Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 grams of hydrogen gas?

The law of constant proportions states that in a chemical substance, the elements are always present in definite proportions by mass. Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water.

1 g of hydrogen reacts with 8 g oxygen

Thus, in order to form water, 3 x 8 = 24 g of oxygen will react completely with 3 g of hydrogen gas.

So, answer is 24 g oxygen.