Moles and molar mass  (Page 2/3)

Moles and mass

1. Complete the following table:

   Element	Relative atomic mass (u)	Sample mass (g)	Number of moles in the sample
   Hydrogen	1.01	1.01
   Magnesium	24.31	24.31
   Carbon	12.01	24.02
   Chlorine	35.45	70.9
   Nitrogen		42.08

2. How many atoms are there in...
   1. 1 mole of a substance
   2. 2 moles of calcium
   3. 5 moles of phosphorus
   4. $24,31\mathrm{g}$ of magnesium
   5. $24,02\mathrm{g}$ of carbon

Molar mass

Molar mass

Molar mass (M) is the mass of 1 mole of a chemical substance. The unit for molar mass is grams per mole or $\mathrm{g}·\mathrm{mol}{}^{-1}$ .

You will remember that when the mass, in grams, of an element is equal to its relative atomic mass, the sample contains one mole of that element. This mass is called the molar mass of that element.

You may sometimes see the molar mass written as $M_m$ . We will use $M$ in this book, but you should be aware of the alternate notation.

It is worth remembering the following: On the periodic table, the relative atomic mass that is shown can be interpreted in two ways.

1. The mass of a single, average atom of that element relative to the mass of an atom of carbon.
2. The mass of one mole of the element . This second use is the molar mass of the element.

Moles and molar mass

1. Give the molar mass of each of the following elements:
   1. hydrogen
   2. nitrogen
   3. bromine
2. Calculate the number of moles in each of the following samples:
   1. $21,62\mathrm{g}$ of boron ( $\mathrm{B}$ )
   2. $54,94\mathrm{g}$ of manganese ( $\mathrm{Mn}$ )
   3. $100,3\mathrm{g}$ of mercury ( $\mathrm{Hg}$ )
   4. $50\mathrm{g}$ of barium ( $\mathrm{Ba}$ )
   5. $40\mathrm{g}$ of lead ( $\mathrm{Pb}$ )

An equation to calculate moles and mass in chemical reactions

The calculations that have been used so far, can be made much simpler by using the following equation: