0.9 Quantitative aspects of chemical change  (Page 6/10)

$\mathrm{Hydrogen}=\mathrm{1,008}\times 2=\mathrm{2,016}\mathrm{u}$

$\mathrm{Sulphur}=\mathrm{32,07}\mathrm{u}$

$\mathrm{Oxygen}=4\times 16=64\mathrm{u}$

Use the calculations in the previous step to calculate the molecular mass of sulphuric acid.

Use the equation:

$\mathrm{Percentage\; by\; mass}=\frac{\mathrm{atomic\; mass}}{\mathrm{molecular\; mass\; of\; H}{}_2\mathrm{SO}{}_4}\times 100\%$

Hydrogen

Sulphur

Oxygen

(You should check at the end that these percentages add up to 100%!)

In other words, in one molecule of sulphuric acid, hydrogen makes up 2,06% of the mass of the compound, sulphur makes up 32,69% and oxygen makes up 65,25%.

Carbon $=\mathrm{52,2}\mathrm{g}$ , hydrogen $=\mathrm{13,0}\mathrm{g}$ and oxygen $=\mathrm{34,8}\mathrm{g}$

Therefore,

In this case, the smallest number of moles is 2.18. Therefore...

Carbon

Hydrogen

Oxygen

Therefore the empirical formula of this substance is: ${\mathrm{C}}_2{\mathrm{H}}_6\mathrm{O}$ . Do you recognise this compound?

Lead

Oxygen

The mole ratio of $\mathrm{Pb}:\mathrm{O}$ in the product is 1:2, which means that for every atom of lead, there will be two atoms of oxygen. The formula of the compound is $\mathrm{PbO}{}_2$ .

In $100\mathrm{g}$ of acetic acid, there is $\mathrm{39,9}\mathrm{g}\mathrm{C}$ , $\mathrm{6,7}\mathrm{g}\mathrm{H}$ and $\mathrm{53,4}\mathrm{g}\mathrm{O}$

$n=\frac{m}{M}$

Empirical formula is $\mathrm{CH}{}_2\mathrm{O}$

The molar mass of acetic acid using the empirical formula is $30\mathrm{g}·\mathrm{mol}{}^{-1}$ . Therefore the actual number of moles of each element must be double what it is in the empirical formula.

The molecular formula is therefore $\mathrm{C}{}_2\mathrm{H}{}_4\mathrm{O}{}_2$ or $\mathrm{CH}{}_3\mathrm{COOH}$