Exercise 3 solution

Balanced equation (given, and balanced: 2 Mg and 2 O on each side): 2Mg + O2 -> 2MgO

Step 1. Mole ratio from the coefficients: Mg : MgO = 2 : 2, which is 1 : 1.

Step 2. Grams to moles of Mg:
  n = 6.00 g / 24.31 g/mol = 0.2468 mol

Step 3. Apply the ratio: 0.2468 mol Mg gives 0.2468 mol MgO.

Step 4. Moles to grams of MgO:
  M(MgO) = 24.31 + 16.00 = 40.31 g/mol
  m = 0.2468 mol x 40.31 g/mol = 9.95 g

Answer: about 9.95 g of magnesium oxide. It is heavier than the 6.00 g of magnesium
because oxygen from the air has joined it (conservation of mass, Chapter 6).

Note: the arithmetic was run in Python (check, my own calculation).