How many grams of oxygen are required for the combustion of 25.5 grams of magnesium? 2mg(s) + o²(g) -> 2mgo(s)

Step 1 : Convert grams of magnesium to moles using n = m/MM.

MM is found in periodic table if you don't know it yet.

So we get :     n(Mg) = 25.5 g  ÷ 24.305 g/mol. = 1.0491... mol.

Step 2: Look at the stoichiometric ratios in the formula. Mg : O = 2:1.

Therefore, the moles of oxygen is half the number of moles of Magnesium.

n(O2) = 0.5 x 1.0491... mol = 0.52458... mol

Step 3: Now convert moles of Oxygen to grams using m = n x MM. Hey presto you have your answer!

m(O2) = 0.52458... x 2(15.999) = 16.7856... grams.

To 3 significant  figures this is 16.8grams.

Let me know if you are confused about anything.