Activity 3.14:
Solving numerical problems involving empirical formulae and molecular formulae
[Relative atomic mass: H = 1, C = 12, N = 14, O = 16, Ca = 40, Zn = 65]
1. Ethanoic acid has a molar mass of 60 g mol–1. If its empirical formula is CH2O, determine the molecular formula of ethanoic acid.
2. Hydrocarbons consist of carbon and hydrogen. 5.7 g of a hydrocarbon contains 4.8 g of carbon.
If the relative molecular mass of the hydrocarbon is 114, determine its molecular formula.
3. What is the mass of zinc required to combine with 0.5 mol of chlorine to produce zinc chloride, ZnCl2?
4. Assume you are a farmer. You want to choose a fertiliser with a high nitrogen content for your plants. Three types of commonly used fertilisers are as follows.
Which fertiliser would you choose? Give reasons for your choice. Show the steps used in the calculation.
Answer:
1. C2H4O2
2. C8H18
3. 16.25 g
4. Urea because the percentage by mass of nitrogen is the highest, that is 46.67%
$$ \begin{aligned} \% \text { nitrogen in ammonium nitrate } & =\frac{2(14)}{14+4(1)+14+3(16)} \times 100 \% \\ & =35 \% \end{aligned} $$
$$ \begin{aligned} \% \text { nitrogen in urea } & =\frac{2(14)}{12+16+2[14+2(1)]} \times 100 \% \\ & =46.67 \% \end{aligned} $$
$$ \begin{aligned} \% \text { nitrogen in calcium nitrate } & =\frac{2(14)}{40+2[14+3(16)]} \times 100 \% \\ & =17.07 \% \end{aligned} $$
Solving numerical problems involving empirical formulae and molecular formulae
[Relative atomic mass: H = 1, C = 12, N = 14, O = 16, Ca = 40, Zn = 65]
1. Ethanoic acid has a molar mass of 60 g mol–1. If its empirical formula is CH2O, determine the molecular formula of ethanoic acid.
2. Hydrocarbons consist of carbon and hydrogen. 5.7 g of a hydrocarbon contains 4.8 g of carbon.
If the relative molecular mass of the hydrocarbon is 114, determine its molecular formula.
3. What is the mass of zinc required to combine with 0.5 mol of chlorine to produce zinc chloride, ZnCl2?
4. Assume you are a farmer. You want to choose a fertiliser with a high nitrogen content for your plants. Three types of commonly used fertilisers are as follows.
Which fertiliser would you choose? Give reasons for your choice. Show the steps used in the calculation.
Answer:
1. C2H4O2
2. C8H18
3. 16.25 g
4. Urea because the percentage by mass of nitrogen is the highest, that is 46.67%
$$ \begin{aligned} \% \text { nitrogen in ammonium nitrate } & =\frac{2(14)}{14+4(1)+14+3(16)} \times 100 \% \\ & =35 \% \end{aligned} $$
$$ \begin{aligned} \% \text { nitrogen in urea } & =\frac{2(14)}{12+16+2[14+2(1)]} \times 100 \% \\ & =46.67 \% \end{aligned} $$
$$ \begin{aligned} \% \text { nitrogen in calcium nitrate } & =\frac{2(14)}{40+2[14+3(16)]} \times 100 \% \\ & =17.07 \% \end{aligned} $$