Chapter 3 Amount of substance Flashcards
amount of substance n
The quantity whose unit of the mole, used as a means of counting any species such as atoms, ions and molecules.
n=m/M amount=mass/molar mass
Mole
The amount of any substance containing as many elementary particles as there are carbon
atoms in exactly 12 g of the carbon-12 isotope, that is, 6.02 × 10^23 particles.
Avogadro constant NA
The number of atoms per mole of the carbon-12 isotope. (6.02 × 1023 mol–1)
Molar mass
M The mass per mole of a substance, in units of g mol–1.
Molecular formula
molecular formula A formula that shows the number and type of atoms of each element present in a molecule.
Empirical formula
The formula that shows the simplest whole-number ratio of atoms of each element present in a compound.
Relative molecular mass
Mr The weighted mean mass of a molecule of a compound compared with one-twelfth of the mass of an atom of carbon-12.
Relative formula mass
The weighted mean mass of the formula unit of a compound compared with one-twelfth of the mass of an atom of carbon-12.
Hydrated
A crystalline compound containing water molecules.
Water of crystallisation
Water molecules that are bonded into a crystalline structure of a compound.
Anhydrous
Containing no water molecules.
Concentration.
The amount of solute, in moles, dissolved in 1dm3 (1000 cm3) of solution.
Standard solution
n A solution of known concentration.
Molar gas volume
Vm The volume per mole of gas molecules at a stated temperature and pressure
At RTP molar gas volume is 24.0dm3 mol-1
n=volume/molar gas volume
RTP
Room temp 20 degrees Celsius
Pressure 101kPa
Ideal gas
Molecules have: Random motion Elastic collisions Negligible size No intermolecular forces
Equation
pV=nRT
R ideal gas constant = 8.314
V=m^3 p=Pa temp=K
Conversions Cm^3 to m^3 Dm3 to m3 C to K KPa to Pa
X10^-6
X10^-3
+273
X10^3
Stoichiometry
The ratio of the amount, in moles, of each substance in a chemical equation (essentially the ratio of the balancing numbers).
Chemists use balanced equations to find
- the quantities of reactants required to prepare a required quantity of a product
- the quantities of products that should be formed from certain quantities of reactants
Theoretical yield
The yield resulting from complete conversion of reactants into products.
Theoretical yield is difficult to achieve
- the reaction may not have gone to completion
- other reactions may have taken place with the main reactions
- purification of the product may result in loss of some product
Actual yield
The amount of product obtained from a reaction.
Percentage yield
% yield = actual amount,inmol,of product / theoretical amount,inmol,of product x100
Limiting reagent
The reactant that is not in excess, which will be used up first and stop the reaction.
Atom economy
(Sum of molar masses of desired products)/(sum of molar masses of all products) x100%.
Measure of how well atoms have been utilised
take account of balancing numbers
Reactions with high atom economies
- Produce a large proportion of desired products and few unwanted waste products
- are important for sustainability as they make the best use of natural resources
To work out mass concentration
e.g conc is 0.250 mol dm^-3 then do 0.250 x molar mass to get grams per dm^3
moles of a solution
n=cV
other way of showing concentration
e.g 0.250 mol dm-3 in gym-3
use m= n x M
