how many moles of oxygen are required for the combustion of 25.5 g of magnesium

Answer 1

write the balanced equation:

2Mg+O2>>2MgO So according to the equation, figure the moles of Mg (25.5/atomicmassMg), then half that is required number of moles of O2.

Answer 2

Here is a step by step example of how to work stoichiometry problems.
http://www.jiskha.com/science/chemistry/stoichiometry.html