Calculating moles Flashcards
Define mass number
Sum of protons and neutrons in the nucleus
Define isotope
Atoms of the same element with different masses, same protons and different amounts of nuetrons
Why are mass numbers decimals on the periodic table?
Due to the presence of isotopes
What does the abundance of an isotope tell us?
How common each isotope is
Give the uses of a TFMS
Finding the abundance and mass of each isotope in an element allowing us to determine the relative atomic mass
-Find the relative molecular mass of substances made of molecules
Describe what happens in stage 1 of TFMS ( electron impact)
Ionisation stage
Electron impact- Sample is vaporised and an electron gun ( a hot wire filament with a current running through it that emits electrons) fires high energy electrons at it knocking one electron from each particle forming a 1+ ion. These positive ions are then attracted towards a negative electric plate where they are accelerated.
Describe what happens during electrospray ionisation in stage 1 of TFMS
Sample is dissolved in a volatile solvent e.g methanol, and is injected through a fine hypodermic needle to give a fine mist ( aerosol) The tip of this needle is then attached to the positive terminal of a high voltage power supply. The particles are then ionised by gaining a proton from the solvent as they leave the needle forming XH+ ions. This solvent then evaporates away while XH+ ions are attracted towards a negative plate where they are accelerated.
What happens during stage 2 of TFMS?
The positive ions are accelerated using an electric field so that they have the same kinetic energy.
This means that because all the particles have the same kinetic energy, the velocity will depend on the masses of the particles, lighter particles will have a faster velocity than heavy particles.
What happens during stage 3 of TFMS?
The positive ions travel through a hole in a negatively charged plate into a tube. The time of flight in the tube of each particle depends on the velocity which in turn depends on its mass.
Time is proportional to the square root of the mass of the ions. This means the lightest isotope will reach the detector first.
What happens during stage 4 of TFMS?
The positive ions hit a negatively charged electric plate. When they hit the detector plate, the positive ions are discharged by gaining electrons from the plate, generating a movement of electrons and hence an electric current that is measured. The size of the current gives a measure of the number of ions hitting the plate.
How is relative molecular mass found by using a mass spectrum graph?
For molecules that are ionised using electron impact, the signal with the highest mass charge ratio gives the relative molecular mass.
Why might there be smaller peaks on a mass spec graph produced by electron impact?
Due to the fragmentation of molecular ions
Define relative atomic mass
Average mass of an elementd atoms compared to one twelth the mass kf carbon 12
What specific condition does time of flight mass spec need to be under?
-A vacumn otherwise air particles would ionise and register on the detector
What is electron impact more commonly used for?
- Elements and substances with a low formula mass, as it can cause larger organic molecules to fragment
What is electrospray ionisation used for
-Larger molecules, softer conditions means molecules dont fragment
Give first equation used in mass spec
T= D X square root of mass over (kinetic energy)2
How to find mass in mass spec?
Mass of one mole divided by the avagadros constant
Give equation of the relative atomic mass
RAM= ( isotopic mass X % abundance or relative abundance) / 100
How do we measure the mr of a molecule that has been ionised by electrospray ionisation from a spectra graph?
- Fragmentation does not occur , one peak that is equivelent to the mass of the MH+ ion
- This means we subtract 1 from this peak number to get the mr
How do we measure the mr of a molecule that has been ionised by electron impact from a spctra graph
-We get fragments giving a series of peaks
- Highest peak is equal to the mr, this peak is the molecular ion
