using the methods of stoichiometry, we can measure the amounts of substances involved in chemical...

• 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.

Topic 16Topic 16

• 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.

Topic 16Topic 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


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

Topic 16Topic 16

• 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.

Topic 16Topic 16

• 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.

Topic 16Topic 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


• 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 16Topic 16

• 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.”

Topic 16Topic 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.





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

Topic 16Topic 16

• 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.

Topic 16Topic 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.



• Therefore, the molar mass of oxygen molecules is twice the molar mass of oxygen atoms: 2 x 16.00 g = 32.00 g.

Topic 16Topic 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


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

Topic 16Topic 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.


• 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.

using the methods of stoichiometry, we can measure the amounts of substances involved in chemical...

Documents

molar mass stoichiometry

molar mass of iron

formula mass

average mass

basic concepts topic

mass of hydrogen

molecular mass

samples mass