Atoms, bonds and groups

Definitions from unit 1 of AS chemsitry OCR textbook: Atoms, bonds and groups

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Isotopes- are atoms of the same element with different numbers of neutrons

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Relative Isotopic mass


is the mass of an atom of an isotope compared with 1/12 the mass of an atom of carbon-12

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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

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Is the amount of any substance containing as many particles as there are carbon atoms in exactly 12g of carbon-12 isotope.

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Avogadro's constant

Avogadro's constant:

Is the number of atoms per mole of the CARBON-12 isotope

(6.02 x 10^23)

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Moles equations

The 3 moles equations!

n:moles c:concentration 

m: mass v:volume

M: molar mass 24 dm^3- at RTP

  • n=m/M
  • n=v/24(dm^3) -used in gas volumes
  • n=vxc - used in solutions

(1000cm^3 = 1dm^3)

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Empirical formula

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

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Molecular formula

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
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Moles and Gas Volumes

n= v/24

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

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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

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Balancing equations

Balancing Equations

  • useful in moles equations and stoichiometry
  • Get same amount of element on both sides

These video tutorials may help:


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Stoichiometry- is themolar relationship between the relative quantities of substances taking part in a reaction

check out this helpful video on problems involving stoichiometry:

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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  
  • alkali 
  • 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
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SALT formation

  • Acid + carbonate ---> salt + water + CO2

  • Acid + base ---> salt + water

  • Acid + alkali ---> salt + water
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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)

= 80g/mol

mass of N in NH4NO3: (There are two N so:) 14 + 14=


% mass= (28/80) x 100%

=35.0% N in NH4NO3

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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

=0.0175 mol

  • Calculate moles of water, molar mass= 18g/mol              n=m/M 2.205/18

=0.1225 mol

ratio 0.0175: 0.1225 divide both by smallest which is 0.0175

      1: 7        (number of water molecules is 7, X=7)

MgSO4: 7H2O

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