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## Stoichiometry Introduction

Stoichiometry is an area of chemistry that examines the relative amounts of reactants and products in a chemical reaction. In a balanced chemical equation, coefficients show us molecular ratios of the substances involved. Lets examine the following reaction-

N2 + 3 H2 --> 2 NH3

We could read this chemical equation as saying that “one molecule of nitrogen reacts with three molecules of hydrogen to produce two molecules of ammonia.” The ratio of nitrogen : hydrogen : ammonia is 1:3:2. As long as our ratio stays constant we can manipulate this equation and still be balanced-

100 N2 + 300 H2 --> 200 NH3

This equation remains balanced and our ratio stays constant when we multiply all quantities by the same number- in this case 100. We have discussed how tiny atoms and molecules are. For our purposes it is unrealistic to think of these reactions happening between only hundreds of molecules- this is far too tiny of a reaction to be noticed. Can we multiply this equation by a larger number of particles to make it more relevant?

1(6.02 x 1023) N2 + 3(6.02 x 1023) H2 --> 2(6.02 x 1023) NH3

In this case we multiple each quantity by Avogadro’s number. Because this number represents a mole, we can now read this equation as saying that “one mole of nitrogen reacts with three moles of hydrogen to produce two moles of ammonia.” Therefore, the coefficients in a balanced chemical equation show us BOTH molecular and molar ratios of substances. If we start with three moles of hydrogen we can expect to produce two moles of ammonia. If we start with six moles of hydrogen we can expect to produce four moles of ammonia. Using molar mass as a conversion factor we can now relate grams of our reactants to grams of products. Lets try an example. Start by balancing the following equation:

___ H2SO4 + ___ NaHCO3 --> ___ H2O + ___ CO2 + ___ Na2SO4

In this reaction we are neutralizing sulfuric acid with baking soda to produce water, carbon dioxide and sodium sulfate. We want to predict how much of the products will be made from a known amount of reactants. How can we use this chemical equation and molar conversions to get from a reaction to a measurement that we can actually see and verify?

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___ H2SO4 + ___ NaHCO3 --> ___ H2O + ___ CO2 + ___ Na2SO4

If you have not already done so, balance the chemical equation above.

1) Give the molar masses of each substance-

H2SO4                          NaHCO3                      H2O                 CO2                 Na2SO4

2) We are going to start this reaction with 4.9g of sulfuric acid and excess baking soda. How many moles of sulfuric acid are we starting with?

3) Using the balanced chemical equation- what is the molar ratio of sulfuric acid : carbon dioxide?

4) Using your answers to questions two and three, determine the number of moles of carbon dioxide that you would expect this reaction to produce.

5) Using your answer to question four, determine the mass (g) of carbon dioxide that you would expect this reaction to produce.

6) What is the mass of the reactants before the reaction is allowed to take place?

7) What is unique about carbon dioxide with relation to the other two products in this reaction?

8) How can we use our answers from questions five, six, and seven to make a prediction about the mass of the products once this reaction is allowed to take place? What is your prediction?

9) What is the mass of the products after the reaction is allowed to take place? Was any mass “lost?” Was your prediction correct? Explain.

Demonstration: Weigh a 250-mL beaker along with a weighing dish, a glass cover the for the beaker, 50 mL of 1.0 M H2SO4 and 10 g of baking soda on a three-beam balance. Slowly add baking soda to acid bit by bit while covering to prevent splatter. While waiting for reaction to subside, replace weighing dish on balance pan. When complete, stir the mixture and place the dish on the pan again. The balance beam will clearly show that the materials have lost mass. The CO2 will have escaped into the air. For the amounts mentioned here the amount of CO2 lost will be about 4.4 g. (The baking soda is in excess and the resulting solution is safe to go down the drain.)

These demonstration notes and questions written by Nathan Guerin.
Basic Stoichiometry Activity
homework for Stoichiometry Activity.
Lab: Stoichiometry
Last updated: May 02, 2011        Home