AQA GCSE FOUNDATION Chemistry: Moles and Avogadro’s Number

Moles and Avogadro's Number

What is a Mole?

A mole is a unit of measurement used in chemistry to represent a specific number of particles (atoms, molecules, ions, etc.). This number is Avogadro's Constant, which is approximately 6.022 x 1023 particles per mole.

Think of it like a "chemist's dozen." Just like a dozen always means 12, a mole always means 6.022 x 1023.

Why Use Moles?

• Convenient for dealing with large numbers: Atoms and molecules are incredibly tiny. Using moles allows us to work with more manageable numbers in chemical reactions.
• Directly relates mass and number of particles: The molar mass of a substance tells us the mass of one mole of that substance. This allows us to convert between mass and the number of particles.

Calculating Moles

1. From Mass:

• Formula: moles = mass (g) / molar mass (g/mol)
• Example: What is the number of moles in 10 grams of sodium chloride (NaCl)?
  • Molar mass of NaCl = 23 g/mol (Na) + 35.5 g/mol (Cl) = 58.5 g/mol
  • moles = 10 g / 58.5 g/mol = 0.17 moles

2. From Number of Particles:

• Formula: moles = number of particles / Avogadro's Constant
• Example: How many moles are in 3.011 x 1023 molecules of water (H2O)?
  • moles = 3.011 x 1023 / 6.022 x 1023 mol-1 = 0.5 moles

3. From Concentration:

• Formula: moles = concentration (mol/dm3) x volume (dm3)
• Example: How many moles of sugar are in 250 cm3 of a 0.2 mol/dm3 solution?
  • Volume in dm3 = 250 cm3 / 1000 cm3/dm3 = 0.25 dm3
  • moles = 0.2 mol/dm3 x 0.25 dm3 = 0.05 moles

Using Moles in Chemical Equations

Chemical equations represent the ratios of moles of reactants and products in a reaction.

• Example: The equation for the reaction of hydrogen gas with oxygen gas to form water is:
  • 2H2 (g) + O2 (g) ? 2H2O (l)
  • This tells us that 2 moles of hydrogen react with 1 mole of oxygen to produce 2 moles of water.

Using moles to calculate mass or volume of reactants/products:

1. Balance the chemical equation.
2. Convert given mass or volume to moles.
3. Use the mole ratio from the balanced equation to find the moles of the desired substance.
4. Convert the moles of the desired substance back to mass or volume.

Example: How many grams of water are produced when 4 grams of hydrogen gas reacts completely with oxygen?

1. Balanced Equation: 2H2 (g) + O2 (g) ? 2H2O (l)
2. Moles of H2: moles = mass / molar mass = 4 g / 2 g/mol = 2 moles
3. Mole Ratio: From the equation, 2 moles of H2 produce 2 moles of H2O. Therefore, 2 moles of H2O are produced.
4. Mass of H2O: mass = moles x molar mass = 2 moles x 18 g/mol = 36 grams

Therefore, 36 grams of water are produced.

Key Points to Remember

• Avogadro's Constant: 6.022 x 1023 particles per mole.
• Molar Mass: The mass of one mole of a substance.
• Moles are the "link" between mass and number of particles.
• Chemical equations show mole ratios for reactants and products.

By understanding the concept of moles and Avogadro's Number, you'll be able to solve a wide range of calculations in chemistry!