# Stoichiometry: Basic Concepts

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Stoichiometry: Basic Concepts
Topic 16 Stoichiometry Using the methods of stoichiometry, we can measure the amounts of substances involved in chemical reactions and relate them to one another. 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.

Even grouping them by millions would not help.
Stoichiometry: Basic Concepts Topic 16 Stoichiometry Atoms are so tiny that an ordinary-sized sample of a substance contains so many of these submicroscopic particles that counting them by grouping them in thousands would be unmanageable. Even grouping them by millions would not help.

Stoichiometry: Basic Concepts
Topic 16 Stoichiometry The group or unit of measure used to count numbers of atoms, molecules, or formula units of substances is the mole (abbreviated mol). The number of things in one mole is 6.02 x This big number has a short name: the Avogadro constant. The most precise value of the Avogadro constant is x For most purposes, rounding to 6.02 x 1023 is sufficient.

Stoichiometry: Basic Concepts
Topic 16 Molar Mass Methanol is formed from CO2 gas and hydrogen gas according to the balanced chemical equation below.

Suppose you wanted to produce 500 g of methanol.
Stoichiometry: Basic Concepts Topic 16 Molar Mass Suppose you wanted to produce 500 g of methanol. How many grams of CO2 gas and H2 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.

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

Stoichiometry: Basic Concepts
Topic 16 Molar Mass Like Avogadro, you need to relate the macroscopic measurements—the masses of carbon dioxide and hydrogen—to the number of molecules of methanol. 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.

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

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

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

Molar Mass of an Element
Stoichiometry: Basic Concepts Topic 16 Molar Mass of an Element 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. Therefore, the molar mass of oxygen molecules is twice the molar mass of oxygen atoms: 2 x g = g.

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

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

Molar Mass of a Compound
Stoichiometry: Basic Concepts Topic 16 Molar Mass of a Compound Covalent compounds are composed of molecules, and ionic compounds are composed of formula units. 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.

Molar Mass of a Compound
Stoichiometry: Basic Concepts Topic 16 Molar Mass of a Compound The formula mass of an ionic compound is the mass in atomic mass units of one formula unit. 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.

Molar Mass of a Compound
Stoichiometry: Basic Concepts Topic 16 Molar Mass of a Compound Ethanol, C2H6O, a covalent compound.

Molar Mass of a Compound
Stoichiometry: Basic Concepts Topic 16 Molar Mass of a Compound Calcium chloride, CaCl2, an ionic compound.

Number of Formula Units in a Sample of a Compound
Stoichiometry: Basic Concepts Topic 16 Number of Formula Units in a Sample of a Compound The mass of a quantity of iron(III) oxide is 16.8 g. How many formula units are in the sample? Use the periodic table to calculate the mass of one formula unit of Fe2O3.

Number of Formula Units in a Sample of a Compound
Stoichiometry: Basic Concepts Topic 16 Number of Formula Units in a Sample of a Compound Therefore, the molar mass of Fe2O3 (rounded off) is 160 g.

Number of Formula Units in a Sample of a Compound
Stoichiometry: Basic Concepts Topic 16 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.

Mass of a Number of Moles of a Compound
Stoichiometry: Basic Concepts Topic 16 Mass of a Number of Moles of a Compound What mass of water must be weighed to obtain 7.50 mol of H2O? The molar mass of water is obtained from its molecular mass. The molar mass of water is 18.0 g/mol.

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

Mass of a Number of Moles of a Compound
Stoichiometry: Basic Concepts 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. The concept is also useful in relating masses of reactants and products in chemical reactions. Cinda, this was highlighted below basic assessment question #3. It didn’t have a slide number so I put it here.

Predicting Mass of a Reactant
Stoichiometry: Basic Concepts Topic 16 Predicting Mass of a Reactant 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 Predicting Mass of a Reactant How many grams of hydrogen gas are required for 3.75 g of nitrogen gas to react completely? Find the number of moles of N2 molecules by using the molar mass of nitrogen.

Predicting Mass of a Reactant
Stoichiometry: Basic Concepts Topic 16 Predicting Mass of a Reactant To find the mass of hydrogen needed, first find the number of moles of H2 molecules needed to react with all the moles of N2 molecules. The balanced chemical equation shows that 3 mol of H2 molecules react with 1 mol of N2 molecules.

Predicting Mass of a Reactant
Stoichiometry: Basic Concepts Topic 16 Predicting Mass of a Reactant Multiply the number of moles of N2 molecules by this ratio. The units in the expression above simplify to moles of H2 molecules.

Predicting Mass of a Reactant
Stoichiometry: Basic Concepts Topic 16 Predicting Mass of a Reactant To find the mass of hydrogen, multiply the number of moles of hydrogen molecules by the mass of 1 mol of H2 molecules, which is 2.00 g.

Predicting Mass of a Product
Stoichiometry: Basic Concepts Topic 16 Predicting Mass of a Product What mass of ammonia is formed when 3.75 g of nitrogen gas react with hydrogen gas according to the balanced chemical equation below? 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.

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

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