Name Stoichiometry © Van Der Sluys, 2004 in Order to Maintain Mass Balance in a Chemical Reaction, St

Name ______Stoichiometry

In order to maintain mass balance in a chemical reaction, stoichiometric coefficients are used to equate the number of atoms of each element on both reactant and product sides of a reaction. The stoichiometric coefficients are essentially multiplication factors, which are the lowest whole number ratios of compounds or moles of compounds required to balance the reaction. For example, in the combustion of methane (CH4), equation 1, the stoichiometric ratios are 1:2:1:2, methane to oxygen to carbon dioxide to water, respectively.

CH4 (g) + 2 O2 (g)  CO2 (g) + 2 H2O (g) (1)

This means that one mole of methane requires two moles of oxygen to completely react, and exactly one mole of carbon dioxide and two moles of water will be produced. The dimensional analysis approach is useful in these types of calculations. In these cases, one should include the number, unit and formula of the compounds used in the calculation. For example, the calculation of the number of moles of water produced when 1.0 mole of methane burns is

1.0 mole CH4 x 2 H2O = 2 mole H2O 1 CH4

Previously we have learned how to convert mass to moles using the gram formula mass (g/mole) of a compound. Therefore, we can combine this type of calculation with the stoichiometric ratios in a reaction to predict the mass of a product that will be produced in a reaction. For example, how many grams of water vapor will be produced when 16 g of methane are burned in an excess of oxygen?

16 g CH4 x 1 mole x 2 H2O x 18 g = 32 g H2O 16 g 1 CH4 1 mole

Step 1 Step 2 Step 3

This calculation involves solving three sets of proportions: (1) The mass of methane is converted to moles using the gram formula mass, 16 g/mole; (2) The stoichiometric ratio of water to methane, 2:1, is used; and (3) The conversion of the moles of water to mass of water using the gram formula mass, 18 g/mole. This type of calculation could be used to determine the mass:mass relationships for any combination of reactants or products in a balanced chemical reaction.

Write balanced reactions and using dimensional analysis, answer the following questions.

1. How many grams of carbon dioxide will be formed if 20. g of methane are burned in an excess of oxygen?

2. How many grams of methane, CH4, must be burned to form 20. g of carbon dioxide?

3. If 15. g of water are formed when methane is burned in an excess of oxygen, how many grams of carbon dioxide must also have been formed?

4. What mass of methane can be burned by 20. g of oxygen?

5. The complete combustion of 20. g of butane, C4H10, requires how many grams of oxygen?

6. How many grams of carbon dioxide will be formed when 25 g of methanol,

CH3OH, is burned using an excess of oxygen?

Answers

1. 55 g 2. 7.3 g 3. 18 g 4. 5.0 g 5. 72 g 6. 34 g