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Chapter 8 Solutions
8.4Percent Concentration
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The concentration of a solution is the amount of solute dissolved in a specific amount
of solution
concentration = amount of soluteamount of solution
Concentration
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Mass percent (m/m) concentration is the percent by mass of solute in a solution calculated using the formula:
Mass = mass of solute(g) x 100%percent mass of solute(g) + mass of solvent(g)
Mass percent = mass of solute(g) x 100% mass of solution(g)
Mass Percent Concentration
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Mass of Solution
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The calculation of mass percent (m/m) requires the grams of solute (g of KCl) and grams of solution (g of KCl solution)
g of KCl = 8.00 g
g of solvent (water) = 42.00 g
g of KCl solution = 50.00 g
8.00 g of KCl (solute) x 100% = 16.0% (m/m) 50.00 g of KCl solution
Calculating Mass Percent
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Guide to Calculating Solution Concentrations
6
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A solution is prepared with 15.0 g of Na2CO3 and 235 g of H2O. What is the mass percent (m/m) of the solution?
1) 15.0%(m/m) Na2CO3
2) 6.38%(m/m) Na2CO3
3) 6.00%(m/m) Na2CO3
Learning Check
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3) 6.00% (m/m) Na2CO3 STEP 1 Given: 15.0 g of Na2CO3; 235 g of H2O
Need: mass percent (m/m) of Na2CO3 solution
STEP 2 Plan: mass percent(m/m) = g of solute x 100% g of solution
Calculate mass of solution
= 15.0 g + 235 g
= 250 g of Na2CO3 solution
Solution
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STEP 3 Write equalities and conversion factors: 15.0 g of Na2CO3 = 250 g of solution
15.0 g Na2CO3 and 250 g solution
250 g solution 15.0 g of Na2CO3
STEP 4 Set up problem:
mass = 15.0 g Na2CO3 x 100 = 6.00%(m/m) Na2CO3
percent 250 g solution(m/m)
Solution (continued)
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The volume percent (v/v) is percent by volume of solute (liquid) in a solution volume % (v/v) = volume of solute x 100%
volume of solution volume solute (mL) in 100 mL of solution.
volume % (v/v) = mL of solute 100. mL of solution
Volume Percent
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The mass/volume percent (m/v) is
mass/volume % (m/v) = grams (g) of solute x 100% volume(mL) of solution
mass of solute(g) in 100 mL solution = g of solute 100. mL of solution
Mass/Volume Percent
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Preparation of a Solution: Mass/Volume Percent
12
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Percent Conversion Factors
Two conversion factors can be written for each type of percent concentration.
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Write two conversion factors for each solution:
A. 8.50% (m/m) NaOH
B. 5.75% (v/v) ethanol
C. 4.8% (m/v) HCl
Learning Check
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A. 8.50% (m/m) NaOH 8.50 g NaOH and 100 g solution 100 g solution 8.50 g NaOH
B. 5.75% (v/v) ethanol 5.75 mL ethanol and 100 mL solution
100 mL solution 5.75 mL ethanol
C. 4.8% (m/v) HCl 4.8 g HCl and 100 mL HCl
100 mL solution 4.8 g HCl
Solution
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Guide to Using Concentration to Calculate Mass or Volume
16
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How many grams of NaCl are needed to prepare225 g of a 10.0% (m/m) NaCl solution?
STEP 1 Given: 225 g of solution; 10.0% (m/m) NaCl Need: g of NaCl (solute)
STEP 2 Plan: g of solution g of NaCl
Example of Using Percent Concentration (m/m) Factors
17
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Solution
STEP 3 Write equalities and conversion factors:
10.0 g of NaCl = 100 g of NaCl solution
10.0 g NaCl and 100 g NaCl solution
100 g NaCl solution 10.0 g NaCl
STEP 4 Set up problem to cancel grams of solution:225 g NaCl solution x 10.0 g NaCl = 22.5 g of NaCl
100 g NaCl solution
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How many grams of NaOH are needed to prepare 75.0 g of 14.0% (m/m) NaOH solution?
1) 10.5 g of NaOH
2) 75.0 g of NaOH
3) 536 g of NaOH
Learning Check
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Solution
STEP 1 Given: 75.0 g of NaOH solution; 14.0% (m/m) NaOH solution Need: g of NaOH (solute)
STEP 2 g of solution g of NaOH
STEP 3 Write equalities and conversion factors:
14.0 g of NaOH = 100 of NaOH solution
14.0 g NaOH and 100 g NaOH solution 100 g NaOH solution 14.0 g NaOHSTEP 4 Set up problem to cancel the grams of solution:
75.0 g NaOH solution x 14.0 g NaOH = 10.5 g of NaOH 100 g NaOH solution
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How many milliliters of a 5.75% (v/v) ethanol solution can be prepared from 2.25 mL of ethanol?
1) 2.56 mL
2) 12.9 mL
3) 39.1 mL
Learning Check
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Solution
STEP 1 Given: 2.25 mL of ethanol (solute)
5.75% (v/v) ethanol solution
Need: mL of ethanol solution
STEP 2 Plan: mL of ethanol mL of solution
STEP 3 Write equalities and conversion factors:
100 mL of solution = 5.75 mL of ethanol 5.75 mL ethanol and 100 mL solution
100 mL solution 5.75 mL ethanolSTEP 4 Set up problem to cancel the mL of ethanol:2.25 mL ethanol x 100 mL solution = 39.1 mL of solution
5.75. mL ethanol
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How many mL of a 4.20% (m/v) will contain 3.15 g KCl?
STEP 1 Given: 3.15 g of KCl (solute) 4.20% (m/v) KCl solution Need: mL of KCl solution
STEP 2 Plan: g of KCl mL of KCl solution
Example Using Percent Concentration (m/v) Factors
23
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STEP 3 Write equalities and conversion factors:
4.20 g KCl = 100 mL of KCl solution
4.20 g KCl and 100 mL KCl solution
100 mL KCl solution 4.20 g KCl
STEP 4 Set up the problem:
3.15 g KCl x 100 mL KCl solution = 75.0 mL of KCl 4.20 g KCl
Using Percent Concentration(m/v) Factors (continued)
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Learning Check
How many grams of NaOH are needed to prepare 125mL of a 8.80% (m/v) NaOH solution?
1) 7.04 g of NaOH
2) 11.0 g of NaOH
3) 14.2 g of NaOH
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Solution
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2) 11.0 g of NaOH
STEP 1 Given: 125 mL of NaOH (solution) 8.80% (m/v) NaOH solution Need: g of NaOH (solute)
STEP 2 Plan: mL of NaOH solution g of NaOH
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Solution (continued)
27
STEP 3 Write equality and conversion factors:
8.80 g of NaOH = 100 mL of NaOH solution
8.80 g NaOH and 100 mL NaOH solution 100 mL NaOH solution 8.80 g NaOH
STEP 4 Set up the problem:
125 mL NaOH solution x 8.80 g NaOH
100 mL NaOH solution
= 11.0 g of NaOH