Hydrogen and oxygen combine in the ratio of 1 : 8 by mass to from water.

Here we have been given that hydrogen and oxygen always combine in the fixed ratio of 1 : 8 by mass. This means that:

1 g of hydrogen gas requires = 8 h of oxygen gas

So, 3 g of hydrogen gas requires = 8 × 3 g of oxygen gas

= 24 g of oxygen gas

Thus, 24 grams of oxygen gas would be required to react completely with 3 grams of hydrogen gas.