) of product obtained/maximum theoretical yield (or moles) x100

Percentage error

• Error/reading x100

• Error; one half of the smallest division of the apparatus scale

• Two measurements —> double error

• Values with approx 1% error = 3sf

• Values with approx 10% error = 2sf

Relationship between mass, moles and relative atomic mass

• n= m/Mr

• m=n x Mr

• Mr=m/n