% purity question?

Question

0.125g of a piece of impure magnesium ribbon is heated in air until there is no change in mass. 0.200g of magnesium oxide is obtained. What is the % purity of the magnesium?

Answer 1

Find moles of magnesium and theoretical amount of MgO that should have been produced:

0.125g/24.3g/mol = 0.00514mol

0.00514mol x [24.3 + 16]g/mol = 0.207g

determine the percentage of theoretical amount that was recovered:

0.200g/0.207g x 100% = 96.6%

This assumes the impurity does not interfere with the mass of recovered oxide