Moles, mass and gas calculations

Moles, mass and gas calculations


In this lesson, we will learn:
  • To recognize the format of stoichiometry test questions and calculations.
  • To recall the molar volume of gas at standard temperature and pressure and its meaning.
  • Methods to calculate number of moles of chemicals in reactions using mass, moles and volume of gas.


  • The units of mass is g, the units of amount of atoms or molecules is mol, and the units of atomic or molecular mass (MRM_R) is g/mol (pronounced "grams per mole" sometimes written gmol1^{-1}).

  • The formula: n(mol)=mass(g)MR(gmol)n(mol) = \frac{mass(g)}{M_R (\frac{g}{mol})} can be used to calculate the number of moles of a substance when you know the mass, and you can use the periodic table to find atomic or molecular mass MR of that substance.

  • With the moles formula above, you can remember it using the units in brackets too, like you would in general algebra: ggmol\large\frac{g}{\frac{g}{mol}} = mol mol and the g will cancel out. You are then left with 11mol\large\frac{1}{\frac{1}{mol}} =mol mol which cancels for mol

  • The molar volume of gas at standard temperature (0°C, 273K) and pressure is 22.4 litres per mole (22.4 L/mol).
    The molar volume of gas at room temperature (25°C, 298K) and pressure is 24 litres per mole (24 L/mol).
    • Whenever answering test questions involving gas volume, check the conditions of the reaction. DO NOT CONFUSE THE TWO!

  • Stoichiometry calculations involve unit conversions from one quantity given in the question to an unknown quantity:
    • To get to moles, use the equation and the molar ratios shown.
    • To get to volume, use the molar volume of gas constants.
    • To get to mass, use the atomic/molecular masses shown in the periodic table.
  • Introduction
    Recap of stoichiometry: Introduction
    Recap stoichiometry basics; what a chemical equation tells us.

    Using number of moles in stoichiometry calculations.

    Molar volume: its meaning and use.

    A molar volume tip to remember.

  • 1.
    Calculate the masses and volumes of reactants and products used in chemical reactions.
    2C6_6H14(l)+_{14\;(l)} +19O2(g)_{2\;(g)} →12CO2(g)+_{2\;(g)} + 14H2_2O(g)_{\;(g)}
    If 75g of C6_6H14_{14} is burned, what mass of CO2_2 would get produced from this reaction?

    If 240g of H2_2O is produced from this reaction, how many moles of C6_6H14_{14} would be required?

    At STP, what volume of O2_2 would be required to produce 85 L of CO2_2 in this process?

    What mass of C6_6H14_{14} would be required to produce 15 moles of CO2_2?

  • 2.
    Calculate the masses and volumes of reactants and products used in chemical reactions.
    P4(s)+_{4\;(s)} + 5O2(g)_{2\;(g)} →P4_4O10(s)_{10\;(s)}
    At STP, what volume of O2_2 gas is needed to completely combust 2kg of P4_4?

    If 25 L of O2_2 were available, how much mass of P4_4 could be reacted with?

    What mass of P4_4O10_{10} would be produced by this?

  • 3.
    Calculate the volume and number of moles of reactants involved in chemical reactions.
    2NH3(aq)+_{3\;(aq)} + NaOCl(aq)_{\;(aq)} →N2_2H4(aq)+_{4\;(aq)} + NaCl(aq)+_{\;(aq)} + H2_2O(l)_{\;(l)}
    If 5000 kg of hydrazine (N2_2H4_{4}) is required from this industrial process, how much ammonia gas (in L completely dissolved in solution at STP) is required as starting material?

    How many moles of hydrazine could be produced if only 5 kg of NaOCl was available?

  • 4.
    Calculate the volume of reactants required in a chemical process.
    SiCl4(g)+_{4\;(g)} + 2H2(g)_{2\;(g)} →Si(s)+_{\;(s)} + 4HCl(g)_{\;(g)}

    500 mg of Si is required from this process. What is the total volume, in L, of H2_2 and SiCl4_4 required to produce this?