Moles, mass and gas calculations - Stoichiometry

Moles, mass and gas calculations

Lessons

Notes:
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.

Notes:
  • 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 given mass, and using the periodic table to find atomic or molecular mass MRM_R of that substance.
  • With the moles formula above, you can treat the unit terms like you would in general algebra: ggmol=mol\frac{g}{\frac{g}{mol}}=mol, where g cancels out.
  • 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 the reaction is occurring under. DO NOT CONFUSE THE TWO TERMS.
  • 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.
  • Intro Lesson
    Recap of stoichiometry: Introduction
  • 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)}
  • 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)}
  • 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)}
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Moles, mass and gas calculations

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