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Avogadro & the Mole (Edexcel International AS Chemistry): Revision Note

Richard Boole

Last updated

The Mole & Avogadro Calculations

  • The Avogadro constant (NA or L) is the number of particles equivalent to the relative atomic mass or molecular mass of a substance

    • The Avogadro constant applies to atoms, molecules, ions and electrons

  • The value of NA is 6.02 x 1023 g mol-1

  • The mass of a substance with this number of particles is called a mole (mol)

    • This can be called the molar mass 

    • This is the mass of substance that contains the same number of fundamental units as exactly 12.00g of carbon-12

  • The amount / number of moles of a substance, n, the mass of the substance, m, and the molar mass, M, are linked by the equation:

n = fraction numerator m a s s comma space m over denominator M o l a r space m a s s comma space M end fraction

  • One mole of any element is equal to the relative atomic mass of that element in grams

    • If you had one mole of carbon in your hand, that is 6.02 x 1023 atoms of carbon, you would have a mass of 12.00 g

    • One mole of water would have a mass of (2 x 1 + 16) = 18 g

Worked Example

  1. What is the molar mass of water?

  2. How many moles are there in 100 g of water?

  3. How many water molecules are there in 100g of water?

Answers

  1. Molar mass of water, H2O = (2 x 1.0) + 16.0 = 18.0 g mol-1 

  2. Moles equals fraction numerator m a s s over denominator m o l a r space m a s s end fraction space equals space fraction numerator 100 over denominator 18.0 end fraction space equals5.56 moles (to 3 s.f.)

  3. Number of molecules = number of moles x Avogadro's constant = 5.56 x (6.02 x 1023) = 3.35 x 1024 molecules

Worked Example

What is the mass of the following:

  1. Five hundred million atoms of platinum

  2. (1.31 x 1022) molecules of ethanol

Answer 1:

Number of moles equals fraction numerator number space of space particles over denominator Avogadro apostrophe straight s space constant comma space straight N subscript straight A end fraction equals fraction numerator 500 space cross times 10 to the power of 6 over denominator 6.02 space cross times 10 to the power of 23 end fraction equals8.31 x 10-16 moles

Mass = moles x molar mass = (8.31 x 10-16) x 195.1 = 1.62 x 10-13 g (lots of atoms, a tiny mass)

Answer 2:

Molar mass of ethanol, C2H5OH = (2 x 12.0) + (5 x 1.0) + 16.0 + 1.0 = 46.0 g mol-1 

Number of moles equals space fraction numerator number space of space particles over denominator Avogadro apostrophe straight s space constant comma space straight N subscript straight A end fraction space equals space fraction numerator 1.31 space cross times space 10 to the power of 22 over denominator 6.02 space cross times 10 to the power of 23 end fraction space equals0.0218 moles

Mass = moles x molar mass = 0.0218 x 46.0 = 1.00 g (an exceptionally large number of molecules in just 1 g)

Examiner Tips and Tricks

  • When you are completing calculations using Avogadro's constant, you may end up with answers that seem very large or very small - don't automatically assume that they must be wrong

  • Remember, Avogadro's constant is a VERY large number:

    • 6.02 x 1023 or 602 000 000 000 000 000 000 000

  • So when you multiply or divide by Avogadro's constant, your answers will, naturally, become very large or very small

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    Richard Boole

    Author: Richard Boole

    Expertise: Chemistry

    Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.