Atoms, molecules and stoichiometry Flashcards
Unified Atomic Mass
Masses of atoms and molecules
one twelfth of the mass of a carbon-12 atom
Relative Atomic Mass
Masses of atoms and molecules
the weighted average mass of atoms in a given sample of an element compared to the value of the unified atomic mass unit
Define
Isotopes
Masses of atoms and molecules
atoms of the same element having
* same atomic number
* different mass number
* same proton number
* different neutron number
Relative Isotopic Mass
Masses of atoms and molecules
the mass of a particular atom of an element compared to the value of the unified atomic mass
Relative Molecular Mass
Masses of atoms and molecules
the weighted average mass of a molecule in a given sample of that molecule compared to the value of the unified atomic mass unit
Relative Formula Mass
Masses of atoms and molecules
the weighted average mass of one formula unit compared to the value of the unified atomic mass unit
What are Hydrated Compounds ?
compounds which contain a definite number of moles of water in their structure
e.g : CuSO4.H2O
Define ‘anhydrous’
containing no water of crystallisation
Mass Spectrometry
Accurate relative isotopic masses
the use of a mass spectrometer to measure the mass of each isotope present in an element
also compares how much of each isotope is present
Relative Isotopic Abundance
Accurate relative isotopic masses
the proportion of one particular isotope in a mixture of isotopes, usually expressed as a percentage
Molecular ion, M+
Accurate relative isotopic masses
the ion that is formed by the loss of an electron from the original complete molecule during mass spectrometry
this gives us the relative formula mass of an unnown compound
Fragmentation
Accurate relative isotopic masses
the breaking up of a covalent compound during mass spectrometry into smaller positively charged species
How does fragmentation take place ?
Accurate relative isotopic masses
it takes place when the single bonds of a covalent compound break up due to an electron bombardment, giving fragments
What is the most common cause of fragmentation ?
Accurate relative isotopic masses
the breaking of single bonds
* C-C
* C-H
* C-N
What does the molecular ion peak represent ?
Accurate relative isotopic masses
- the heaviest ion
- has the highest m/e ratio
- not necessarily the most abundant ion
What does the base peak represent ?
Accurate relative isotopic masses
- always the tallest peak
- most common and abundant ion
How are fragmentation patterns useful ?
Accurate relative isotopic masses
to determine the molar weight and structural formula of unknown molecules
What is the importance of high resolution spectra ?
Accurate relative isotopic masses
allows us to distinguish between ions that appear to have the same mass on a low-resolution spectrum
Molecular Formula
Accurate relative isotopic masses
shows the number and type of each atom in a molecule
What is the [ M+1 ] peak ?
Accurate relative isotopic masses
a very small peak just beyond the molecular ion peak, caused by molecules in which one of the carbon atoms is the 13C isotope
What is the [ M +2 ] peak ?
Accurate relative isotopic masses
helps us identify whether chlorine or bromine is present in an organic compound by comparing the relative heights of the peaks
When is it one chlorine atom ?
[ M+2 ] peak
Accurate relative isotopic masses
if the [ M+2 ] peak is 1/3 of the M peak
When is it one bromine atom ?
[ M+2 ] peak
Accurate relative isotopic masses
if the [ M+2 ] peak is equal to the height of the M peak
Avogadro Constant
Amount of substance
the number of specified particles equivalent to the relative atomic mass or the relative molecular mass of a substance in grams
Symbol of Avogadro Constant
Amount of substance
L
Numerical Value of Avogadro Constant
Amount of substance
6.02 x 10^23
Define the ‘mole’
Amount of substance
the amount of substance which contains 6.02 x 10^23 specified particles
Specified particles
mole
- atoms
- molecules
- ions
- electrons
Define ‘molar mass’
Amount of substance
the mass of a mole of substance in grams
Stoichiometry
Mole calculations
The mole ratios of reactants and products shown in a balanced equation
Empirical Formula
Mole calculations
The simplest whole number ratio of the elements present in one molecule or formula unit of a compound
Molecular Formula
- shows the actual number of each of the different atoms present in a molecule
- used to write balanced equations and to calculate molar masses
- it is always a multiple of the empirical formula
Oxidation number ( oxidation state )
a number given to an ion/atom in a compound to represent how oxidised/reduced it is
Compound ion
ions that contain more than one type of atom
Spectator ions
Ions present in a reaction mixture which do not take part in the reaction
Ionic equation
A balanced equation showing only those ions, atoms or molecules taking part in the reaction
- often written for reactions involving a change in oxidation state
Titre
final reading - initial reading
burette
Solution concentration
the amount of solute ( in moles ) dissolved in a stated volume of solution ( usually 1.00 dm3 )
Molar gas volume
The volume occupied by one mole of any gas at r.t.p
