Topic 16 Topic 16

Topic 16: Stoichiometry Basic Concepts Additional Concepts Table of Contents Topic 16 Topic 16

Using the methods of stoichiometry, we can measure the amounts of substances involved in chemical reactions and relate them to one another. Stoichiometry Stoichiometry: Basic Concepts For example, a sample’s mass or volume can be converted to a count of the number of its particles, such as atoms, ions, or molecules. Topic 16 Topic 16

The group or unit of measure used to count numbers of atoms, molecules, or formula units of substances is the mole (abbreviated mol). Stoichiometry Stoichiometry: Basic Concepts The number of things in one mole is 6.02 x This big number has a short name: the Avogadro constant. Topic 16 Topic 16

Methanol is formed from CO 2 gas and hydrogen gas according to the balanced chemical equation below. Molar Mass Stoichiometry: Basic Concepts Topic 16 Topic 16

Molar Mass Stoichiometry: Basic Concepts Suppose you wanted to produce 500 g of methanol. Topic 16 Topic 16 How many grams of CO 2 gas and H 2 gas would you need? How many grams of water would be produced as a by-product? Those are questions about the masses of reactants and products.

But the balanced chemical equation shows that three molecules of hydrogen gas react with one molecule of carbon dioxide gas. Molar Mass Stoichiometry: Basic Concepts The equation relates molecules, not masses, of reactants and products. Topic 16 Topic 16

Like Avogadro, you need to relate the macroscopic measurements—the masses of carbon dioxide and hydrogen—to the number of molecules of methanol. Molar Mass Stoichiometry: Basic Concepts To find the mass of carbon dioxide and the mass of hydrogen needed to produce 500 g of methanol, you first need to know how many molecules of methanol are in 500 g of methanol. Topic 16 Topic 16

Average atomic masses of the elements are given on the periodic table. Molar Mass of an Element Stoichiometry: Basic Concepts For example, the average mass of one iron atom is 55.8 u, where u means “atomic mass units.” Topic 16 Topic 16

The atomic mass unit is defined so that the atomic mass of an atom of the most common carbon isotope is exactly 12 u, and the mass of 1 mol of the most common isotope of carbon atoms is exactly 12 g. Molar Mass of an Element Stoichiometry: Basic Concepts Topic 16 Topic 16

Molar Mass of an Element Stoichiometry: Basic Concepts The mass of 1 mol of a pure substance is called its molar mass. Topic 16 Topic 16

The molar mass is the mass in grams of the average atomic mass. Molar Mass of an Element Stoichiometry: Basic Concepts If an element exists as a molecule, remember that the particles in 1 mol of that element are themselves composed of atoms. Topic 16 Topic 16

For example, the element oxygen exists as molecules composed of two oxygen atoms, so a mole of oxygen molecules contains 2 mol of oxygen atoms. Molar Mass of an Element Stoichiometry: Basic Concepts Therefore, the molar mass of oxygen molecules is twice the molar mass of oxygen atoms: 2 x g = g. Topic 16 Topic 16

The mass of an iron bar is 16.8 g. How many Fe atoms are in the sample? Number of Atoms in a Sample of an Element Stoichiometry: Basic Concepts Use the periodic table to find the molar mass of iron. Topic 16 Topic 16 Use the periodic table to find the molar mass of iron. The average mass of an iron atom is 55.8 u. Then the mass of 1 mol of iron atoms is 55.8 g.

To convert the mass of the iron bar to the number of moles of iron, use the mass of 1 mol of iron atoms as a conversion factor. Stoichiometry: Basic Concepts Topic 16 Topic 16 Now, use the number of atoms in a mole to find the number of iron atoms in the bar. Number of Atoms in a Sample of an Element

Simplify the expression above. Stoichiometry: Basic Concepts Topic 16 Topic 16 Number of Atoms in a Sample of an Element

Covalent compounds are composed of molecules, and ionic compounds are composed of formula units. Molar Mass of a Compound Stoichiometry: Basic Concepts Topic 16 Topic 16 The molecular mass of a covalent compound is the mass in atomic mass units of one molecule. Its molar mass is the mass in grams of 1 mol of its molecules.

The formula mass of an ionic compound is the mass in atomic mass units of one formula unit. Molar Mass of a Compound Stoichiometry: Basic Concepts Topic 16 Topic 16 Its molar mass is the mass in grams of 1 mol of its formula units. How to calculate the molar mass for ethanol, a covalent compound, and for calcium chloride, an ionic compound, is shown.

Ethanol, C 2 H 6 O, a covalent compound. Molar Mass of a Compound Stoichiometry: Basic Concepts Topic 16 Topic 16

Calcium chloride, CaCl 2, an ionic compound. Molar Mass of a Compound Stoichiometry: Basic Concepts Topic 16 Topic 16

The mass of a quantity of iron(III) oxide is 16.8 g. How many formula units are in the sample? Number of Formula Units in a Sample of a Compound Stoichiometry: Basic Concepts Topic 16 Topic 16 Use the periodic table to calculate the mass of one formula unit of Fe 2 O 3.

Stoichiometry: Basic Concepts Topic 16 Topic 16 Therefore, the molar mass of Fe 2 O 3 (rounded off) is 160 g. Number of Formula Units in a Sample of a Compound

Now, multiply the number of moles of iron oxide by the number in a mole. Stoichiometry: Basic Concepts Topic 16 Topic 16 Number of Formula Units in a Sample of a Compound

What mass of water must be weighed to obtain 7.50 mol of H 2 O? Mass of a Number of Moles of a Compound Stoichiometry: Basic Concepts Topic 16 Topic 16 The molar mass of water is obtained from its molecular mass. The molar mass of water is 18.0 g/mol.

Use the molar mass to convert the number of moles to a mass measurement. Stoichiometry: Basic Concepts Topic 16 Topic 16 Mass of a Number of Moles of a Compound

The concept of molar mass makes it easy to determine the number of particles in a sample of a substance by simply measuring the mass of the sample. Stoichiometry: Basic Concepts Topic 16 Topic 16 The concept is also useful in relating masses of reactants and products in chemical reactions. Mass of a Number of Moles of a Compound

Ammonia gas is synthesized from nitrogen gas and hydrogen gas according to the balanced chemical equation below. Predicting Mass of a Reactant Stoichiometry: Basic Concepts Topic 16 Topic 16

Predicting Mass of a Reactant Stoichiometry: Basic Concepts Topic 16 Topic 16 How many grams of hydrogen gas are required for 3.75 g of nitrogen gas to react completely? Find the number of moles of N 2 molecules by using the molar mass of nitrogen.

To find the mass of hydrogen needed, first find the number of moles of H 2 molecules needed to react with all the moles of N 2 molecules. Predicting Mass of a Reactant Stoichiometry: Basic Concepts Topic 16 Topic 16 The balanced chemical equation shows that 3 mol of H 2 molecules react with 1 mol of N 2 molecules.

Multiply the number of moles of N 2 molecules by this ratio. Predicting Mass of a Reactant Stoichiometry: Basic Concepts Topic 16 Topic 16 The units in the expression above simplify to moles of H 2 molecules.

To find the mass of hydrogen, multiply the number of moles of hydrogen molecules by the mass of 1 mol of H 2 molecules, which is 2.00 g. Predicting Mass of a Reactant Stoichiometry: Basic Concepts Topic 16 Topic 16

What mass of ammonia is formed when 3.75 g of nitrogen gas react with hydrogen gas according to the balanced chemical equation below? Predicting Mass of a Product Stoichiometry: Basic Concepts Topic 16 Topic 16 The amount of ammonia formed depends upon the number of nitrogen molecules present and the mole ratio of nitrogen and ammonia in the balanced chemical equation.

The number of moles of nitrogen molecules is given by the expression below. Predicting Mass of a Product Stoichiometry: Basic Concepts Topic 16 Topic 16

Predicting Mass of a Product Stoichiometry: Basic Concepts Topic 16 Topic 16 To find the mass of ammonia produced, first find the number of moles of ammonia molecules that form from 3.75 g of nitrogen. Use the mole ratio of ammonia molecules to nitrogen molecules to find the number of moles of ammonia formed.

Predicting Mass of a Product Stoichiometry: Basic Concepts Topic 16 Topic 16 Use the molar mass of ammonia, 17.0 g, to find the mass of ammonia formed.

Basic Assessment Questions Question 1 Determine the number of atoms in 45.6 g gold, Au. Topic 16 Topic 16

Basic Assessment Questions Answer 1.39 x Au atoms Topic 16 Topic 16

Basic Assessment Questions Question 2 Determine the number of atoms in 17.5 g copper(II) oxide, CuO. Topic 16 Topic 16

Basic Assessment Questions Answer 2.65 X Atoms Topic 16 Topic 16

Basic Assessment Questions Question 3 Determine the mass of 1.25 mol aspirin C 9 H 8 O 4. Topic 16 Topic 16

Basic Assessment Questions Answer 225g C 9 H 8 O 4 Topic 16 Topic 16

Basic Assessment Questions Question 4 What mass of sulfur must burn to produce 3.42 L of SO 2 at 273°C and 101 kPa? The reaction is Topic 16 Topic 16

Basic Assessment Questions Answer 2.45 g S Topic 16 Topic 16

Stoichiometry: Additional Concepts Topic 16 Topic 16 Additional Concepts

Stoichiometry: Additional Concepts Stoichiometric Calculations There are three basic stoichiometric calculations: mole-to-mole conversions, mole-to-mass conversions, and mass-to- mass conversions. All stoichiometric calculations begin with a balanced equation and mole ratios. Topic 16 Topic 16 Stoichiometric mole-to-mole conversion How can you determine the number of moles of table salt (NaCl) produced from 0.02 moles of chlorine (Cl 2 )?

Stoichiometry: Additional Concepts Stoichiometric mole-to-mole conversion First, write the balanced equation. Then, use the mole ratio to convert the known number of moles of chlorine to the number of moles of table salt. Use the formula below. Topic 16 Topic 16

Stoichiometry: Additional Concepts Stoichiometric Mole- to-Mass Conversion A mole-to-mass conversion allows you to calculate the mass of a product or reactant in a chemical reaction given the number of moles of a reactant or product. The following reaction occurs in plants undergoing photosynthesis. Topic 16 Topic 16 Stoichiometric Mole-to-Mass Conversion

Stoichiometry: Additional Concepts Stoichiometric Mole-to-Mass Conversion How many grams of glucose (C 6 H 12 O 6 ) are produced when 24.0 moles of carbon dioxide reacts in excess water? Determine the number of moles of glucose produced by the given amount of carbon dioxide. Topic 16 Topic 16

Stoichiometry: Additional Concepts Stoichiometric Mole-to-Mass Conversion Multiply by the molar mass. 721 grams of glucose is produced from 24.0 moles of carbon dioxide. Topic 16 Topic 16

Stoichiometry: Additional Concepts Stoichiometric Mass-to-Mass Conversion In this calculation, you can find the mass of an unknown substance in a chemical equation if you have the balanced chemical equation and know the mass of one substance in the equation. Topic 16 Topic 16

Stoichiometry: Additional Concepts Stoichiometric Mass-to-Mass Conversion How many grams of sodium hydroxide (NaOH) are needed to completely react with 50.0 grams of sulfuric acid (H 2 SO 4 ) to form sodium sulfate (Na 2 SO 4 ) and water? Topic 16 Topic 16 Write the balanced equation.

Stoichiometry: Additional Concepts Stoichiometric Mass-to-Mass Conversion Convert grams of sulfuric acid to moles NaOH. Topic 16 Topic 16

Stoichiometry: Additional Concepts Stoichiometric Mass-to-Mass Conversion Calculate the mass of NaOH needed. Topic 16 Topic grams of H 2 SO 4 reacts completely with 40.8 grams of NaOH.

Stoichiometry: Additional Concepts Limiting Reactants Rarely are the reactants in a chemical reaction present in the exact mole ratios specified in the balanced equation. Topic 16 Topic 16 Usually, one or more of the reactants are present in excess, and the reaction proceeds until all of one reactant is used up. The reactant that is used up is called the limiting reactant.

Stoichiometry: Additional Concepts Limiting Reactants The limiting reactant limits the reaction and, thus, determines how much of the product forms. Topic 16 Topic 16 The left-over reactants are called excess reactants. How can you determine which reactant in a chemical reaction is limited? First, find the number of moles of each reactant by multiplying the given mass of each reactant by the inverse of the molar mass.

Stoichiometry: Additional Concepts Determining the Limiting Reactant In the reaction below, 40.0 g of sodium hydroxide (NaOH) reacts with 60.0 g of sulfuric acid (H 2 SO 4 ). Topic 16 Topic 16

Stoichiometry: Additional Concepts Determining the Limiting Reactant To determine the limiting reactant, calculate the actual ratio of available moles of reactants. Topic 16 Topic 16

Stoichiometry: Additional Concepts Determining the Limiting Reactant You can see that when 0.5 mol H 2 SO 4 has reacted, all of the 1.00 mol of NaOH would be used up. Topic 16 Topic 16 Some H 2 SO 4 would remain unreacted. Thus, NaOH is the limiting reactant. So, is available. Compare this ratio with the mole ratio from the balanced equation:, or

Stoichiometry: Additional Concepts Determining the Limiting Reactant To calculate the mass of Na 2 SO 4 that can form from the given reactants, multiply the number of moles of the limiting reactant (NaOH) by the mole ratio of the product to the limiting reactant and then multiply by the molar mass of the product. Topic 16 Topic 16

Stoichiometry: Additional Concepts Determining the Limiting Reactant Topic 16 Topic g of Na 2 SO 4 can form from the given amounts of the reactants.

Additional Assessment Questions Question 1 Topic 16 Topic 16 Balance the following equation. How many moles of KClO 3 are needed to produce 50 moles of O 2 ?

Additional Assessment Questions Answer Topic 16 Topic 16

Additional Assessment Questions Question 2 Topic 16 Topic 16 Calculate the mass of sodium chloride (NaCl) produced when 5.50 moles of sodium reacts in excess chlorine gas.

Additional Assessment Questions Answer Topic 16 Topic g NaCl

Additional Assessment Questions Question 3 Topic 16 Topic 16 Determine the mass of copper needed to react completely with a solution containing 12.0 g of silver nitrate (AgNO 3 ).

Additional Assessment Questions Answer Topic 16 Topic g Cu

Additional Assessment Questions Question 4 Topic 16 Topic 16 Aluminum reacts with chlorine to produce aluminum chloride.

Balance the equation. Answer 4a Question 4a Additional Assessment Questions Topic 16 Topic 16

If you begin with 3.2 g of aluminum and 5.4 g of chlorine, which is the limiting reactant? Answer 4b Question 4b Additional Assessment Questions Topic 16 Topic 16 Cl 2

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