If two molecules of hydrogen gas are to react with one molecule of oxygen gas,

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then we obtain the correct ratio of reactants by starting with two volumes of hydrogen and one of oxygen (right). Since the reaction produces two molecules of water, we can predict that if the product obtained is water vapor, there will be two volumes of vapor. The reaction by which ammonia, , is prepared from nitrogen and hydrogen gases is

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Avogadro's principle tells us that if we want to carry out this reaction without waste, we should begin with three times as much hydrogen as nitrogen by volume at the same pressure and temperature. The product, ammonia, will have twice the volume of the starting , or half the volume of the entire starting gas mixture. For gases, equal volumes at the same temperature and pressure contain equal numbers of moles.