Determine the mass of oxygen that will be required to burn 124g of phosphorus.

Phosphorus combustion reaction:
4P + 5O2 = 2P2O5.
124 g of phosphorus, this is the amount of 124 g: 31 g / mol = 4 mol. Oxygen for its complete combustion will need 5/4 * 4 mol = 5 mol of oxygen, this is 32 g / mol * 5 mol = 160 g.

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