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 ($M_R$) is g/mol__ (pronounced "grams per mole" sometimes written gmol$^{-1}$).

- The
__formula__: $n(mol) = \frac{mass(g)}{M_R (\frac{g}{mol})}$ 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 M_{R} of that substance.

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