3.3 Empirical formulae
5 exam questions worth 11 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
0 / 11
What is the empirical formula of butene, C4H8?
Choose one answer.
Correct Not quite: the answer is CH2
Mark scheme
A
Model answer
A: C4H8 simplifies (÷4) to CH2.
1.2 g of carbon reacts with 3.2 g of oxygen.
Calculate the empirical formula of the compound formed. (A_\mathrm{r}: C = 12, O = 16)
Mark scheme
- C: 1.2 ÷ 12 = 0.1; O: 3.2 ÷ 16 = 0.2 (1)
- ratio 1 : 2 (1)
- CO2 (1)
Model answer
Moles of C = 0.1 and moles of O = 0.2, a 1 : 2 ratio, so the empirical formula is CO2.
A hydrocarbon has the empirical formula CH2 and a relative formula mass of 56.
Work out its molecular formula. (A_\mathrm{r}: C = 12, H = 1)
Mark scheme
- M_\mathrm{r} of CH2 = 14; 56 ÷ 14 = 4 (1)
- C4H8 (1)
Model answer
CH2 has M_\mathrm{r} 14, and 56 ÷ 14 = 4, so the molecular formula is C4H8.
In the magnesium oxide experiment, explain why the lid of the crucible is lifted from time to time, and why the crucible is heated until its mass stops changing.
Mark scheme
- lifting the lid lets oxygen / air in to react with the magnesium (1)
- constant mass shows all the magnesium has reacted (1)
Model answer
The lid is lifted to let oxygen in so the magnesium can react. Heating to constant mass makes sure all of the magnesium has reacted.
A compound contains 75% carbon and 25% hydrogen by mass.
Calculate its empirical formula. (A_\mathrm{r}: C = 12, H = 1)
Mark scheme
- C: 75 ÷ 12 = 6.25; H: 25 ÷ 1 = 25 (1)
- ratio 1 : 4 (1)
- CH4 (1)
Model answer
C: 75 ÷ 12 = 6.25 and H: 25 ÷ 1 = 25. Dividing by 6.25 gives 1 : 4, so the empirical formula is CH4.
Found a mistake?