1. Chemistry for Changing Times 12th Edition Hill and Kolb
Chapter 5
Chemical Accounting: Mass and Volume Relationships
John Singer
Jackson Community College, Jackson, MI
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2. Chemical Sentences: Equations
Chemical equations represent the sentences in the language of chemistry. They are the means of communicating a chemical change using the symbols and formulas to represent the elements and compounds involved in a chemical reaction.
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3. Chemical Sentences: Equations
Reactants are the species present before the reaction.
Products are the species present after the reaction:
Reactants → Products
The arrow (→) means “yield(s)” or “react(s) to produce”.
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4. Chemical Sentences: Equations
The following are used to denote the states of matter of a species in an equation:
(s) = solid
(l) = liquid
(g) = gas
(aq) = aqueous solution
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5. Chemical Sentences: Equations
Coefficients are numbers used to balance a chemical equation. Never change the subscripts.
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6. Volume Relationships in Chemical Equations
Law of Combined Volumes: When all measurements are made at the same temperature and pressure, the volumes of gaseous reactants and products are in a small whole-number ratio.
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7. Volume Relationships in Chemical Equations
Avogadro’s hypothesis: When measured at the same temperature and pressure, volumes of all gases contain the same number of molecules.
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Avogadro’s Number
Avogadro’s number is defined as the number of atoms in a 12-g sample of carbon-12 and is:
6.02 x 1023
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The Mole
A mole (mol) is defined as the amount of a substance that contains 6.02 x 1023 particles.
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The Mole
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The Mole
Formula mass is the average mass of a formula unit relative to that of a carbon-12 atom.
It is simply the sum of the atomic masses for all atoms in a formula.
If the formula represents a molecule, often the term molecular mass is used.
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The Mole
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The Mole
Molar volume of a gas: One mole of any gas occupies a volume of 22.4 L at standard temperature and pressure (STP).
STP is defined as 1 atmosphere (atm) of pressure and a temperature of 0 oC.
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The Mole
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15. Mole and Mass Relationships in Chemical Equations
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16. Mole and Mass Relationships in Chemical Equations
Stoichiometry involves the quantitative relationship between reactants and products in a balanced chemical equation.
The coefficients of a balanced chemical equation represent moles.
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17. Mole and Mass Relationships in Chemical Equations
2 H2 + O2  2 H2O
This equation can be read as follows:
2 mol of H2 reacts with one mol O2 to yield
2 mol of H2O.
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18. Mole and Mass Relationships in Chemical Equations
Steps in a Stoichiometric Calculation:
Write and balance the chemical equation for the reaction.
Determine molar masses of substances involved in the calculation.
Use the coefficients of the balanced equation to convert the moles of the given substance to the moles of the desired substance.
Use the molar mass to convert the moles of the desired substance to grams of the desired substance.
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19. Mole and Mass Relationships in Chemical Equations
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Solutions
The amount of solute in a given amount of solvent is defined as solution concentration.
A dilute solution contains relative small amounts of solute in a given amount of solvent.
A concentrated solution contains relatively large amounts of solute in a given amount of solvent.
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Solutions
Molarity (M) is defined as the moles of solute per liter of solution.
M =
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Solutions
Percent Concentration
Percent by volume = x 100
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Solutions
Percent Concentration
Percent by mass = x 100
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