Percentage purity 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:
To work this out, first you need to figure out how much magnesium is in the magnesium oxide and then you need to divide that mass by the mass of magnesium ribbon to get the percent purity.

So:

The molar mass of magnesium oxide = 56.293 ( mass Mg + 2 * mass O)

0.200 g MgO2 / 56.293 = 3.553 * 10^-3 moles MgO2

1mole MgO2 = 1mole Mg

Thus 3.553 * 10^-3 moles Mg present

3.553 * 10^-3 moles Mg * 24.3050 g Mg/mole = 0.0864 g magnesium present

0.0864g magnesium/0.125 g impure magnesium * 100% = 69.1% purity

Please double check the math, but the approach is right.

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