Stoichiometry Module: Solutions
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Calculating with Solutions

Since most chemical reactions actually occur when the chemicals are in solution, it is important to master the calculations reviewed in this module. Once you are comfortable with these calculations, you will be able to solve most any stoichiometry problem.

If you worked through Module 5-1, you may recall a discussion of baking powder. Baking powder contains calcium dihydrogen phosphate, Ca(H2PO4)2, and sodium hydrogen carbonate, NaHCO3 (also called sodium bicarbonate). When the powder is dry, these two compounds do not react. When a liquid is added to the baking powder, the two chemicals dissolve, forming a solution. When the powder is in solution, they quickly react forming carbon dioxide. It is the carbon dioxide gas that causes cakes, cookies, and some breads to rise.

The balanced chemical equation for this process is:

Clicking on formulas will show molecular weights.

Suppose you decided to bake a cake using solutions from your chemistry laboratory instead of baking powder. You find a 0.25 M calcium dihydrogen phosphate, Ca(H2PO4)2, solution and a 0.75 M sodium hydrogen carbonate, NaHCO3, solution in the stock room. Your instructor estimates that you should use .70 grams calcium dihydrogen phosphate and .50 grams sodium hydrogen carbonate.

What volume of each solution should you add to your cake?

Ca(H2PO4)2: 0.18 mL
NaHCO3: 0.38 mL		 Ca(H2PO4)2: 0.75 mL
NaHCO3: 4.5 mL		 Ca(H2PO4)2: 2.8 mL
NaHCO3: 0.67 mL		 Ca(H2PO4)2: 12 mL
NaHCO3: 7.9 mL

Good! The calculation would look like:

Remember that M stands for molarity, or moles per liter. Molarity multiplied by a volume (in liters) would equal some number of moles.

Remember that M stands for molarity, or moles per liter. Molarity multiplied by a volume (in liters) would equal some number of moles.

You must first convert the number of grams needed to the number of moles needed. Then the only missing part of the above equation is the volume.




















After you have added all the ingredients and the baking powder has reacted, the volume of the mixture is 500 mL. What is the concentration of sodium hydrogen phospate, Na2HPO4, in the cake batter?

.003 M .006 M .012 M 1.5 M

Remember molarity is moles per liter. How many liters of batter are there?

Good! The balanced chemical equation shows one mole of Na2HPO4 produced for every one mole of Ca(H2PO4)2. Hence, 0.00299 moles of Na2HPO4 will be produced.

The balanced chemical equation shows that one mole of Na2HPO4 will be produced for every mole of Ca(H2PO4)2 OR every two moles of NaHCO3 used.

Remember molarity is moles divided by liters.