2. Amount Of Substance Flashcards
Why are all mass relative to carbon-12?
1/12 of its relative atomic mass is 1
How can relative atomic mass (Ar) be defined?
The weighted average mass of an atom of an element, taking into account its isotopes, relative to 1/12 of the relative atomic mass of an atom of carbon-12
How can relative atomic mass be calculated?
average mass of one atom of an element / 1/12 mass of one atom of carbon-12
How can relative molecular mass (Mr) be defined?
The mass of that molecule compared to 1/12 of the relative atomic mass of an atom of carbon-12
How can relative molecular mass be calculated?
average mass of one molecule / 1/12 mass of one atom of carbon-12
What do ionic compounds exist as instead of molecules?
Ionic lattices
What is used for ionic compounds instead of relative molecular mass?
Relative formula mass
Why is relative formula mass used for ionic compounds?
They form ionic lattices instead of molecules
What does the formula of an ionic compound show?
The simplest ratio of ions in the compound
instead of the total number of atoms in the compound
What is the mole defined as?
The amount of substance that contains 6.022e23 particles
How many particles are in one mole of a subtance?
6.022e23 (electrons, atoms, molecules, ions or compounds)
What equation is used to work out number of moles (using mass and mr)?
n = mass / mr
What is the final answer usually given to?
3 significant figures
How many decimal places should Mr be calculated to?
1
What is the concentration of a solution?
The amount of solute present in a fixed quantity of solvent
What is concentration measured in?
g/dm³ or mol/dm³
What is the molarity?
The concentration (in mol/dm³)
How can number of moles be calculated (when given conc. and volume)?
n=cv
g/dm³ to mol/dm³?
÷ Mr
mol/dm³ to g/dm³?
x Mr
What is a titration?
The procedure of reacting volumes
During a titration, what is placed in the burette?
The solution of known concentration
During a titration, what is placed in the conical flask?
The solution of unknown concentration
What is the law of conservation of mass?
Mass of reactants = Mass of products
What does the volume a gas occupies depend on?
- temperature
- pressure
- amount of gas
How does the temperature of a gas influence the volume it occupies?
The hotter the gas, the faster the particles move and the more space they occupy
How does the pressure of a gas influence the volume it occupies?
The higher the pressure, the more compressed the gas will be and the less space it will occupy
How does the amount of a gas influence the volume it occupies?
The more gas particles there are, the more space they will occupy
What is the ideal gas law?
One mole of any gas, at the same temperature and pressure, will have the same volume as one mole of any other gas
What is the equation for the ideal gas law?
PV = nRT
What is pressure measured in for the ideal gas equation?
Pa
What is volume measured in for the ideal gas equation?
m³
What is the gas constant measured in for the ideal gas equation?
J/K mol
What is temperature measured in for the ideal gas equation?
kelvin, K
How to convert from degrees to kelvin?
add 273
What is the value of the gas constant, R, in the ideal gas equation?
8.31 J/K mol
What is 1 atm equal to?
100,000 Pa
What is the empirical formula of a compound?
The formula which shows the simplest, whole-number ratio of the atoms in the compound
What is the molecular formula of a substance?
The formula which shows the number of each type of atom in the one molecule of that substance
What is the molecular formula, relative to the empirical formula?
The molecular formula is always a simple, whole-number multiple of the empirical formula (and therefore can be the same as the empirical formula)
Why can the percentage of each element in a compound be used as the mass ratio?
As the empirical formula is a ratio
What can you use if you are not given mass ratios when working out empirical formula?
The percentage of each element in a compound
How to work out empirical formula?
- Find mole ratio by doing mass/mr for each substance in compound
- divide by smallest value
- round if near number, if not multiply all to get a whole number ratio
How can molecular formula be found from empirical formula?
Divide Mr of the compound by Mr of the empirical formula`
What does a chemical reaction equation show?
- the identities of all the reactants and products
- the reaction coefficients
- state symbols
When balancing equations, what should be done with unchanged species?
They should be left out e.g. catalysts
What should be balanced first in equations?
Compounds first; elements second
What does a state symbol show?
The physical state of each species in the reaction e.g. s, l, g, aq
What are compounds of ions known as?
Salts
Do salts form molecules?
No - they form giant ionic lattices
Why is the formula representing salts not a molecular formula?
They form giant ionic lattices instead of molecules, so a represented using a unit formulae
What is the unit formula of an ionic compound?
The formula that shows the simplest whole number ratio in which the ions in the compound exist
What are the ions called that remain in aqueous solution before and after the reaction?
Spectator ions
In ionic equations, if an insoluble ionic compound is produced, is the formula written as ions?
No - the ions are not free to move
What are ionic equations useful for?
- to simplify precipitation reactions
* to simplify acid-base reactions
Do all reactions between strong acids and alkalis have the same ionic equation?
Yes
Can covalent compounds be ‘split up’ in ionic equations?
No - they do not contain ions
What is percentage yield?
A comparison of the theoretical maximum yield and the actual yield
What is atom economy?
The percentage of the total mass of reactants that can be converted into the desired product
How is percentage yield calculated?
(Actual amount of product ÷ theoretical amount of product) x 100
What quantities are used when calculating percentage yield?
mols or grams
Why is percentage yield always less than theoretical maximum?
- some reactions are reversible - not all reactants end up as products
- in some reactions products must be isolated from other unwanted products (and some desired products may be lost in isolation methods)
- some reactions may produce products that go on to produce additional reactions
How is atom economy calculated?
(Mr of desired products ÷ sum of molecular mass of all reactants) x 100
In which of atom economy and percentage yield do you have to pay attention to the coefficient before formulae?
Atom economy
Advantages of having a higher atom economy?
economic - more efficient use of raw materials, and less money wasted from producing unwanted by-products from reactants
environmental - produce less waste, and some products are harmful to the environment (high atom economy means less of these products are produced)
