Isotopes- are atoms of the same element with different numbers of neutrons
Relative Isotopic mass
RELATIVE ISOTOPIC MASS:
is the mass of an atom of an isotope compared with 1/12 the mass of an atom of carbon-12
Relative atomic mass
Relative atomic mass-
The weighted mean mass of an atom of an element compared with 1/12 of the mass of an atom of carbon-12
Is the amount of any substance containing as many particles as there are carbon atoms in exactly 12g of carbon-12 isotope.
Is the number of atoms per mole of the CARBON-12 isotope
(6.02 x 10^23)
The 3 moles equations!
m: mass v:volume
M: molar mass 24 dm^3- at RTP
- n=v/24(dm^3) -used in gas volumes
- n=vxc - used in solutions
(1000cm^3 = 1dm^3)
Empirical formula- the simplest whole number ratio of atoms of each element in a compound.
E.g: 0.6075g of Magnesium combines with 3.995g of Bromine to form a compound, what is it's empirical formula?
find molar ratios: Mg : Br
n=m/M 0.6075/24.3 3.995/79.9
moles: 0.025 : 0.050
Divide by smallest to get whole numbers in this case divide each by 0.025
1 : 2
The formula is MgBr2
Molecular formula- the actual number of atoms of each element in a compound
E.g: A compound has an empirical formula of CH2 and a relative molecular mass of 56, what is it's molecular formula?
- empirical formula's mass of CH2 : 12.0 + (1.0 x2) = 14
- Number of CH2 units in a molecule: 56/14
- everything in empirical formula x 4, CH2 x4 = C4H8
Moles and Gas Volumes
E.g: calculate the amount in moles of gas molecules in 72cm^3 of any gas at rtp?
- n=(72/1000)/24 (get it in dm^3)
n= 0.003 moles
Moles and solutions
n= v x c
E.g : What is the amount in moles of NaOH dissolved in 25.0cm^3 of an aqueous solution of concentration 0.0125 mol/dm^3?
25/1000 (to get dm^3) x0.0125
= 3.125 x 10^-4 moles concentration- amount of solute in mol, dissolved per 1dm^3 of solution.
A standard solution is a solution of a known concentration
- useful in moles equations and stoichiometry
- Get same amount of element on both sides
These video tutorials may help:
Stoichiometry- is themolar relationship between the relative quantities of substances taking part in a reaction
check out this helpful video on problems involving stoichiometry:
Acids, bases and alkalis
Acids: are protons donors
Bases: are proton acceptors
Alkali: a type of base that dissolves in water forming hydroxide ions OH-
Salts can be produced by reacting an ACID with:
- a base
- carbonate SALT- a chemcial compound formed when the H+ from an acid is replaced by a metal ion or a positive ion such as NH4
- Acid + carbonate ---> salt + water + CO2
- Acid + base ---> salt + water
- Acid + alkali ---> salt + water
Ammonium salts and fertilisers
- Ammonium salts are formed when acids are neutralised by aqueous ammonia (base)
- Ammonium salts contain the NH4 + ion
E.g : Calculating % mass of nitrogen in ammonium nitrate NH4NO3
Molar mass of NH4NO3: 14 + (1x4) + 14 + (16 x 4)
mass of N in NH4NO3: (There are two N so:) 14 + 14=
% mass= (28/80) x 100%
=35.0% N in NH4NO3
Water of Crystallisation
Hydrated- contains water molecules Anhydrous- without water molecules.
E.g: experiment to determine the formula of hydrated magnesium sulfate:
mass of hydrated sale MgSO4: XH20 = 4.312g
mass of Anhydrous salt MGSO4 =2.107g
mass of XH2O is (4.312- 2.107) = 2.205g
- Calculate moles of Anhyrous salt: molar mass of salt= 120.4g/mol , n= m/M 2.107/120.4
- Calculate moles of water, molar mass= 18g/mol n=m/M 2.205/18
ratio 0.0175: 0.1225 divide both by smallest which is 0.0175
1: 7 (number of water molecules is 7, X=7)