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

• 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 ConceptsStoichiometry: Basic ConceptsStoichiometry: Basic ConceptsStoichiometry: 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.

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• 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.

Stoichiometry


• Even grouping them by millions would not help.

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• The group or unit of measure used to count numbers of atoms, molecules, or formula units of substances is the mole (abbreviated mol).

Stoichiometry


• The number of things in one mole is 6.02 x 1023. This big number has a short name: the Avogadro constant.

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• The most precise value of the Avogadro constant is 6.0221367 x 1023. For most purposes, rounding to 6.02 x 1023 is sufficient.

• Methanol is formed from CO2 gas and hydrogen gas according to the balanced chemical equation below.

Molar Mass

Stoichiometry: Basic ConceptsStoichiometry: Basic ConceptsStoichiometry: Basic ConceptsStoichiometry: Basic ConceptsTopic 16Topic 16

Molar Mass


• Suppose you wanted to produce 500 g of methanol.

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• 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.

• But the balanced chemical equation shows that three molecules of hydrogen gas react with one molecule of carbon dioxide gas.

Molar Mass


• The equation relates molecules, not masses, of reactants and products.

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• 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


• 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.

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• Average atomic masses of the elements are given on the periodic table.

Molar Mass of an Element


• For example, the average mass of one iron atom is 55.8 u, where u means “atomic mass units.”

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• 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.





• The mass of 1 mol of a pure substance is called its molar mass.

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• 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.

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• 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 16.00 g = 32.00 g.

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• 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


• Use the periodic table to find the molar mass of iron.

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• 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.


• 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 ConceptsStoichiometry: Basic ConceptsStoichiometry: Basic ConceptsStoichiometry: Basic ConceptsTopic 16Topic 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


• 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.



• 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, C2H6O, a covalent compound.



• Calcium chloride, CaCl2, an ionic compound.



• 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


• Use the periodic table to calculate the mass of one formula unit of Fe2O3.


• Therefore, the molar mass of Fe2O3 (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 ConceptsStoichiometry: Basic ConceptsStoichiometry: Basic ConceptsStoichiometry: Basic ConceptsTopic 16Topic 16 Number of Formula Units in a Sample

of a Compound

• What mass of water must be weighed to obtain 7.50 mol of H2O?

Mass of a Number of Moles of a Compound


• 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 ConceptsStoichiometry: Basic ConceptsStoichiometry: Basic ConceptsStoichiometry: Basic ConceptsTopic 16Topic 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.

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




• 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.

• 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.

• Multiply the number of moles of N2 molecules by this ratio.



• The units in the expression above simplify to moles of H2 molecules.

• 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.



• 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


• 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.





• 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.



• Use the molar mass of ammonia, 17.0 g, to find the mass of ammonia formed.

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