Moles, mass and gas calculations - Stoichiometry

Moles, mass and gas calculations


• 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”).

• 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\degreeC, 273K) and pressure is 22.4 litres per mole (22.4 L/mol).

• The molar volume of gas at room temperature (25\degreeC, 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.

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.
  • 1.
    Recap of stoichiometry: Introduction
  • 2.
    2C6_6H14(l)+_{14\;(l)} +19O2(g)_{2\;(g)} →12CO2(g)+_{2\;(g)} + 14H2_2O(g)_{\;(g)}
  • 3.
    P4(s)+_{4\;(s)} + 5O2(g)_{2\;(g)} →P4_4O10(s)_{10\;(s)}
  • 4.
    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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