# CGPwned: Molar Calculations Crash Course!

I don't think this includes electrolysis calculations but you can find that by downloading Chemistry CGPwned :)

Anyways hope this helps with those who have trouble doing molar calculations!

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• Created by: cgpwned
• Created on: 14-03-12 13:08

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Do it...the CGPwned Way!:)
MOLAR CALCULATIONS ­ THE CRASH COURSE
A mole is a measure of the amount of substance. One mole contains 6 x 1023 (also known as
the Avogadro Number) particles (atoms, molecules or formulae) of the substance. For example,
1 mol of sodium contains 6 x 1023 atoms of sodium.
CALCULATING RELATIVE ATOMIC MASS
Chlorine has two isotopes: chlorine-35 and chlorine-37. A typical sample will be 75%
chlorine-35 and 25% chlorine-37.
The RAM = (0.75 x 35) + (0.25 x 37) = 35.5g
CALCULATING RELATIVE FORMULA MASS
H2 Hx2 2x1=2
Ca(OH)2 (1 x Ca) + (2 x O) + (2 x 40 + (2 x 16) + (2 x 1) =
H) 74
CALCULATING THE NUMBER OF MOLES IN A GIVEN MASS OF AN ELEMENT/COMPOUND
Amount in Mass
moles =
RFM
Amount in moles = Mass of Substance (g) /RFM of Element or Compound (g)
a) Calculate the number of moles in 2.3g of sodium:
RFM of sodium = 23g per mole
Given mass = 2.3g
Amount in moles = 2.3g / 23g
= 0.01 mol
b) Calculate the mass of 2 moles of magnesium:
RFM of magnesium = 24g per mole
Given mass = x
Given moles = 2 mol
2 mol = x / 24g
x = 2mol x 24g
= 48g
SIMPLE MOLAR CALCULATIONS
Chemical equations are like, ratios. For example:

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### Page 2

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Mg + S MgS means 1 mole of magnesium will react with 1 mole of sulphur to form
one mole of magnesium sulphide.
What does this mean? It means that 2 moles of magnesium will react to 2 moles of sulphur to
form 2 moles of magnesium sulphide; 0.5 moles of magnesium will react to 0.5 moles of sulphur
to form 0.5 moles of magnesium sulphide and so on.…read more

### Page 4

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Work out the RFM of CaCO3 to convert the given mass into moles:
CaCO3 = 40 + 12 + 3(16) = 100g
Number of moles of 30g of CaCO3 = 30/100 = 0.3 mol
By now you should know how to interpret chemical equations in terms of moles ­ you should
know that using 0.3 moles of calcium carbonate will give you 0.3 moles of carbon dioxide. If

### Page 5

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The concentration of our hydrochloric acid solution is 2 mol dm-3. This means 1000cm3 of this
bottle of HCl solution contains 2 moles of HCl. We only need one mole so:
(1 mol/2mol) x 1000cm3 = 500cm3 (we can do it the easier way by simply dividing 1000cm3 by 2
because we know that 1 mol is half of 2 mol)
CALCULATING THE CONCENTRATION OF A SOLUTION
This is basically, putting given information into the form X mol dm-3.…read more

### Page 6

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The question says that 40 cm3 of HCl neutralized the sodium hydroxide. This means that 40 cm3
of HCl solution contains 0.005 moles of HCl. All we need to do is use this information and put it
in the form X mol dm-3 :
0.005/40 = 0.000125 moles of HCl in 1 cm3
0.000125 x 1000 = 0.125 moles of HCl in 1000 cm3
So the concentration of hydrochloric acid solution was 0.…read more