The Mole

It'd be handy to be able to count out atoms- but they're way too tiny. You can't even see them, never mind get hold of them with tweesers. But not to worry - using the idea of relative mass, you can figure out how many atoms you've got.

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  • Created by: chunks-42
  • Created on: 05-05-15 16:55

A mole is just a very large number

1) Amount of substance is measured using a unit called the mole (mol for short) and given the symbol n.

2) One mole is roughly 6x10 23 particles (Avagadro's constant).

3) It doesn't matter what the particles are. They can be atoms, molecules, electrons, ions, penguins- anything.

In the reaction C + O2 --> CO2

One mole of carbon reacts with 1 molecule of carbon dioxide, so 1 mole of carbon reacts with 1 mole of oxygen to make 1 mole of carbon dioxide.

In the reaction 2Mg + O2 --> 2MgO

2 moles of magnesium react with 1 mole of oxygen molecule to make 2 moles of magnesium oxide.

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Calculating Molar Mass

Molar mass, M, is the mass of one mole of something. But the main thing to remember is: Molar mass is just the same as the relative molecular mass, Mr

The only difference is you stick a 'g mol-1' for grams per mole on the end...

E.g. Find the molar mass of CaCO3

Relative formula mass of CaCOS3 = 40 + 12 + (3X16) = 100

So the molar mass is 100g mol-1

Here's another formula. This one's really important - you need it all the time: Moles = mass/Mr

E.g. How many moles of aluminium oxide are present in 5.1g of Al2O3?

Molar Mass of Al2O3 = (2x27) + (3 x 16) = 102 g mol-1

Number of moles of Al2O3 = 5.1/102 = 0.05 moles

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Calculating Concentration

1) The concentration of a solution is how many moles are dissolved per 1 dm3 of solution. The units are mol dm-3 (or M).

2) Here's the formula to find the number of moles:

Number of moles = concentration x volume (in cm3) / 1000

Number of moles = concentration x volume (in dm3)

E.g. What mass of sodium hydroxide needs to be dissolved in 50 cm3 of water to make 2M solution?

Number of moles = 2 x 50 / 1000= 0.1 moles of NaOH

Molar Mass of NaOH = 23 + 16 + 1 = 40g mol-1

Mass= number of moles x M = 0.1 x 40 = 4g

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Calculating Volume under the same conditions

If temperature and pressue stay the same, one mole of any gas always has the same volume.

At room temperature and pressure (r.t.p.), this happens tobe 24cm3, (r.t.p. is 298 K (25 degrees celsius) and 100kPa).

Here are two formulas for working out the number of moles in a volume of gas. Don't forget- only use them for r.t.p.

Number of moles = volume in dm3/ 24

Number of moles = volume in cm3 / 24,000

E.g. How many moles are there in 6 dm3 of oxygen gas at r.t.p.?

Number of moles = 6/24 = 0.25 moles of oxygen molecules 

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The Ideal Gas Equation - PV=nRT

In the ideal world (and AQA exam questions), it's not always room temperature and pressure. The ideal gas equation lets you find out the number of moles in a caertian volume at any temperature and pressure.

PV=nRT where P = pressure (Pa), V= volume (m3), n= number of moles, R= 8.31 J K-1mol-1, and   T = temperature (K)

E.g. At a temperature of 60 degrees Celsius and a pressure of 25 kPa, a gas occupied a volume of 1100cm3 and had a mass of 1.6g. Find its relative molecular mass.

n=PV/RT = (250x10 3) x (1.1 x 10-3) / 8.31 x 333 = 0.1 moles

If 0.1 moles is 16g, then 1 moles = 1.16/0.1 = 16g. So the relative molecular mass (Mr) is 16.

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