Module 2.1 Flashcards
What is relative atomic mass
The average mass of 1 atom of an element relative to 1/12th the mass of a carbon 12 atom
How do you calculate relative atomic mass
(sum of the mass × abundance of each isotope) ÷ total abundance
How can you check if your calculated Ar is accurate
You can check your calculated value to the accurate periodic table value ( which is compared to 1/12th a carbon atom)
If your calculation is an average of the isotopes you calculated with it could be accurate
What is relative molecular mass
The average mass of one molecule relative to 1/12th the mass of a carbon-12 atom
How do you calculate mR (relative molecular mass)
-Count the number of atoms in the element
-Multiple the ar of each element by the number of times it is present in the molecule
- add all the calculated values together
How do you calculate moles (using mr)
Moles = mass / mr
What is Avogadros constant
A constant to show the number of particles in 1 mol
6.02×10²³
How do you calculate the number of particles in a molecule
Particles = moles × avogadros constant ( 6.022×10²³)
How do you calculate the number of atoms in a molecule
First find the number of particles in the molecule
Then multiply by the number of atoms in the molecule
How to calculate the number of protons / electrons in a set mass of a molecule
First find the number of particles in the molecule. (Moles × avogadros constant)
Then multiply by the number of atoms, to find the number of atoms
Finally multiply by the atomic number
Calculating the number of protons in a molecule example
How many protons in 6.0g of nitrogen gas
N² = 14×2 = 28
Moles = 6/28
Particles in molecule = 6/28 × 6.022×10²³
Particles in atom = 2 × 1.290428571×10²³
Number of protons = 2.580857143×10²³ × 7. (7 is the atomic number/
= 1.8066×10²⁴
What is concentration
The number of particles in a given volume
What are the two main units of concentration
Mol/dm³
g/dm³
Formula for moles using concentration
Moles = concentration × volume
How do you convert cm³ to dm³
Divide the cm³ by 1000
What is the relative charge and relative mass of an electron
Charge = -1
Mass = 1/1836 th the mass of a proton
What is the relative charge and relative mass of a proton
Charge = +1
Mass = 1
What is the relative charge and relative mass of a neutron
Charge = 0
Mass = 1
What is the actual charge on a proton / electron
± 1.60217733×10–¹⁹ C
(±1.6×10-¹⁹)
What the overall charge of an atom
0 (it is neutral)
Where is the majority of an atoms mass
In the nucleus ( where protons + neutrons are located)
What is the atomic number
Number of protons and electrons
It is always smaller than mass number
What is the mass number
The total mass in an atom (protons + neutrons)
What is an isotope
An element with a different number of neutrons
What are cations
Positive ions which have fewer electrons that protons
What are anions
Negative ions with more electrons than protons
How is the mass of carbon 12 defined
It is defined as exactly 12 atomic mass units
What is measured in the relative atomic mass of an element
The isotopic mass of each isotope and the abundance of each isotope
what is the calculation for concentration
c = n/v
(n = moles or grams , v = volume in dm3)
What is water of crystallisation
Water molecules (often seen as crystals) surrounding a metal compound or salt
It means these molecules are hydrated
How do you calculate percentage purity
(pure mr / impure mr) x 100
How do you find moles of water, from knowing the percentage of water in a hydrated substance
e.g k2CO3 . xH2O
it is 11.5% water
k2CO3 = 138.2 mr
100 - 11.5 = 88.5% potassium carbonate
H20 = 18
88.5 / 139.2 = 0.640376
11.5 / 18 = 0.63889
0.640376 / 0.63889 = 1.002326
= 1
so there is 1 mol of water
How do you find moles of water, from knowing the percentage of water in a hydrated substance
Find the percentage mass of each substance
Find the mr of each substance
Divide the percentage mass by the mr to find the moles
Divide the moles by each other to find the molar ratio
What is atom economy
A measure of how efficiently atoms are used up in a chemical reaction
how do you calculate atom economy
(mr of desired product / mr of reactants) x 100
How do you calculate the mol of a gas at room temperature and pressure
mol = volume / 24dm3
n = v/24
the volume and gas constant (24dm3) have to be in the same unit s
What is the ideal gas pressure equation
pV = nRT
p is pressure (in Pa)
v is volume (m³)
n is the number of moles
R is the gas constant - 8.314 J
/mol / k
T is the temperature (K = c° + 273k)
when is the ideal gas equation used
When gases are in the ‘ideal’ state.
- in continuous motion
- Do not experience any intermolecular forces
-Exert pressure when they collide
-Kinetic energy increases with temp
-The sizes are ignored
What is a standard solution
A solution of a known concentration
(in mol/dm3)
How do you make a standard solution
Weigh the mass by difference in a beaker (subtract the mass of the weighing boat from the mass of the weighing boat + substance)
-Fully dissolve the solute in a solvent using a stirring rod to mix
-Transfer the solution to a volumetric flask using a funnel
- rinse the stirring rod and funnel into the volumetric flask using a beaker
- Fill the volumetric flask to 250cm³ with the solvent
- ensure the bottom of the meniscus is at 250cm³
- Invert the flask several times to mix
What is a species (in an equation)
The ion, element or compound that takes part in the reaction
What is a simple molecule
A compound that holds a few atoms together by covalent bonds.
It cant conduct electricity
What is a giant structure
Where many atoms or ions bond together in a repeating pattern.
e.g Giant covalent structure or ionic compounds
Why are giant compounds written using empirical formula
As the number of atoms / ions in the structure depends on the size of the crystalline structure
What is percentage yield
A percentage that shows how much of the product is produced in comparison to its theoretical yield
How do you calculate percentage yield
(actual yield / theoretical yield) x 100
Why is percentage yield not always 100%
The reaction may not go to completion (could be in equilibrium)
Side reactions could occur - making byproducts
The reactants might not be pure
Why do we use process with higher atom economy
A higher atom economy means that less waste will be produced (as the amount of byproducts is lower)
What was Ernest Rutherford’s experiment + results
They fired alpha particles at a thin sheet of gold foil
Most particles were not deflected
A small percentage were deflected at small angles + some where deflected directly back to the source
This proved the positive charge of an atom (and most of its mass) are concentrated in a central nucleus
Negative electrons orbit the nucleus
What was Daltons atomic theory
Atoms are tiny particles that make up elements
Atoms cant be divided
All atoms of a given element are the same
Atoms of one element are different to atoms of another element
What did he discover / what was J J Tomosons model
He found that cathode rays were a stream of particles with:
A negative charge
A very small mass
They could be deflected by magnetic and electric fields
He proposed that atoms were made up of a ‘sea’ of electrons moving in a positive charge
(The plum pudding model)
What did Niels Bohr do
He altered Rutherford’s model and placed electrons in fixed shells
This prevented them from spiraling into the nucleus
What is the value of the gas constant in pv = nRt
R = 8.314 J /mol / k
What is the difference between isotopes of the same atom
They have a different number of neutrons
Therefore they have a different mass number
What is mass spectrometry
A mass spectrometer is a piece of apparatus which can:
- Identify an unknown compound
- Find the relative abundance of each isotope of an element
- Determine structural information about molecules
How does a mass spectrometer work
It determines the mass of a molecule / isotope by measuring the mass to charge ratio of ions
It makes substances positive ions which are then separated based on their mass and charge
How do you determine relative atomic mass from a mass spectrometer
(sum of (percentage abundance × mass : charge) ÷ 100
(The same equation as usual for RAM)
What are molecular ions
Molecules that are covalently bonded and lose / gain electrons to be ions
E g OH-
How do you calculate the moles of a gas at room temperature and pressure
Use the equation
Moles = volume ÷ 24dm³
n= v/24
What is an acid
An acid is a proton (H+) donator
They have a pH of less than 7
All acids contain hydrogen
What is a base
A proton (H+) acceptor
They are used to neutralise acids
E.g
Metal oxides
Metal hydroxides
Metal carbonates
Metal hydrogen carbonates
What is an alkali
An alkali is any substance that when dissolved gives a pH greater than 7
They release OH- ions in water
(Any base that is soluble in water is an alkali)
What are amphoteric substances
Substances that behave as both an acid and bases - they can accept and donate protons
How are salts formed
Neutralisation of an acid with a base
How are titrations carried out
- using a volumetric pipette add a measured volume of one solution into a conical flask
- add a suitable indicator
-place the other solution in a burette
- add the solution in the burette into the conical flask until the reaction has been completed (when the indicator changes colour)
- Repeat more accurately (by droppping the pipette liquid in at a slower place) until you get 3 or more concordant titres
Before using the burette rinse it (using what you will place into the burette) to prevent any reactions from occuring
What are the colour changes in phenolphthalein
Acid + neutral = colourless
Base = pink
What is an oxidation number
The number of electrons an atom uses to bind to another atom
What are the key rules for oxidation numbers
An element in its natural state has an oxidation number of 0
Oxygen has -2 UNLESS IN PEROXIDES
Hydrogen is +1 except in metal hydrides where it becomes -1
In simple ions the oxidation number is the charge of an ion
Combined fluorine is always -1 (e.g CaF2 Ca = 2+ F2 = -2 so F= -1 (except in its natural state)
What is a redox reaction
A reaction with both oxidation and reduction
What is oxidation
The loss of electrons
What is reduction
The gain of electrons
What is an oxidising agent
The species which gains (accepts) electrons
What is a reducing agent
The species which loses (donates) electrons
What is a half equation
An equation showing how the oxidation state of an atom / molecular ion changes
How do you write a half equation
Balance the equation on each side
Then write the electron difference on the side that is more positive
E.g O2 +4e- → 2O 2-
E.g Li →Li+ + e-
How do you balance half equations when oxygen is involved (acidic conditions)
Balance the equation
Add H+ to the more negative side and H2O to the positive
Balance the equation
Put the electron difference on the more positive side
How do you combine two half equation reactions to create 1 equation
Multiply the half equations to have the same number of electrons
The electrons then cancel out
How do you create a half equation in cold alkaline conditions
First (if oxygen is present)
Balance the equation
Add H+ to the negative side and H2O to the positive
Add the electron difference
Then add OH- to both sides
Balance this with the H+ ions
Add the OH- and H+ on one side to make water
Simplify the water on both sides (one should cancel out)
What is Disproportionation
When an atom is both oxidised and reduced in a reaction
What are the stages to follow in complex mole calculations (calculations where a solution is dissolved, made into a standard solution and titred)
Draw out the equations transfers
1) Find the moles in the beaker that has been titred against
2) Divide the moles by the volume in the burette, and multiply by the volume in the volumetric beaker (typically 250)
3) apply the molar ratio
This gives you the moles of the dissolved solution
How do you calculate percentage uncertainty
(uncertainty × trials) ÷ (absolute value) × 100
The absolute value for titrations is (final - initial)
The uncertainty is the error on the readings
