3.1 Mole (continued)

Recall that the atomic mass, formula mass, and molecular mass of a substance expressed in grams is called Mole.

For Example,

One mole of O (atom) $=16\mathrm{g}$$= 16\mathrm{g}$ One mole of ${\mathrm{O}}_2$$\mathrm{O_2}$ (molecule) $=32\mathrm{g}$$= 32\mathrm{g}$ One mole of ${\mathrm{H}}_2\mathrm{O}$$\mathrm{H_2O}$ (molecule) $=18\mathrm{g}$$= 18\mathrm{g}$ One mole of $\mathrm{N}{\mathrm{a}}^{+}$$\mathrm{Na^+}$ (ion) $=23\mathrm{g}$$= 23\mathrm{g}$ One mole of NaCl (Formula unit) $=58.5\mathrm{g}$$= 58.5\mathrm{g}$

Mole ratios in Stoichiometric Calculations:

Let's look at the following chemical reaction.

What does this chemical equation tell us? It tells us that 2 moles of hydrogen + 1 moles of oxygen will form 2 moles of water. Any chemical reaction can be interpreted in terms of a mole. Moles can easily be converted into the mass of reactants/products. A balanced chemical equation will tell you the ratio of the reactants/products to the mole. The mole is the unit of measurement used to express the amount of a substance.

The mole is the SI unit to express the amount of a substance. It is symbolized as mol.

Example 3.1

Methanol burns according to the following equation.

If 3.50 moles of methanol are burnt in oxygen, calculate

(a) How many moles of oxygen are used? (b) How many moles of water are produced?

Solution

(a) Moles of methanol $= 3.50$ moles Moles of oxygen $= ?$ According to the balanced chemical equation 2 moles of ${\mathrm{C}\mathrm{H}}_3\mathrm{O}\mathrm{H}=3$$\mathrm{CH}_3\mathrm{OH} = 3$ moles of ${\mathrm{O}}_2$$\mathrm{O_2}$ 1 moles of ${\mathrm{C}\mathrm{H}}_3\mathrm{O}\mathrm{H}=3\mathrm{/}2$$\mathrm{CH}_3\mathrm{OH} = 3 / 2$ moles of ${\mathrm{O}}_2$$\mathrm{O_2}$

3.5 moles of ${\mathrm{C}\mathrm{H}}_3{\mathrm{O}\mathrm{H}}_2=\frac{3.50\times 3}{2}$$\mathrm{CH}_{3}\mathrm{OH}_{2} = \frac{3.50 \times 3}{2}$ moles of ${\mathrm{O}}_2$$\mathrm{O}_{2}$ $= 5.25$ moles of ${\mathrm{O}}_2$$\mathrm{O}_{2}$

So the number of moles of ${\mathrm{O}}_2$$\mathrm{O}_{2}$ consumed $= 5.25$ moles

(b) No. of Moles of ${\mathrm{C}\mathrm{H}}_3\mathrm{O}\mathrm{H}=3.5$$\mathrm{CH}_{3}\mathrm{OH} = 3.5$ moles

No. of Moles of ${\mathrm{H}}_2\mathrm{O}=?$$\mathrm{H}_{2}\mathrm{O} = ?$

According to the balanced chemical equation

2 moles of ${\mathrm{C}\mathrm{H}}_3\mathrm{O}\mathrm{H}=4$$\mathrm{CH}_{3}\mathrm{OH} = 4$ moles of ${\mathrm{H}}_2\mathrm{O}$$\mathrm{H}_{2}\mathrm{O}$

1 moles of ${\mathrm{C}\mathrm{H}}_3\mathrm{O}\mathrm{H}=4\mathrm{/}2$$\mathrm{CH}_{3}\mathrm{OH} = 4 / 2$ moles of ${\mathrm{H}}_2\mathrm{O}$$\mathrm{H}_{2}\mathrm{O}$

3.5 moles of ${\mathrm{C}\mathrm{H}}_3\mathrm{O}\mathrm{H}=\frac{3.50\times 4}{2}$$\mathrm{CH}_{3}\mathrm{OH} = \frac{3.50 \times 4}{2}$ moles of ${\mathrm{H}}_2\mathrm{O}$$\mathrm{H}_{2}\mathrm{O}$

$= 7.00$ moles of ${\mathrm{H}}_2\mathrm{O}$$\mathrm{H}_{2}\mathrm{O}$

So the number of moles of ${\mathrm{H}}_2\mathrm{O}$$\mathrm{H}_{2}\mathrm{O}$ produced $= 7.00$ moles

Concept Assessment Exercise 3.1

NH is an important raw material in the manufacture of fertilisers. It is obtained by the combination of ${\mathrm{N}}_2$$\mathrm{N}_{2}$ and ${\mathrm{H}}_2$$\mathrm{H}_{2}$ as shown by the following balanced equation.

How many moles of the following are required to manufacture 8.0 moles of ${\mathrm{N}\mathrm{H}}_3$$\mathrm{NH}_{3}$ ?

(a) Nitrogen (b) Hydrogen