0.9 Quantitative aspects of chemical change  (Page 4/10)

Some simple calculations

1. Calculate the number of moles in each of the following samples:
   1. $5,6\mathrm{g}$ of calcium
   2. $0,02\mathrm{g}$ of manganese
   3. $40\mathrm{g}$ of aluminium
2. A lead sinker has a mass of $5\mathrm{g}$ .
   1. Calculate the number of moles of lead the sinker contains.
   2. How many lead atoms are in the sinker?
3. Calculate the mass of each of the following samples:
   1. $\mathrm{2,5}\mathrm{mol}$ magnesium
   2. $12\mathrm{mol}$ lithium
   3. $4,5\times 10{}^{25}$ atoms of silicon

Molecules and compounds

So far, we have only discussed moles, mass and molar mass in relation to elements . But what happens if we are dealing with a molecule or some other chemical compound? Do the same concepts and rules apply? The answer is 'yes'. However, you need to remember that all your calculations will apply to the whole molecule . So, when you calculate the molar mass of a molecule, you will need to add the molar mass of each atom in that compound. Also, the number of moles will also apply to the whole molecule. For example, if you have one mole of nitric acid ( $\mathrm{HNO}{}_3$ ), it means you have $6,022\times {10}^{23}$ molecules of nitric acid in the sample. This also means that there are $6,022\times {10}^{23}$ atoms of hydrogen, $6,022\times {10}^{23}$ atoms of nitrogen and ( $3\times 6,022\times {10}^{23}$ ) atoms of oxygen in the sample.

In a balanced chemical equation, the number that is written in front of the element or compound, shows the mole ratio in which the reactants combine to form a product. If there are no numbers in front of the element symbol, this means the number is '1'.

e.g. ${\mathrm{N}}_2+3{\mathrm{H}}_2\to 2\mathrm{N}{\mathrm{H}}_3$

In this reaction, 1 mole of nitrogen reacts with 3 moles of hydrogen to produce 2 moles of ammonia.