Magnesium burns in air to give magnesium oxide as shown below. If 8g of magnesium oxide product is produced, calculate how much magnesium metal must have been used.

Question

Answer 1

2Mg(s) + O2(g) ==> 2MgO(s) ... balanced equation

? g...........xs..............8 g......

We use grams to moles and then convert back to grams.

8 g MgO x 1 mol MgO = 0.1985 moles MgO

40.3 g MgO

moles Mg needed = 0.1985 moles MgO x 2 moles Mg = 0.397 moles Mg needed

mol MgO

grams Mg needed = 0.397 moles Mg x 24.3 g Mg/mol = 9.65 g = 10 g (1 sig.fig.)