Lowell High School Chemistry

Stoichiometry Online Tutorial — scroll down for sample problem

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What is Stoichiometry?

The word stoichiometry derives from two Greek words: stoicheion (meaning "element") and metron (meaning "measure"). Stoichiometry deals with calculations about the masses (sometimes volumes) of reactants and products involved in a chemical reaction. It usually determines how much of a substance is expected to react or be produced in a chemical reaction. It is a very mathematical part of chemistry, so get your calculator ready!

 

Jeremias Benjamin Richter (1762-1807) was the first to describe the law of definite proportions, or stoichiometry. (He also came up with the concept of titrations.)

In 1792, he wrote: "Die stšchyometrie (Stšchyometria) ist die Wissenschaft die quantitativen oder MassenverhŠltnisse zu messen, in welchen die chymischen Elemente gegen einander stehen." (German translated into English: Stoichiometry is the science of measuring the quantitative proportions or mass ratios in which chemical elements stand to one another.)

Key Terms

stoichiometry: mathematical process used to calculate how much you expect to produce from a chemical reaction
molar mass: used in stoichiometry to convert from grams to moles and vice versa; units are g/mol
limiting reagent: the reactant that limits the amount of products made — based on the moles and the mole ratio of the reactants
theoretical yield: calculated amount from stoichiometry you expect to get from the reaction
actual yield: real amount from you got from the reaction after carried out by experiment
percent yield: how close your actual yield is to what you expected to get


The key steps (in order) to answering a stoichiometry problem:

1. Balance your equation first.

2. Convert from grams to moles using molar mass.

3. Determine the limiting reagent [if necessary] (Use mole ratios to figure out.)

4. Use ratios to find the moles of the reactant or product you need to find.

5. Convert from moles back to grams of the new substance using that substance's molar mass.

6. Calculate percent yield [if necessary] (Divide actual yield by theoretical yield X 100%)

 

 

Sample Example Problem [to show you how it's done]:

Problem: How many grams of silicon carbide are produced when 50.0 g of silicon dioxide is heated with 32.0 g of carbon?

 

            SiO2 (s)  +  C (s) -->  SiC (s)  +  CO (g)                   [unbalanced]

 

 

1. Balance your equation first.

            SiO2 (s)  +  3C (s)  -->   SiC (s)  +  2CO (g)             [balanced]

 

 

2. Convert from grams to moles using molar mass.

 

            To get moles from grams of silicon dioxide (SiO2):    [molar mass = 60 g/mol]

 

                        50.0 g SiO2  X  ( 1 mol  /  60 g ) = 0.833 mol SiO2

 

            To get moles from grams of carbon (C):         [molar mass = 12 g/mol]

 

                        32.0 g C  X  ( 1 mol  /  12 g ) = 2.667 mol C

 

3. Determine the limiting reagent [if necessary] (Use mole ratios to figure out.)

 

            Figure out how many moles of C would need to react with 0.833 moles SiO2:

                        * From your balanced equation, the ratio is 3 mol C : 1 mol SiO2.

 

                        0.833 mol SiO2  X  ( 3 mol C / 1 mol SiO2 )  = 2.5 mol C needed

 

                        * But you have 2.667 mol C to start with and need 2.5 mol C to react, so you have extra C and limiting reagent is SiO2

                        * To calculate excess: 2.667 - 2.500 = 0.167 mol C in excess

 

            OR

 

            Figure out how many moles of SiO2 would need to react with 0.833 moles C:

                        * From your balanced equation, the ratio is 1 mol SiO2 : 3 mol C.

 

                        2.667 mol C  X  ( 1 mol SiO2 / 3 mol C )  = 0.889 mol SiO2 needed

 

                        * But you have 0.833 mol SiO2 to start with and need 0.889 mol SiO2 to react, so you have SiO2 limiting reagent because you don't have enough SiO2 needed to react

* Compare 0.889 mol SiO2 needed vs. 0.833 mol SiO2 you started with

 

 

4. Use ratios to find the moles of the reactant or product you need to find.

            Since SiO2 is the limiting reagent, you use the moles of SiO2 if you have to calculate how many grams of silicon carbide (the product you're trying to find) are being produced. Use your mole ratios from your balanced equation:

 

                        0.833 mol SiO2  X  ( 1 mol SiC / 1 mol SiO2 )  = 0.833 mol SiC produced

 

 

5. Convert from moles back to grams of the new substance using that substance's molar mass.

 

            To get grams from moles of carbon (SiC):      [molar mass = 40 g/mol]

 

                        0.833 mol SiC  X  ( 40 g  /  1 mol ) = 33.33 g SiC produced

 

 

Answer to Problem: 33.33 g SiC produced = the theoretical yield

 

Practice Problems Here:

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