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.

the enthalpy of formation of water is given as -285.8kj/mol, which states that the enthalpy of formation is negative.

negative enthalpy of formation indicates that the heat is released during the reaction process. the heat is released from reaction to the surroundings the enthalpy is expressed with a negative sign with the energy given out in the reaction process.

the formation of water takes place by giving out heat which is -285.8 kj/mol.