Question

Would the following experimental errors give a mass percent of magnesium too high or too low?

incomplete combustion of magnesium

not heating the crucible to dryness

spilled some ashes

Answer 1

For combustion of magnesium, the reaction is----

2Mg (s) + O₂ (g) = 2MgO (s)

Incomplete combustion of magnesium means MgO with unreacted Mg. So, the error will come from unreacted Mg and hence total mass due to incomplete combustion of magnesium will be lower than that of complete combustion. So, mass percent of Mg in MgO gives too high as experimental error.

If crucible was not heated to dryness, there will be error in initial weight of Mg as considering H₂O i.e, experiencing higher initial weight of Mg as expected. But, after complete combustion, H₂O will be vaporised, so, here, mass percent of Mg in MgO gives too high as experimental error.

Spilling some ashes (MgO) means that considering less weight of MgO as final weight for calculation as experimental error. so, here also mass percent of Mg in MgO gives too high.