Lab: How would your calculated value for the percent composition of magnesium oxide be affected if all the magnesium in the crucible had not reacted.
Calculation-${\% Mg = }\dfrac{{{\text{mMg}}}}{{{\text{mMgO}}}}{ \times 100\% }$
Will the ${\% Mg}$ be over-reported? Under-reported?

Hint:The given problem is based on the percent composition of the substance.
The percent composition is calculated by taking a percentage of the mass of the component to the total mass of all the components present in the mixture or given compounds.

Complete answer:
Here, in the question the percentage composition formula for the magnesium is given which is as follows:
${\% Mg = }\dfrac{{{\text{mMg}}}}{{{\text{mMgO}}}}{\times 100\% }$
Here, m represents the mass of the species.
Here, magnesium is converted into magnesium oxide. It is given that all magnesium is not converted into magnesium oxide.
When we calculate the percent of magnesium the numerator that is the mass of the magnesium remains the same but the mass of the magnesium formed changes.
As the magnesium oxide is formed the ratio of the mass of magnesium to the mass of magnesium oxide gets reduced.
Here, all magnesium is not converted into magnesium oxide hence the percentage of the magnesium is under-reported.

Note:The percentage composition is calculated simply by taking the ratio of the mass of the component to the total mass of all the components and this ratio is then multiplied by 100.
Purity indicates the actual amount of the compound present in the sample. This is because impurities are present along with the desired compound. Hence, in many chemical reactions, the percent purity of the final product is determined. Higher is the purity more will be the amount of the desired compound in the formed product.