What is a possible cause of a large percentage of error in an experiment where mgo is produced from the combustion of magnesium?

A possible cause of a large percentage of error in an experiment where MgO is produced from the combustion of magnesium would be not all of the Mg has completely reacted. 

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Not all the magnesium has completely reacted.

Explanation:

In a laboratory experiment to produce MgO through burning of a Magnesuim ribbon is achieved when all conditions are observed.

Opening the crucible at intervals slows the rate of the burning magnesium.

The magnesium reacts with air to form MgO.

In most experiments, not all the magnesium gets to completely react.

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explanation:

covalent bonds

consist of pairs of electrons shared by two atoms, and bind the atoms to fixed orientation and requre high energies to break them

ionic bonds

when two atoms differ considerably in their electronegativity, then one of the atoms will lose its electron to the other atom. this results in a positively charged ion(cation) and a negatively charged ion (anion)