If you burn 31.4 g of hydrogen and produce 281 g of water, how much oxygen reacted?

Question

In the combustion of hydrogen gas, hydrogen reacts with oxygen from the air to form water vapor.

If you burn  g of hydrogen and produce  g of water, how much oxygen reacted?

Answer 1

First, write the balance equation:

2H₂ + O₂ ==> 2H₂O ... balanced equation

31.4 g....x g...........281 g

Conservation of mass tells us that mass on the left side of the equation must equal mass on right side.

31.4 g + x g = 281 g

x g = 281 g - 31.4 g

x g = 249.6 g of O₂

This assumes that no reactants or products were lost during the reaction.

You can verify this answer as follows:

250 g O₂ x 1 mol O₂ / 32 g = 7.8 moles O₂

31.4 g H₂ x 1 mol H₂ / 2 g = 15.7 moles H₂

Note that there are twice as many moles of H₂ as there are O₂ just as in H₂O.