# OCR AS Chemistry (F321.1) - Atoms and Reactions Revision Notes

These revision notes are for Atoms and Reactions - the first module of F321 (Atoms, bonds and groups) from OCR AS Chemistry A.

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OCR AS Chemistry Notes
ATOMIC MASSES
RELATIVE ATOMIC MASS
Weighted average mass of an atom of an element (compared to 1/12 of the mass of an atom of carbon-12, which
is given the mass of 12)
o (This takes into account the presence of isotopes and their amounts)
n= relative atomic mass (mass number) = protons + neutrons
m= atomic number = protons
Calculating
Use mass spectrometry:
1. Using the ratio from the graph heights, work out average sample contents as percentage
2. 1st relative isotopic 100
mass × % contents 2nd relative isotopic mass × % contents
× 100
ISOTOPES
Atoms of the same element (same protons) but with a different number of neutrons
o Isotopes have the same chemical properties (same number of electrons) but different physical properties
o E.g.
RELATIVE ISOTOPIC MASS
Mass of an atom of an isotope of an element (compared to 1/12 of the mass of an atom of carbon-12, which is
given the mass of 12)
RELATIVE MOLECULAR MASS
Weighted average mass of a molecule (compared to 1/12 of the mass of an atom of carbon-12, which is given the
mass of 12)
THE MOLE
THE AVAGADRO CONSTANT (NA)
The number of atoms of carbon-12 in exactly 12g of it
6.02 x1023
MOLE (mol)
The amount of a substance containing as many particles as there are atoms in exactly 12g of carbon-12
Mass of 1 mole of any substance is equal to its relative molecular mass in grams
Mass of one mole of an element = molar mass (A) [g/mol OR gmol-1]
We can see moles in equations:
o 2H 2 + O2 2H 2O
o 2 moles + 1 mole 2 moles

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N = Mass
Mr OR N = Mass
Ar
EMPIRICAL + MOLECULAR FORMULA
MOLECULAR FORMULA = shows the types of atoms present and their number
o C 2H 6
EMPIRICAL FORMULA = the simplest whole number ratio of atoms
o C 2H 6 CH 2
To find empirical formula:
1. Work out the moles of each element
2. Make a whole number molar ratio (divide all by smallest value)

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BaCl22H 2O
REACTING VOLUMES OF GASES
Equal volumes of gases measured at same temperature and pressure contain the same number of molecules
50cm3 of O2 All have the same number of molecules
50cm3 of CO2
50cm3 of C3H8
MOLAR VOLUME
= Volume taken by 1 mole of any gas (it is at a constant temperature and pressure)
@ Standard temperature + pressure (STP)
0oC / 273K 1 atm / 101 kPa
1 mole of any gas takes volume of 22.…read more

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Add acid to water
ACIDS, BASES AND SALTS
ACID: a proton donor:
o H +(aq) + Cl-(aq)
o Strong acid: fully ionised / dissociated
H Cl H + + Cl-
o Weak acid: partially ionised / dissociated
C H 3COOH CH 3OO- + H +
BASE: substance which is able to react with H+ ions / proton donor:
o Typically metal oxide / metal hydroxide
o If it dissolves in water = alkali
N H 3(g) + H 2O (l)NH 4OH (aq)
Strong alkali: fully…read more

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STANDARD SOLUTION: solution of known concentration
EQUIVALENCE POINT: reached when both of them react completely and neither is in excess
STOICHIOMETRIC AMOUNT (titre): volume of one solution required to completely react with another solution
(measured using a burette)
Recording results:
o Rough titration is not used in calculation
o Must obtain 3 consistent results which agree to 0.1 cm3
o All results to 2 d.p
Calculation:
1. Balanced equation
2. Moles of standard solution
3. Molar ratio moles of unknown solution