<p>What mass of aluminum will be completely oxidized by 2 moles of oxygen at STP? </p>
<p>The answer is 72 grams, and the formula used, according to the explanation, is:
4Al + 3O2 ---> 2Al2O3</p>
<p>I understand how they got the 72 from the reaction, but I don't understand how they got the actual reaction. I thought it was 2Al + 2O2 ---> 2AlO2</p>
<p>Can someone explain why its 4Al + 3O2 ---> 2Al2O3?</p>
<p>The oxidation number of of Aluminum is +3 (it is a metal and therefore a cation–cations are positively charged ions) and the oxidation of number of Oxygen is -2 because it gains two electrons to form a stable octet (it is a nonmetal and therefore an anion). When you crisscross the two numbers to attain a net charge of 0, the compound is Al203.</p>