Ch. 11: Stoichiometry

Sec. 11.2: Stoichiometric Calculations

Objectives

• List the sequence of steps used in solving stoichiometric problems.

• Solve stoichiometric problems.

Using Stoichiometry

• Stoichiometry is the study of quantitative relationships between the amounts of reactants used and the amounts of products formed by a chemical reaction.

• In Action: Start with a balanced equation; develop mole ratios between “given” (or “known”) and “unknown”; use molar conversions to convert to appropriate unit.

Mole to Mole Conversions

Problem: Potassium reacts with water to produce potassium hydroxide and hydrogen gas. Determine the number of moles of hydrogen gas produced when 0.04 mol of K is used?

Mole to Mole Conversions

• Step 1: Write a balanced equation. 2K(s) + 2H2O(l) 2KOH(aq) + H2(g)

• Step 2: Identify the known and unknown in the problem.– 0.04 mol K is known; mol H2 is unknown.

• Step 3: Use the balanced equation to determine the mole ratio in which mol unknown is the numerator and mol known is the denominator. mol unknown = 1 mol H2

mol known 2 mol K

Mole to Mole Conversions

• Step 4: Multiply the known by the mole ratio. Note that known substance & its units will cancel if it is correctly done. 0.04 mol K x 1 mol H2 = 0.02 mol H2

2 mol K

We have gone from moles of known

to moles of unknown!

Practice Problems• How many moles of carbon dioxide gas are

produced when 10.0 mol of propane (C3H8) are burned in an excess of oxygen?

• Sulfuric acid is produced when sulfur dioxide reacts with oxygen gas and water. If 12.5 mol SO2 reacts, many many mol of H2SO4 are produced? How many mol of O2 are needed?

Mole to Mass Conversions

• Follow steps 1 - 4 (for mole to mole conversions).

• Add an extra step: Use a molar conversion to convert from moles to mass or from mass to moles where needed in the problem.

Practice Problems• Determine the mass of NaCl produced

when 1.25 mol Cl2 reacts with Na.

• How many grams of chlorine gas can be obtained from the decomposition of 2.50 mol of NaCl?

* Determine the number of moles of NaCl required to obtain 3.4 g of sodium when NaCl is decomposed.

Mass to Mass Conversions• What do we need to do in a mass to mass

conversion?– Convert a given mass to moles.– From moles known, calculate moles unknown.– Convert moles unknown to mass unknown.

29

29 29

29 8.6g

In Summary1 - Convert mass of “known” to moles.

2 - Do a mole to mole conversion between

moles of known & moles of unknown.• Write a balanced equation.• Identify the known and unknown.• Use the balanced equation to determine the mole ratio in which

mol unknown is the numerator and mol known is the denominator.

• Multiply the known by the mole ratio.

3 - Convert moles of unknown to mass.

Practice Problems• Ammonium nitrate, an important fertilizer,

produces dinitrogen oxide and water when it decomposes. Determine the mass of water produced from the decomposition of 25.0 g of ammonium nitrate.

• The decomposition of NaN3 produces the nitrogen gas that inflates air bags. Determine the mass of N2 produced if

100. g NaN3 is decomposed.

Gas Stoichiometry (pgs. 440-1)

• Objectives– Determine volume ratios for gaseous

reactants and products by using coefficients from a chemical equation.

– Calculate amounts of gaseous reactants and products in a chemical reaction

Volume to Volume Conversions

• Recall that coefficients in chemical reactions can be interpreted as particles, moles, & mass.

• Since Avogadro’s principle states that equal volumes of gases at the same temperature & pressure contain equal numbers of particles, the coefficients can also represent relative volumes when gases are involved.

Volume to Volume Conversions

• 2C4H10(g) + 13O2(g) 8CO2(g) + 10H2O(g)

• Interpreted in terms of volumes:– 2 L butane reacts with 13 L of oxygen gas to

produce 8 L of carbon dioxide and 10 L of water vapor.

OR– 2 mL butane reacts with 13 mL of oxygen gas to

produce 8 mL of carbon dioxide and 10 mL of water vapor.

Volume to Volume Conversions• In order to find the volume of a reactant or

product in a reaction, you will need a balanced chemical equation and the volume of at least one gas involved in the reaction.

• Because the coefficients represent volumes, we can form volume ratios (much like mole ratios) from the balanced equation.

• We will then follow essentially the same steps as in mole-mole stoichiometric calculations.

CH4(g) + 2O2(g) CO2(g) + 2H2O(g)• What volume of methane is needed to produce

26L of water vapor?1 -Equation is balanced.

2 -Known: 26L H2O; Unknown: Vol of methane.

3 -Volume ratio between unknown and known is: 1 L CH4

2 L H2O

4 -Multiply known by ratio: 26 L H2O x 1 L CH4 =

2 L H2O

13 L CH4

Practice Problems• What volume of oxygen is needed to react with

solid sulfur to form 3.5 L of sulfur dioxide?

• Determine the volume of hydrogen gas needed to react completely with 5.00 L of oxygen to form water.

• How many liters of propane gas (C3H8) will undergo complete combustion with 34.0 L of oxygen gas?

Advanced Gas Stoichiometry

• If conditions are at STP, it is possible, using gas stoichiometry and mole conversions, to calculate moles, mass, and/or #’s of particles of gas from a given volume of gas in a chemical reaction.

• (We can also calculate volume from moles, mass, and/or #’s of particles.)

NH4NO3(s) N2O(g) + 2H2O(g)• Calculate the mass of ammonium nitrate that must

be used to obtain 0.100 L of dinitrogen gas at STP.– Equation is balanced.

– Known: 0.100 L N2O;

Unknown: mass of NH4NO3

* Since it is not possible to get a volume ratio with a solid, we will need to use a mole ratio between the unknown and known (1 mol NH4NO3/1 mol N2O) AFTER we convert 0.100 L to moles!

NH4NO3(s) N2O(g) + 2H2O(g)• Convert L to moles

0.100 L N2O x 1 mole = 0.00446 mol N2O 22.4 L• Use the mole ratio between known & unknown

0.00446 mol N2O x 1 mol NH4NO3 = 1 mol N2O 0.00446 mol NH4NO3

• Convert from moles to grams

0.00446 mol NH4NO3 x 80 g = 0.357 g NH4NO3

1 mol

Practice Problems• When calcium carbonate is heated, it

decomposes to form solid calcium oxide and carbon dioxide gas. How many liters of gas are produced at STP if 2.38 kg of calcium carbonate reacts completely?

• Iron rusts by reacting with oxygen to form iron (III) oxide. Calculate the volume of oxygen gas at STP that is required to completely react with 52.0 g of iron.