MYP Chemistry sample question: Chemical equations and conservation of mass

Magnesium ribbon burns in oxygen with a bright white flame to form a white powder.

1. (a)Write a word equation for the reaction.[1]
   Show the mark scheme for (a)

   • • magnesium + oxygen → magnesium oxide (1)
2. (b)Write a balanced symbol equation, including state symbols.[2]
   Show the mark scheme for (b)

   • • Correct formulae: Mg, O₂, MgO (1)
   • • 2Mg(s) + O₂(g) → 2MgO(s) (balanced with state symbols) (1)
3. (c)The white powder has a greater mass than the original magnesium. Explain why.[2]
   Show the mark scheme for (c)

   • • The magnesium has combined with oxygen from the air (1)
   • • so the product contains the mass of the magnesium plus the mass of the oxygen that joined it (1)

Balance by changing the numbers in front of formulae, never the small numbers inside them. Then count the atoms of each element on both sides.