Stoichiometry

Last updated Jun 27, 2023 Edit Source

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• Relative atomic mass $(A_r)$ is a given unit calculated from the average atomic mass of all an element’s isotopes in the proportion they appear in nature
  • e.g. chlorine is made up of two isotopes
    • $75%$ chlorine-35 and $25%$ chlorine-37
    • The relative atomic mass of chlorine is 35.5
      • $A_r(Cl) = 0.75 \times 35 + 0.25 \times 37 = 35.5$
  • The atomic mass constant is equal to $\frac{1}{12}$ the mass of carbon-12
  • Molar mass is equal to an elements relative atomic mass
    • Thus molar mass is always given and can be used with another value to find a substances mass or moles
• Moles are used to represent a quantity of atoms
  • One mole of a substance is its molar mass in grams
    • e.g. 12.01 grams of carbon is equal to 1 mole of carbon
  • One mole is equal to $6.022 \times 10^{23}$ atoms
    • e.g. one mole of carbon is $6.022 \times 10^{23}$ atoms of carbon
    • $6.022 \times 10^{23}$ is Avogadro’s number, it is a special pre-defined quantity
  • Moles are used as a universal unit for quantifying atoms allowing us to find values, such as mass, volume and concentration, and convert between them
• Converting between moles, mass and molar mass

• Stoichiometry is the relationship between the relative quantities of substances taking part in a reaction or forming a compound
• In stoichiometry, we need to be able to convert between many different units
  • Lets say you want to find the mass of $NaCl$ in a solution, if you are given the concentration of $NaCl$ and the volume of solution, how do you find the mass of $NaCl$?
• There are three main groups of units used for different states of matter:
  • Solid → converting between moles, mass and molar mass
  • Solution → converting between volume, moles and concentration
  • Gas → converting between moles, volume and molar volume
• Moles can be calculated no matter what state a substance is in allowing us to calculate any unit for any substance
  • We can find the mass $NaCl$ in a solution using its concentration by converting concentration to moles and dividing moles by $NaCl$’s molar mass!

• STP → Standard Temperature and Pressure
  • STP is $0\degree C$ and $100\\ kPa$
• Number of moles of gas at STP:
  • $n = \frac{V}{22.71}$
  • 1 mole of any gas occupies $22.71$ L at standard temperature and pressure