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Moles and Molarity

Jamie Z 2024-03-05

Learning Goals

  • Describe a mole in terms of Avagad’s number of particles and mass
  • Calculate molar mass, moles, mass and number of particles

Resources

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Moles

  • M = Moles per Litre
  • A chemical unit/amount of a substance
  • 1 mole contains \(6.022*10^{23}\) atoms
  • 1 mole has a mass equal to the molar mass of a substance

\[ Moles(n) = \frac{mass(m)}{molecular\ mass(mm)} \]

Particles

  • To calculate the number of particles in a substance, you multiply the number of moles by

\[ 6.022*10^{23} \]

Molarity

  • Molarity is the number of moles of solute per litre of solution
  • The concentration of moles in a liquid

\[ Concentration(C) = \frac{n(moles)}{volume(v)} \]

Example

Calculate the volume of 1.5M hydrochloric acid needed when 20g HCL is required

  • This question isn’t really phrased well so I will rewrite it
  • Calculate the volume of a solution that has 20g of 1.5M HCL

To calculate the volume of the solution, we can use the following steps:

  1. Calculate the number of moles of HCl:

    • Molarity (M) = moles of solute (mol) / litres of solution (L)

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    • Rearrange the equation to find moles of solute: moles of solute = M * L

    • We are given the molarity (M = 1.5 M) and need to find the litres of solution (L). However, we are given the mass of solute (20 g) instead of litres.

  2. Convert mass of HCl to moles of HCl:
    • moles of solute (mol) = mass of solute (g) / molar mass (g/mol)
    • Molar mass of HCl = 36.46 g/mol
    • moles of HCl = 20 g / 36.46 g/mol = 0.548 moles
  3. Now that you have the moles of HCl, use the original equation to find the litres of solution:
    • moles of solute (mol) = M * L
    • L = moles of solute (mol) / M
    • L = 0.548 moles / 1.5 M = 0.365 L

Therefore, the volume of the solution is 0.365 litres.