Equation Stoichiometry.notebook November 21, 2014 · Solve for the mass of one component (usually...

Equation Stoichiometry.notebook

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November 21, 2014


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November 21, 2014

The coefficients in a balanced chemical equation represent moles of reactants and moles of products. The mole ratio (or proportion) of reactants and products in a balanced equation must remain constant. This is how they react stoichiometrically, so to speak.

A simple analogy would be in baking a cake. Let’s say to bake one cake you need the following ingredients:

2 eggs1 cup flour1 teaspoon vanilla all on left = 1 cake½ cup of sugar½ cup of milks by yeast:

If you we wanted to bake three cakes,

How many eggs would we need?

What about the sugar?

How many teaspoons of vanilla?

Get the idea?

The same idea applies to the mole ratios of reactants and products in a balanced equation.


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MoleMole Calculations

Given the reaction for the fermentation of simple sugar:

C6H12O6(aq) à 2 C2H5OH(l) + 2 CO2(g)

glucose alcohol carbon dioxide

mole ratio: ___ ___ ___

or ___ ___ ___

or ___ ___ ___

Problem 1

How many moles of ethanol (the alcohol) can be produced by the complete fermentation of 12 moles of glucose?

Problem 2

How many moles of glucose must be fermented to produce 5 moles of carbon dioxide?


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MoleMass and MassMole Calculations

If you understand molemole idea, a molemass or massmole simply involves an additional step of converting mole to mass or mass to mole.

Write a balanced equation for the complete combustion of acetylene gas, C2H2(g):

Problem 3

Calculate the mass of carbon dioxide gas that can be produced by the complete combustion of 10.0 moles of acetylene:

Problem 4 How many moles of water can be produced if 104. grams of acetylene completely reacts with oxygen gas?


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Problem 5

Calculate the mass of hydrogen produced by the electrolysis of 25.0 moles of water.

Problem 6

Determine the mass of glucose produced by a plant using the photosynthesis reaction if it absorbs 2.00 moles of water.


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MoleVolumeMolecules Calculations

Recall we learned that one mole of any gas at STP (0oC and1 atmosphere pressure) occupies a volume of 22.4 liters and that one mole contains Avogadro’s number of particles. Let’s try some stoichiometry problems involving molecules and gas volumes at STP.

Problem 7

Calculate the volume of oxygen gas produced from the decomposition of 2.50 moles of sodium chlorate when the gas is measured at STP:

Problem 8

Determine the moles of calcium carbonate that must be decomposed to produce 28.0 L of carbon dioxide gas when the gas is measured at STP:


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Problem 9

Calculate the number of oxygen molecules required to react with 0.750 g of magnesium metal to produce magnesium oxide.

Problem 10

Elemental phosphorous (which occurs as a tetratomic molecule, P4) reacts with chlorine gas to produce phosphorous trichloride gas. Determine the number of phosphorous molecules and the volume of chlorine gas, when measured at STP, needed to produce 79.5 L of phosphorous trichloride.


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November 21, 2014

MassMass Calculations

In this type of problem, we are given the mass of one component in the reaction and asked to calculate the mass of another. These types of problems involve the following conversions:

mass A à mole A à mole B à mass B (from coefficients

in equation)

Problem 11

How many grams of ammonia can be made by reacting 25.0 g of nitrogen with sufficient hydrogen?


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Problem 12

Write a balanced equation for the neutralization of solid magnesium hydroxide by hydrochloric acid. If 5.00 g of solid magnesium hydroxide is swallowed, what mass of hydrochloric can be neutralized? (Caution: do not ingest magnesium hydroxide directly – take an antacid tablet containing MOM).

Problem 13

If 3.20 g of sodium hydroxide is dissolved in water and reacts with sufficient aqueous copper(II)chloride, calculate the mass of precipitate that will form.


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November 21, 2014

Limiting Reactant Problems

Thus far we have done stoichiometry problems where we always assumed that the given quantities of reactants in a reaction occurred in “stoichiometric amounts.” This is not always the case. In a limiting reactant problem, one of the reactants “runs out” first and is termed the limiting reactant (LR). The LR will thus dictates the amount of products formed. The other reactant(s) is therefore in “excess” (XS).

There are different ways to solve limiting reactant problems, but what most students find easiest is just to apply the massmass idea in solving these problems. Solve for the mass of one component (usually a product) by assuming the first quantity of reactant given is limiting reactant. Do the same for the second quantity of reactant given. Whichever reactant results in the lower amount of product is the limiting reactant!


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reactant results in the lower amount of product is the limiting reactant!

Problem 14

Ethanol, C2H5OH, reacts with oxygen gas to produce carbon dioxide and water vapor. If 50.0 g of ethanol reacts with 72.0g of oxygen gas, what mass of carbon dioxide gas will be produced?

1. mass ethanol à mole ethanol à mole carbon dioxide à mass carbon dioxide if ethanol is limiting reactant:

2. mass oxygen à mole oxygen à mole carbon dioxide à mass carbon dioxide if oxygen is limiting reactant:

3. Which reactant will run out first? Which will be in excess (XS)?

Now let’s calculate the mass of excess reactant left over after the LR runs out. One way to do this is to determine the mass of excess reactant that reacts and subtract that mass from the original mass of the excess reactant:


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Problem 15

If 25.4 grams of hydrogen gas reacts with 64.8 g of nitrogen gas to produce ammonia gas (NH3),

1. write a BME2. determine the LR3. determine the mass of ammonia produced4. determine the mass of XS reactant left over


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November 21, 2014

Percent Yield

The percent yield of a reaction is the amount of substance (product) you actually obtain in a reaction divided by the theoretical amount you should obtain X 100. The theoretical amount is the stoichiometric amount we have been calculating in our problems thus far.

Problem 16

Hydrogen peroxide can be decomposes to water and oxygen gas. Write a balanced equation for this decomposition below:

a) Calculate the volume of oxygen gas that would be produced if 175 g of hydrogen peroxide is decomposed at STP:

b) If 51.4 L of oxygen gas is actually obtained, determine the percent yield:


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Problem 17

In the decomposition of exactly 12.50 g of copper(II)carbonate, 7.50 g of copper(II)oxide was obtained (along with the oxygen gas). Calculate the percent yield:


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Energy in Calculations

In these problems the energy term (in kilojoules) is in the same proportion as the moles of reactants and moles of products. Let’s take an example.

Problem 18

According to Reference Table I (found on the last page), 3351 kJ of energy is released when 2 moles of aluminum oxide is formed from its elements at 101.3 kPa and 298 K (these are standard conditions). Write the balanced equation (including the energy term), and calculate the amount of energy that will be released when 408.0 g of aluminum oxide is formed from its elements.

Problem 19

According the Reference Table I, determine the amount of heat energy involved when 200.0 g of solid sodium hydroxide is dissolved in water. Include a balanced equation.


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Problem 20

Calculate the amount of heat energy involved in the reaction of 5.00 g of iodine with excess hydrogen gas. Is the heat released or absorbed? Explain.


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Equation Stoichiometry.notebook November 21, 2014 · Solve for the mass of one component (usually...

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Transcript of Equation Stoichiometry.notebook November 21, 2014 · Solve for the mass of one component (usually...