Using Mole Ratios Remember the balanced chemical equation for the formation of water. You can read it as, “Two moles of hydrogen react with one mole of oxygen and form two moles of water.” Because each mole of oxygen that reacts will yield two moles of water, you can write the following conversion factors, or mole ratios.
The mole ratio on the left allows you to convert moles of water to moles of oxygen. Now you can calculate how many moles of oxygen are required to produce eight moles of water:
Converting Moles to Mass
The last step is to convert moles of O2 to grams of O2 by using the molar mass of O2 as a conversion factor.
So, in order to produce 144 grams of H2O, you must supply 128 grams of O2. Notice that you used the concept of a mole in two ways to solve this problem. In the first and last step, you used a molar mass to convert between mass and moles. In the middle step, you used the mole ratio to convert moles of a product into moles of a reactant.
Section 7.1 Assessment
Reviewing Concepts
What is the law of conservation of mass?- Why does a chemical equation need to be balanced?
- Why do chemists use the mole as a counting unit?
- What information do you need to predict the mass of a reactant or product in a chemical reaction?
What is a mole ratio?
Critical Thinking
Applying Concepts The following equation describes how sodium and chlorine react to produce sodium chloride.
Is the equation balanced? Explain your answer.
Calculating Ammonia, NH3, can be made by reacting nitrogen with hydrogen.
How many moles of NH3 can be made if 7.5 moles of H2 react with enough N2?
Calculating What mass of NH3 can be made from 35.0 g of N2?
Math Practice
Balance the following equation.
Write a balanced chemical equation for the formation of magnesium oxide, MgO, from magnesium and oxygen.
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