Calcium and Oxygen are combined in the rates of 5:4 by mass to form calcium oxide.

What mass of Oxygen gas would be required to react with 2.5g of calcium ?

calcium and oxygen combine in the rates of 5:4 by mass to form calcium oxide

let x gram of oxygen is required to react with 2.5 g of calcium to form calcium oxide

therefore 5/4  = 2.5 / x

  5x = 2.5 * 4

  x = (2.5 *4 ) / 5

  x = 2

therefore 2 grams of oxygen is required to react with 2.5 grams of calcium to form calcium oxide .

hoping that my answer helps  you  :)