OCR- chemistry Formula's and calculations for EL&DF (F331)

Basic formulas and calculations needed in AS chemistry with some worked examples

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  • Created by: Chemzoik
  • Created on: 10-04-11 14:35

Avogadro constant

6.02*10^23 given the symbol Na is the number of particles in one mole of a substance.

Relative atomic mass (Ar)

tells you the number of times an atom of an element is heavier that one-twelfth of an atom of C-12. Ar values have no units.

Relative formula mass (Mr)

is the sum of the relative atomic masses for each of the atom in the formula. for a simple compound it is sometimes called relative molecular mass. Mr values have no units. Eg. CH4= 12+(1*4)=16.

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Empirical Formula

tells you the simplest ratio of atoms of each element in a compound.

Molecular Formula

tells you the actual number of atoms of each element in a molecule. This may be either the same as or a whole number multiple of the emprical formula.

To work out the amount of substance, use Amount= Mass(g) / Ar

molecule/compound : amount= Mass(g) / Mr

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Example 1(working out the formula from reacting masses)

2.43g of magnesium reacts with 0.20g of hydrogen.What is the formula of magnesium hydride?

Step 1: 2.43g of Mg= 2.43 / 24.3=0.100 moles.

Step 2: 0.20g of H= 0.20 / 1.0= 0.20 moles

Step 3: dividing through by the smallest number of moles (0.1 in this case) gives 1Mg :2H. the empirical formula is therefore MgH2.


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Example 2: (working out the percentage by mass of an element)

Calculate the percentage by mass of nitrogen in ammonium sulfate, (NH4)2 SO4.

Step 1: Mr (NH4)2 SO4= 2* (14+1+1+1+1)+32.1+(4*16)=132.1

Step 2: Mass of nitrogen in 1mole of (NH4)2 SO4= 14+14=28

Step 3: % by mass= (mass of an element in 1 mole/ Mr of compund) * 100

                             = 28/132.1*100

                             =  21.2%

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Danica Arthur-Asante


Thank you amigo! :)

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