Empirical and molecular formulaeAQA A-Level Chemistry: Subtopic test

1. 1
   Chemists in a food laboratory identify sugars and acids from their formulae. They record that glucose has the molecular formula C₆H₁₂O₆.

   (a)

   Which statement correctly defines the empirical formula of a compound?

   [1 mark]

   • AThe actual number of atoms of each element in one molecule
   • BThe number of moles of each element in 100 g of the compound
   • CThe simplest whole number ratio of atoms of each element in the compound
   • DThe ratio of the masses of each element in the compound

   (b)

   Which of these compounds has the empirical formula CH₂O?

   [1 mark]

   • AC₂H₄O₂
   • BC₂H₆O
   • CC₃H₈O
   • DC₂H₂O₄

   (c)

   State the empirical formula of glucose and explain how it differs from its molecular formula.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   An organic compound X contains only carbon and hydrogen. Analysis shows it contains 85.7% carbon and 14.3% hydrogen by mass, and its relative molecular mass is 56.0. Use Ar values: C = 12.0, H = 1.0.

   (a)

   What amount, in mol, of carbon is present in 100 g of compound X?

   [1 mark]

   • A0.140 mol
   • B14.3 mol
   • C1.20 mol
   • D7.14 mol

   (b)

   What is the empirical formula of compound X?

   [1 mark]

   • ACH
   • BCH₂
   • CCH₃
   • DC₂H₃

   (c)

   Calculate the molecular formula of compound X.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A 2.00 g sample of an iron oxide is heated in a stream of hydrogen until it is completely reduced to iron. The mass of iron produced is 1.40 g. Use Ar values: Fe = 55.8, O = 16.0.

   (a)

   Calculate the amount, in mol, of iron and the amount, in mol, of oxygen in the sample.

   [3 marks]

   (b)

   Use your answers to (a) to deduce the empirical formula of the iron oxide. Explain why the formula cannot be given as FeO₁.₅.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A vitamin C tablet contains an organic acid composed only of carbon, hydrogen and oxygen. Analysis shows 40.9% carbon, 4.6% hydrogen and 54.5% oxygen by mass, and mass spectrometry shows that its relative molecular mass is 176. Use Ar values: C = 12.0, H = 1.0, O = 16.0.

   (a)

   Calculate the empirical formula and the molecular formula of the organic acid.

   [6 marks]

   (b)

   A student divides by the smallest value as above but rounds the ratio 1 : 1.35 : 1 to 1 : 1 : 1. She gives the empirical formula as CHO and the molecular formula as C₆H₆O₆. Evaluate the student's answer and state the correct molecular formula.

   [6 marks]

   Total for question 4: 12 marks

End of questions