What is the symbol for the element in this problem?

When a certain nonmetal whose formula is X4 burns in air, X4O10 forms. If 120.0 g of oxygen gas is consumed completely, along with 92.93 g X4, identify element X. What is the symbol for the element?

Answer:
The element you are looking for is Phosphorus (P).

120 g of oxygen in a X4O10 while oxygen weights 16 g per atom implies that we are dealing with 7.5 moles of atomic oxygen.
But the molecule has 4 atoms of X for every 10 of O, so you should be having 7.5 * 4 / 10 or 3 moles of X. You have 92.93 g of X, divide by 3 and you get 30.976 g of X per mole. Look into the periodic table for an element weighting that much, and this is P.
Tell me , interesting.
From a periodic table, we know that oxygen weighs 15.99 g/mole. So if we have 120.0 grams of oxygen, the we have 120 / 15.99 = 7.50 moles of oxygen. The question gives us the stoichiometry of the product, which is 4 X's for every 10 oxygen's. That means that we have 4/10 moles of X. 0.4 times 7.5 = 3 moles of X. Since we know we have 92.93 grams of X, we can figure out that X must weigh 92.93 / 3 = 30.98 grams/mole. This is the formula weight of X. If you go to a periodic table, you see that P (phosphorous) has this molecular weight. So X is P, phosphorous.
Good luck!

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