Chemical Calculations Prentice-Hall Chapter 12.2 Dr. Yager

Objectives  Construct mole ratios from balanced chemical equations and apply these ratios in stoichiometric calculations.  Calculate stoichiometric quantities from balanced chemical equations using units of moles, mass, representative particles and volume of gases at STP.

A mole ratio is a conversion factor derived from the coefficients of a balanced chemical equation and interpreted in terms of moles. In chemical calculations, mole ratios are used to convert between moles of reactant and moles of product, between moles of reactants, or between moles of products.

Mole Ratios N 2 (g) + 3H 2 (g) 2NH 3 (g) N 2 (g) + 3H 2 (g) 2NH 3 (g) 1 mole of nitrogen reacts with 3 moles of hydrogen Mole ratios are used to convert between any two compounds:

Mole-Mole Calculation If you have 0.60 mol of N 2, how much NH 3 is produced? The ratios are:

Write the six mole ratios for: 4Al(s) + 3O 2 (g) 2Al 2 O 3 (s)

Write the six mole ratios for: 4Al(s) + 3O 2 (g) 2Al 2 O 3 (s)

How many moles of aluminum are needed to form 3.7 mol of Al 2 O 3 ?

Mass-Mass Calculation

Calculate the number of grams formed by reacting 5.4 grams of H 2 with excess N 2. N 2 (g) + 3H 2 (g) 2NH 3 (g) N 2 (g) + 3H 2 (g) 2NH 3 (g)

Calculate the number of grams formed by reacting 5.4 grams of H 2 with excess N 2.

Other Stoichiometric Calculations  It all comes down to moles.  If you want atoms, particles, etc. then multiply moles by 6.02x10 23  If you want gas volume at STP then multiply moles by 22.4 liters.  If you have volume divide by 22.4 L/mol to get moles  Always use dimensional analysis to check units!

Solution Diagram Solution Diagram 12.2

Problem-Solving Approach Problem-Solving Approach 12.2

1. How many moles of water are produced when 2.5 mol of O 2 react according to the following equation? C 3 H 8 + 5O 2  3CO 2 + 4H 2 O C 3 H 8 + 5O 2  3CO 2 + 4H 2 O a) 2.0 b) 2.5 c) 3.0 d) 4.0

1. How many moles of water are produced when 2.5 mol of O 2 react according to the following equation? C 3 H 8 + 5O 2  3CO 2 + 4H 2 O C 3 H 8 + 5O 2  3CO 2 + 4H 2 O a) 2.0 b) 2.5 c) 3.0 d) 4.0

2. Nitrogen gas reacts with hydrogen gas to produce ammonia gas. produce ammonia gas. N 2 (g) + 3H 2 (g)  2NH 3 (g) N 2 (g) + 3H 2 (g)  2NH 3 (g) What volume of H 2 is required to react with 3.00 L of N 2, and what volume of NH 3 is produced at 0°C? a) volume of H 2 = 9.00 L, volume of NH 3 = 6.00 L b) volume of H 2 = 3.00 L, volume of NH 3 = 3.00 L c) volume of H 2 = 3.00 L, volume of NH 3 = 6.00 L d) volume of H 2 = 1.00 L, volume of NH 3 = 1.50 L

2. Nitrogen gas reacts with hydrogen gas to produce ammonia gas. produce ammonia gas. N 2 (g) + 3H 2 (g)  2NH 3 (g) N 2 (g) + 3H 2 (g)  2NH 3 (g) What volume of H 2 is required to react with 3.00 L of N 2, and what volume of NH 3 is produced at 0°C? a) volume of H 2 = 9.00 L, volume of NH 3 = 6.00 L b) volume of H 2 = 3.00 L, volume of NH 3 = 3.00 L c) volume of H 2 = 3.00 L, volume of NH 3 = 6.00 L d) volume of H 2 = 1.00 L, volume of NH 3 = 1.50 L

3. Automotive airbags inflate when sodium azide, NaN 3, rapidly decomposes to its component elements via this reaction: 2NaN 3  2Na + 3N 2. 2NaN 3  2Na + 3N 2. How many grams of sodium azide are required to form 5.00 g of nitrogen gas? How many grams of sodium azide are required to form 5.00 g of nitrogen gas? a)11.61 g b)17.41 g c)7.74 g d)1.36 g

3. Automotive airbags inflate when sodium azide, NaN 3, rapidly decomposes to its component elements via this reaction: 2NaN 3  2Na + 3N 2. 2NaN 3  2Na + 3N 2. How many grams of sodium azide are required to form 5.00 g of nitrogen gas? How many grams of sodium azide are required to form 5.00 g of nitrogen gas? a)11.61 g b)17.41 g c)7.74 g d)1.36 g