2. (a) The molecular formula of butene is C₄H₈ - Edexcel - GCSE Chemistry Combined Science - Question 2 - 2018 - Paper 1

To calculate the relative formula mass of C₄H₈:

• The mass of carbon (C) is 12, and there are 4 carbon atoms:

  $4 imes 12 = 48$

• The mass of hydrogen (H) is 1, and there are 8 hydrogen atoms:

  $8 imes 1 = 8$

• Adding these together:

  $48 + 8 = 56$

Thus, the relative formula mass of butene is 56.

The balanced chemical equation for the combustion of butene can be written as:

C₄H₈ + 5 O₂ → 4 CO₂ + 4 H₂O.

To test for carbon dioxide, one can use limewater (a dilute solution of calcium hydroxide). When carbon dioxide is bubbled through limewater, it reacts to form calcium carbonate, which causes the solution to turn cloudy. This cloudiness indicates the presence of carbon dioxide.