What percentage mass of magnesium oxide is magnesium?

What is the percent by mass of oxygen in magnesium oxide?

• Magnesium combines with oxygen to form magnesium oxide. If 17.56 g of magnesium reacts completely with 7.83 g of oxygen, what is the percent by mass of oxygen in magnesium oxide?  

• Answer:

  Magnesium oxide is MgO Percent O = 16.0 / (24.3 + 16.0) x 100 = 39.7% You wrote "17.56 g of magnesium reacts completely with 7.83 g of oxygen." If your statement is true, then... percent oxygen = 7.83 / (7.83 + 17.56) x 100 = 30.8% But your data produces an "unusual" formula. 17.56g Mg x (1 mol Mg / 24.3g Mg) = 0.723 mol Mg 7.83g O x (1 mol O / 16.0g O) = 0.491 mol O 0.723 mol Mg / 0.491 = 1.47 ≈ 1.5 0.491 mol 0 / 0.491 = 1 Empirical formula = Mg3O2 This is the case only under extreme conditions. A team of researchers led by Artem R. Oganov said, "We have predicted that two new compounds, MgO2 and Mg3O2, become stable at pressures above one and five million atmospheres, respectively." http://commcgi.cc.stonybrook.edu/am2/publish/General_University_News_2/SBU_Geoscientists_Predict_New_Compounds_Could_Change_Our_View_of_What_Planets_are_Made_of.shtml The question is, did your question writer know about the ultra-high pressure experiments, or merely "make up" a compound? Mg3O2 would not be called simply "magnesium oxide." That would be confused with the magnesium oxide (MgO) that we have produced many, many times by burning magnesium ribbon in air. My point is that some teachers and even textbook authors write things that aren't true because either they don't care or simply don't know any better. I would like teachers to let students know if they "make stuff up" so that their students won't be confused by misinformation.