Phosphorus will react with oxygen to produce diphosphorous pentoxide. How many grams of oxygen are required to produce the diphosphorous pentoxide if 15g of phosphorus are used?

Answer 1

Balanced chemical eqn 4P + 5O₂ .--> 2 P₂O₅

15 g P is provided. First covert the gms to moles

Strategy map . g P ----> moles P ---> moles of O2-----> g O₂.

STEPWISE METHOD 1

15 g P x 1 mole P/ 30.97 g P =0.48 moles P

but reacting ration of P to oxygen according to balanced eqn of rxn is 4:5

so to calculate moles of oxygen used use the conversion factor 5 mole O2/ 4 mole of P as shown below

0.48 moles P x 5 mole O2/ 4 mole of P = 0.60 moles O₂.

Finally calculate mass of O2 using the conversion factor 32 g O₂/1 mole O₂

0.60 moles O₂ x 32 g O₂/1 mole O₂ = 19.2 g O₂

METHOD 2 : Using dimension analysis and following the strategy map...

Strategy map . g P ----> moles P ---> moles of O2-----> g O₂.

15 g P x (1 mole P/ 30.97 g P) x (5 mole O2/ 4 mole of P) x (32 g O₂/1 mole O₂) = 19.37 = 19 O₂