C3-Quantitive Chemistry

Compounds have a Relative Formula Mass

Relative Formula Mass = Mr

Relative Atomic Mass = Ar

To find the Mr = add up all the Relative Atomic Masses of the atoms in the compound.

To find the Ar = The top number (mass number on a element in the periodic table.

If there is more than one atom you must multiply the Ar by how many atom there is.

Find the Relative Formula Mass of MgCl2

Ar of Mg = 24 , Ar of Cl2 = (35.5 X 2 ) = 71

Mg(24) + Cl2 (71) = 24 + 71 = 95

So Mr of MgCl2 is 95

  Pecentage Mass of an element in a compound = 

( Ar X number of atoms in the element ) divide by ( Mr of the compound) X 100

Find the Percentage Mass of sodium in sodium carbonate = Na2CO3

Ar of sodium = 23  Ar of carbon = 12   Ar of oxygen = 16

Mr of Na2CO3 = (2 X 23)  + 12 + (3 X 16) = 106

Percentage Mass of Sodium ( Ar X number of atoms of that element )divide( Mr of the compound ) X100 = (23x2) / 106 X 100 = 43%

NUMBER OF MOLES = MASS divide Mr

Mg + 2HCL = MgCl2 + H2

In the above reaction 1 mole of Mg and 2 moles of HCL reacts to form 2 moles of MgCl and 2 moles of H

Balance Equations using Reacting Masses

• Divide the mass of each substance by its Relative Formula Mass to find number of moles.
• Divide the number of moles of each substance by the smallest number of moles in the reaction.
• If any of the numbers aren't whole multiply all numbers by the same amount to make whole numbers
• Write balanced symbol equation by putting these numbers in front of the chemical formulas.

EXAMPLE ON NEXT CARD

8.1g of Zinc Oxide (ZnO) reacts completely with 0.60g of carbon to form 2.2g of carbon dioxide and 6.5g of Zinc. Write a balanced symbol equation.

1. Work out the Mr of substances in the reaction.

ZnO = 65+16=81      C=12     CO2= 12+(2x16=32)=44       Zn=65

2. Divide the mass of each substance by its Mr to calculate how many moles of each substance reacted or were produced.

ZnO = 8.1 / 81 = 0.10 mol                    C= 0.60 / 12 = 0.050 mol

CO2 = 2.2 / 44 = 0.050 mol                   Zn = 6.5 / 65 = 0.10 mol

3. Divide by smallest number of moles which is 0.050

ZnO=0.10/0.050=2.0 ,  C=0.050/0.050=1.0 ,  CO2=0.050/0.050=1.0 , Zn=0.10/0.050=2.0

4. Symbol Equation = 2ZnO + C = CO2 + 2Zn

Mass (g) = No.of moles X Mr

No.of Moles = Mass / Mr

Mr = Mass / No.of moles

Moles = Volume (dm3) X Conc (M)

Volume (dm3) = Moles / Conc

Conc = Moles / Volume

To convert cm3 into dm3 divide by a 1000 ( 500 cm3 /1000 = 0.5 dm3)