1.3 Chemical Calculations Flashcards
Define relative atomic mass
The average mass of one atom of the element relative to one-twelfth the mass of one atom of carbon-12
Define relative isotopic mass
The mass of an atom of an isotope relative to one-twelfth the mass of an atom of carbon-12
What is atomic mass measured in?
Unified atomic mass, u.
The equation for calculating relative atomic mass
Ar = (mass x %) + (mass x %) + …
————————————
100
What is a mass spectrometer used for?
Measuring the relative abundance and atomic masses of isotopes
How does a mass spectrometer work?
Vapourisation
Ionisation
Acceleration
Deflection
Detection
What happens at vaporisation?
- sample is heated and turned into a gas before it enters the ionisation chamber
What happens at ionisation?
- gaseous sample os bombarded with high energy electrons from an electron gun, forming +ve ions
What happens at acceleration?
- the +ve ions are accelerated using an electric field and a vacuum pump
What happens at deflection?
- ions are deflected by a magnetic field
- amount of deflection depends on the mass and charge of the ion (light = deflected more)
What happens at detection?
- the abundance and mass/charge (m/z) ratio of the ions is read and a mass spectrum graph is produced
Outline the mass spectrum of bromine
- 2 isotopes of equal abundance: 79Br and 81Br at 1:1
- peak @ 79 and 81
- peak @ 158, 160 and 162 (1:2:1)
Outline the mass spectrum of chlorine
- peak @ 35 and 37 (3:1 ratio)
- peak @ 70, 72 and 74 (9:6:1)
Equation for moles in a solid
Moles = mass
———
Mr
Moles in a solution
Moles = vol x conc
—————
1000
Moles in a gas
Moles = volume
———-
24.5
Define a mole
- the amount of a substance which contains as many particles as there are carbon atoms in 12g of the carbon-12 isotope
Define avogadros constant
- the number of atoms per mole of the carbon-12 isotope
Define concentration
How many moles of solute are dissolved in 1dm3 of solution
3 concentration equations
- n = (v x c) / 1000
- v = (n x 1000) / c
- c = (n x 1000) / v
- only divide by 1000 if vol given in cm3
mass / charge =
Relative mass of ions detected
Define relative formula mass
The sum of the relative atomic masses of all atoms present in its formula
What are some uses of mass spectrometry?
- identifying unknown compounds
- identifying trace compounds in forensics
- analysing molecules in space
What is fragmentation?
The splitting of molecules in a mass spectrometer to smaller parts
Define molar volume
- volume per mole of a gas at rtp (24.5dm^3)
What is the ideal gas equation
pV = nRT
What is each letter of the ideal gas equation
p - pressure in Pa
V - volume in m^3
n - moles
R - gas constant (8.31 JK-1 mol-1)
T - temperature (K)
Percentage yield equation
= actual amount in mol of product
————————————————— x100
theoretical amount in mol of product
Atom economy equation
= mass of desired products
————————————— x100
total mass of reactants
Define a procedural error and precision error
- procedural : error due to the procedure
- precision : error due to inaccuracy with equipment
( uncertainty is half of the smallest reading )
Define the water of crystallisation
- water molecules that form an essential part of the crystalline structure of a compound
Define molecular formula
True number of each atom in a molecule
Define empirical formula
Simplest whole number ratio of atoms in each element of a compound
Percentage error equation
Error
—————- x 100
Reading
