03 - atoms, molecules and stoichiometry Flashcards
what is a unified atomic mass unit?
- one twelfth of an unbound (isolated), neutral atom of the carbon-12 isotope in its ground state (lowest energy state)
- one twelfth of the mass of a carbon-12 atom (SIMPLE DEF.)
- ‘standard’ atom’s mass
- defined in terms of isotope carbon-12
- symbol = u/ Da ( Dalton )
1u = 1.66 * 10^-27
what is the relative atomic mass?
- ratio of average mass of the atoms of an element to the unified atomic mass unit
Ar = weighted avg. mass of atoms in a give sample of an element / unified atomic mass unit
- has no units simce it’s a ratio
what is the relative isotopic mass?
- the mass of a particular atom of an isotope compared to the value of the unified atomic mass unit
what is relative molecular mass?
- the ratio if the weighted avg. mass of a molecule of a molecular compound to the unified atomic mass unit
- found by adding up the relative atomic masses of all the atoms in one molecule
- when calculating, the simplest formula (formula unit) of compound is used
- symbol = Mr
- has no units since it’s a ratio
Mr = weighted average mass of molecules in a given sample of a molecular compound / unified atomic mass unit
what is a formula unit?
the simplest formula for a covalent/ionic giant structure
eg: SiO2, MgCl2
what is relative formula mass?
- weighted average mass of one formula unit compared to the value of the unified atomic mass unit
- symbol = Mr
- calculated the same way as relative molecular mass
what is water of crystallisation?
- a specific number of moles of water associated with a crystal structure
- seperated from main formula with a dot
- number of moles is usually a whole number
with = hydrated compound
without = ahydrous compound
what is a hydrated compound?
- a compound which contains a definite number of moles of water in their structure (water of crystallisation)
- can be different degrees of hydration
what is an anhydrous compoubd?
- a compound containing no water of crystallisation
- can be converted to hydrous compound by adding water
what is mass spectroscopy?
- form of molecular chemical analysis
- mass spectrometer: used to measure the mass of each isotope present in an element, and how much of each isotope is present (relative abundance)
- PROCESS:
- sample is vaporised
- atoms become ions
- ion stream deflected by strong magnetic field
- magnetic field increases, heavier isotopes move to detector
- detector connected to computer
- computer displays mass spectrum (x = m/e ratio, y = relative abundance)
what does mass spectrometry do?
- provide info on structure
- indentify unknown compounds
- determine relative abundance of each isotope of an element
how do you find the Ar from a mass spectra?
(isotopic mass * % abundance) + (isotopic mass * % abundance) +…. / 100
what is fingerprinting?
- technique where substance is identified by matching its spectrum along the spectra of known substances stored in a database
what is a molecular ion (M+)
- ion formed when one electron is removed from a molecule to form an ion with a single positive charge during mass spectrometry (NO ATOM REMOVED)
whta is fragmentation?
- breaking apart of a covalent compound into smaller +ively charged species in a mass spectrometer
- can find fragment by adding masses to the m/e ratio
- breaking of single bonds (C-C, C-H) is most common cause
what is an M+1 peak?
- a small peak beyond the molecular ion peak at a mass of [M + 1]
- caused by molecules in which one of the carbon atoms is the carbon-13 isotope
- any organic compound has 1.1% carbon-13
FINIDING NO. OF CARBON ATOMS (N) IN A MOLECULE
n = 100/1.1 * abundance of [M+1]+ ion/abundance of M+ ion
