SAMPLE PROBLEM
8.1
Solutions of Electrolytes and Nonelectrolytes
Indicate whether solutions of each of the following contain only ions, only molecules, or mostly molecules and a
few ions:
a. Na2SO4, a strong electrolyte
b. CH3OH, a nonelectrolyte
SOLUTION
a. A solution of Na2SO4 contains only the ions Na+ and SO42–.
b. A nonelectrolyte such as CH3OH dissolves only as molecules.
STUDY CHECK
Boric acid, H3BO3, is a weak electrolyte. Would you expect a boric acid solution to contain
only ions, only molecules, or mostly molecules and a few ions?
Timberlake: General, Organic & Biological Chemistry
Copyright ©2010 by Pearson Education, Inc.
SAMPLE PROBLEM
8.2
Electrolyte Concentration
In body fluids, concentrations of electrolytes are often expressed as milliequivalents (mEq) per liter. A typical
concentration for Ca2+ in the blood is 8.8 mEq/L.
a. How many moles of calcium ion are in 0.50 L of blood?
b. If chloride ion is the only other ion present, what is its concentration in mEq/L?
SOLUTION
a. Using the volume and the electrolyte concentration (in mEq/L), we can find the number of equivalents in
0.50 L of blood:
We can then convert equivalents to moles (for Ca2+ there are 2 Eq per mole):
b. If the concentration of Ca2+ is 8.8 mEq/L, then the concentration of Cl– must be 8.8 mEq/L to balance the
charge.
STUDY CHECK
A Ringer’s solution for intravenous fluid replacement contains 155 mEq Cl– per liter of solution. If a patient
receives 1250 mL of Ringer’s solution, how many moles of chloride were given?
Timberlake: General, Organic & Biological Chemistry
Copyright ©2010 by Pearson Education, Inc.
SAMPLE PROBLEM
8.3
Saturated Solutions
At 20 ˚C, the solubility of KCl is 34 g/100 g of water. In the laboratory, a student mixes 75 g of KCl with 200. g
of water at a temperature of 20 ˚C.
a. How much of the KCl can dissolve?
b. Is the solution saturated or unsaturated?
c. What is the mass of any solid KCl in the bottom of the container?
SOLUTION
a.
KCl has a solubility of 34 g of KCl in 100 g of water. Using solubility as a
conversion factor, the maximum amount of KCl that can dissolve in 200. g of water is calculated as follows:
b.
Because 75 g of KCl exceeds the amount that can dissolve in 200. g of water,
the KCl solution is saturated.
c. If we add 75 g of KCl to 200. g of water and only 68 g of KCl can dissolve, there is 7 g of solid (undissolved)
KCl on the bottom of the container.
STUDY CHECK
At 50 ˚C, the solubility of NaNO3 is 110 g/100 g of water. How many grams of NaNO3 are needed to make a
saturated NaNO3 solution with 50. g of water at 50 ˚C?
Timberlake: General, Organic & Biological Chemistry
Copyright ©2010 by Pearson Education, Inc.
SAMPLE PROBLEM
8.4
Factors Affecting Solubility
Indicate whether the solubility of the solute will increase or decrease in each of the following situations:
a. dissolving sugar using 80 ˚C water instead of 25 ˚C water
b. effect on the dissolved O2 in a lake as it warms
SOLUTION
a. An increase in the temperature increases the solubility of the sugar.
b. An increase in the temperature decreases the solubility of O2 gas.
STUDY CHECK
At 10 ˚C, the solubility of KNO3 is 30 g/100 g H2O. Would you expect the solubility of KNO3 to be higher or
lower at 40 ˚C? Explain.
Timberlake: General, Organic & Biological Chemistry
Copyright ©2010 by Pearson Education, Inc.
SAMPLE PROBLEM
8.5
Formation of an Insoluble Salt
Solutions of BaCl2 and K2SO4 are mixed and a white solid forms.
a. Write the net ionic equation.
b. What is the white solid that forms?
SOLUTION
a. STEP 1 STEP 2
BaSO4(s) is insoluble.
STEP 3
STEP 4
b. BaSO4 is the white solid.
STUDY CHECK
Predict whether a solid might form in each of the following mixtures of solutions. If so, write the net ionic
equation for the reaction.
a. NH4Cl(aq) + Ca(NO3)2(aq)
b. Pb(NO3)2(aq) + KCl(aq)
Timberlake: General, Organic & Biological Chemistry
Copyright ©2010 by Pearson Education, Inc.
SAMPLE PROBLEM
8.6
Calculating Mass Percent
What is the mass percent of a solution prepared by dissolving 30.0 g of NaOH in 120.0 g of H2O?
SOLUTION
STEP 1 Given 30.0 g of NaOH and 120.0 g of H2O
Need mass percent (m/m) of NaOH
STEP 2 Plan The mass percent is calculated by using the mass in grams of the
solute and solution in the definition of mass percent.
STEP 3 Equalities/Conversion Factors
STEP 4
Set Up Problem The mass of the solute and the solution are
obtained from the data:
Timberlake: General, Organic & Biological Chemistry
Copyright ©2010 by Pearson Education, Inc.
SAMPLE PROBLEM
8.6
Calculating Mass Percent (Continued)
STUDY CHECK
What is the mass percent (m/m) of NaCl in a solution made by dissolving 2.0 g of NaCl in 56.0 g of H2O?
Timberlake: General, Organic & Biological Chemistry
Copyright ©2010 by Pearson Education, Inc.
SAMPLE PROBLEM
8.7
Calculating Percent Concentration
A student prepared a solution by dissolving 5.0 g of KI in enough water to give a final volume of 250 mL. What is
the mass/volume percent (m/v) of the KI solution?
SOLUTION
STEP 1 Given 5.0 g of KI and 250 mL of solution
Need mass/volume percent (m/v) of KI
STEP 2 Plan The mass/volume percent is calculated by using the mass in grams of the solute and the volume
in mL of the solution in the definition of mass/volume percent.
STEP 3 Equalities/Conversion Factors Write the mass/volume percent expression.
STEP 4
Set Up Problem Substitute solute and solution quantities into the mass/volume percent expression.
STUDY CHECK
What is the mass/volume percent (m/v) of Br2 in a solution prepared by dissolving 12 g of bromine (Br2) in
enough carbon tetrachloride to make 250 mL of solution?
Timberlake: General, Organic & Biological Chemistry
Copyright ©2010 by Pearson Education, Inc.
SAMPLE PROBLEM
8.8
Using Mass/Volume Percent to Find Mass of Solute
A topical antibiotic is 1.0% (m/v) Clindamycin. How many grams of Clindamycin are in 60. mL of the 1.0% (m
/v) solution?
SOLUTION
STEP 1 Given 1.0% (m/v) Clindamycin
Need grams of Clindamycin
STEP 2 Plan
STEP 3 Equalities/Conversion Factors The percent (m/v) indicates the grams
of a solute in every 100 mL of a solution. The 1.0% (m/v) can be
written as two conversion factors:
STEP 4 Set Up Problem The volume of the solution is converted to mass
of solute using the conversion factor:
STUDY CHECK
Calculate the grams of KCl in 225 g of an 8.00% (m/m) KCl solution.
Timberlake: General, Organic & Biological Chemistry
Copyright ©2010 by Pearson Education, Inc.
SAMPLE PROBLEM
8.9
Calculating Molarity
What is the molarity (M) of 60.0 g of NaOH in 0.250 L of solution?
SOLUTION
STEP 1 Given 60.0 g of NaOH in 0.250 L of solution
Need molarity (moles/L)
STEP 2 Plan The calculation of molarity requires the moles of NaOH and the
volume of the solution in liters.
STEP 3 Equalities/Conversion Factors
Timberlake: General, Organic & Biological Chemistry
Copyright ©2010 by Pearson Education, Inc.
SAMPLE PROBLEM
8.9
Calculating Molarity (Continued)
What is the molarity (M) of 60.0 g of NaOH in 0.250 L of solution?
SOLUTION
STEP 4 Set Up Problem
The molarity is calculated by dividing the moles of NaOH by the
volume in liters.
STUDY CHECK
What is the molarity of a solution that contains 75.0 g of KNO3 dissolved in 0.350 L of solution?
Timberlake: General, Organic & Biological Chemistry
Copyright ©2010 by Pearson Education, Inc.
SAMPLE PROBLEM
8.10
Using Molarity to Find Volume
How many liters of a 2.00 M NaCl solution are needed to provide 67.3 g of NaCl?
SOLUTION
STEP 1 Given 67.3 g of NaCl from a 2.00 M NaCl solution
Need liters of NaCl solution
STEP 2 Plan The volume of the NaCl solution is calculated using the moles of NaCl and molarity of the NaCl
solution:
STEP 3 Equalities/Conversion Factors
The molarity of any solution can be written as two conversion factors:
Timberlake: General, Organic & Biological Chemistry
Copyright ©2010 by Pearson Education, Inc.
SAMPLE PROBLEM
8.10
Using Molarity to Find Volume (Continued)
How many liters of a 2.00 M NaCl solution are needed to provide 67.3 g of NaCl?
SOLUTION
STEP 4 Set Up Problem
STUDY CHECK
How many moles of HCl are present in 750 mL of a 6.0 M HCl solution?
Timberlake: General, Organic & Biological Chemistry
Copyright ©2010 by Pearson Education, Inc.
SAMPLE PROBLEM
8.11
Molarity of a Diluted Solution
What is the molarity of a solution prepared when 75.0 mL of a 4.00 M KCl solution is diluted to a volume of
0.500 L?
SOLUTION
STEP 1 Give Data in a Table We make a table of the molar concentrations
and volumes of the initial and diluted solutions. For the calculation,
units must be the same.
STEP 2 Plan The unknown molarity can be calculated by solving the dilution
expression for M2:
STEP 3 Set Up Problem The values from the table are placed into the dilution expression:
STUDY CHECK
You need to prepare 600. mL of 2.00 M NaOH solution from a 10.0 M NaOH solution. What volume of the
10.0 M NaOH solution do you use?
Timberlake: General, Organic & Biological Chemistry
Copyright ©2010 by Pearson Education, Inc.
SAMPLE PROBLEM
8.12
Volume of a Solution in a Reaction
Zinc reacts with HCl to produce ZnCl2 and hydrogen gas H2:
How many liters of a 1.50 M HCl solution completely react with 5.32 g of zinc?
SOLUTION
STEP 1 Given 5.32 g of Zn and a 1.50 M HCl solution
Need liters of HCl solution
STEP 2 Plan We start the problem with the grams of Zn given and use its
molar mass to calculate moles. Then we can use the mole–mole factor
from the equation and the molarity of the HCl as conversion factors:
Timberlake: General, Organic & Biological Chemistry
Copyright ©2010 by Pearson Education, Inc.
SAMPLE PROBLEM
8.12
Volume of a Solution in a Reaction (Continued)
Zinc reacts with HCl to produce ZnCl2 and hydrogen gas H2:
How many liters of a 1.50 M HCl solution completely react with 5.32 g of zinc?
SOLUTION
STEP 3 Equalities/Conversion Factors
Timberlake: General, Organic & Biological Chemistry
Copyright ©2010 by Pearson Education, Inc.
SAMPLE PROBLEM
8.12
Volume of a Solution in a Reaction (Continued)
Zinc reacts with HCl to produce ZnCl2 and hydrogen gas H2:
How many liters of a 1.50 M HCl solution completely react with 5.32 g of zinc?
SOLUTION
STEP 4 Set Up Problem We can write the problem setup as seen in our plan:
STUDY CHECK
Using the reaction in Sample Problem 8.12, how many grams of zinc can react
with 225 mL of 0.200 M HCl solution?
Timberlake: General, Organic & Biological Chemistry
Copyright ©2010 by Pearson Education, Inc.
SAMPLE PROBLEM
8.13
Isotonic, Hypotonic, and Hypertonic Solutions
Describe each of the following solutions as isotonic, hypotonic, or hypertonic. Indicate whether a red blood cell
placed in each solution will undergo hemolysis, crenation, or no change.
a. a 5.0% (m/v) glucose solution
b. a 0.2% (m/v) NaCl solution
SOLUTION
a. A 5.0% (m/v) glucose solution is isotonic. A red blood cell will not undergo any change.
b. A 0.2% (m/v) NaCl solution is hypotonic. A red blood cell will undergo hemolysis.
STUDY CHECK
What is the effect of a 10% (m/v) glucose solution on a red blood cell?
Timberlake: General, Organic & Biological Chemistry
Copyright ©2010 by Pearson Education, Inc.