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Chapter 4Chemical Reactions

Working with Solutions

Molar ConcentrationDefine molarity or molar concentration of a solution.Calculate molarity from mass and volume.Use molarity as a conversion factor.

Diluting SolutionsDescribe what happens to the concentration of a solution when it is diluted.Perform the calculations associated with dilution.Describe the process for diluting a solution.

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Quantitative Analysis

Gravimetric AnalysisDetermine the amount of a species by

gravimetric analysis.Volumetric Analysis

Calculate the volume of reactant solution needed to perform a reaction.

Understand how to perform a titration.Calculate the quantity of substance in a

titrated solution.

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Preparing Solutions

To prepare a solution, add the measured amount of solute to a volumetric flask and then add water to bring the solution to the mark on the flask.

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Preparing solutions of accurate concentrations

• Beakers hold approximate volumes

• Volumetric flasks hold accurate volumes

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ngageLearning

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Solubility Rules

http://www.dynamicscience.com.au/tester/solutions1/chemistry/solutions/solubilitycurves.html

1. Sodium (Na+), potassium (K+), and ammonium (NH4+)

salts are soluble2. Nitrates (NO

3–), acetates (C

2H

3O

2–), and perchlorates

(ClO4–) are soluble

3. Silver (Ag+), lead (Pb2+), and mercury(I) (Hg+) salts are insoluble

4. Chlorides (Cl–), bromides (Br-), and iodides (I–) are soluble

5. Carbonates (CO32-), sulfides (S2-), oxides (O2–),

phosphates (PO43–), and hydroxides (OH–) are

insoluble6. Sulfates (SO

42–) are soluble except for calcium sulfate

and barium sulfate

• Solubility rules tell us when to expect a precipitate, but not how much remains dissolved in solution

• Can solute precipitate or dissolve just by heating/cooling?

Practice

Describe what happens when the following solutions are prepared in the following scenarios:

• 25 g potassium nitrate mixed with 100 g of water, then placed into the refrigerator (5 degrees C)

• 60 g sodium sulfate mixed with 100 g of water, then heated to 65 degrees C

• A 65 wt% solution of lead (II) nitrate prepared one hot summer day. That next week, the air conditioning was turned on. The solution got cloudy.

Solutions

• Solutions are homogeneous mixtures of two or more substances.

• The solvent is the substance in greatest quantity.

• Solutes are dissolved in the solvent

Unsaturated/Saturated/Supersaturated Solutions• An unsaturated solution does not contain as much

solute as it can dissolve.

• A saturated solution contains the maximum concentration of solute that can dissolve in it.

• A supersaturated solution contains more than the quantity of a solute that is predicted to be soluble in a given volume of solution at a given temperature.

Supersaturated Solution

Expressions of Concentration

• Percent by mass =

• Percent by volume =

• Molarity =

• Parts per million (ppm) =

´mass of solute 100%

mass of solution

´volume of solute 100%

volume of solution

moles of soluteL of solution

´ 6g of solute 10 ppmg of solution

Practice CalculationsIf there's 0.000575 g of NaCl dissolved in 975 g of water, what is the concentration in parts per million?

0.500 moles of hydrochloric acid are dissolved in 2 liters of water. Calculate the molarity of this solution.

If there's 6.24 x 10-6 g of PbSO4

dissolved in 1 liter of water, what is the concentration in parts per billion?

The EPA has set a standard of 10 ppb for arsenic in drinking water. A certain sample of water contains ow many grams of arsenic 0.00025 g of arsenic in a sample of water that's 100 mL. Is this concentration above or below the acceptable level?

If the molarity of a solution is 0.15 M, what is the pH? Is this solution acidic or basic?

Molar Concentration, Molarity, (M)Moles of solute per liter of solution

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solutionofliterssoluteofmoles)(Molarity =M

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Which of the following three solutions has the highestconcentration?

1.5 10-3 M NaOH

2.25 10-6 M H2SO4

9.1 10-12 M HCl

Diluting SolutionsPreparing a solution with specific concentration requires diluting it with a certain volume of water. This requires knowledge of the relationship between the molarity of the solution before and after dilution.

It is known that:

Rearrange:

Before and after dilution:

Moles of solute have not changed, so4 | 16

solutionofliterssoluteofmoles)(Molarity =M

moles of solute = molarity × liters of solution

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Moles of solute = ×i iM V

Moles of solute = f fM V´

= i i f fM V M V´ ´

Determine the milliliters of a solution of 14.8 MNH3 required to produce 100 mL of NH3 with a molarity of 1.00 M.

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The initial volume can be determined by rearranging the dilution formula.

=

=

i i f f

f fi

i

M V M VM V

VM

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1.00 =iMV 100.0 mL14.8 ´M

6.76 mL=

Gravimetric AnalysisIt is a type of quantitative analysis in which the amount of a species in a material is determined by converting the species to a product that can be isolated completely and weighed.

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Volumetric AnalysisIt is a type of quantitative analysis based on titration.

Quantitative AnalysisIt is the determination of the amount of a substance or species present in a material.

The figure on the right shows the reaction between an unknown amount of barium ion and K2CrO4 (yellow), forming the yellow BaCrO4 precipitate.

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The figure to the right shows BaCrO4 precipitate being filtered. It can then be dried and weighed to determine the mass of the precipitate.

Gravimetric Analysis

´207.2 g/mol% Pb = 100% = 68.32%303.3 g/mol

4229.8 mg PbSO 0.6832 = 157.0 mg Pb´

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Therefore, a sample of water contains:

The water sample contains 157.0 mg Pb per liter.

Obtaining the mass percentage of Pb in PbSO4:

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A 1.000-L sample of polluted water was analyzed for lead (II) ion, Pb2+, by adding an excess of sodium sulfate to it. The mass of lead (II) sulfate that precipitated was 229.8 mg. What is the mass of lead in a liter of water? Give the answer as milligrams of lead per liter of solution.

A flask contains an unknown amount of HCl. This solution is titrated with 0.207 M NaOH. It takes 4.47 mL of the NaOH solution to complete the reaction. Determine the mass of HCl.

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Determine the stoichiometry of the reaction by writing the balanced equation.

Determining the mass of HCl:

– 3 0.207 mol NaOH 1 mol HCl 36.5 HCl4.47 10 L NaOH soln = 0.0338 g HCl1 L NaOH soln 1 mol NaOH 1 mol HCl

´ ´ ´ ´

)¾¾® 2( ) ( ) NaOH + HCl NaCl + H( (O )aq aq aq l

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• when a less concentrated solution is desired, a dilution is performed

• add more solvent to the original solution – volume increases

• the number of moles does not change

Dilution

molarityinit · volumeinit = molarityfinal · volumefinal

• when a less concentrated solution is desired, a dilutionis performed

• add more solvent to the original solution – volume increases

• the number of moles does not change

Dilutionsame

concentration

same number of moles

M1 M2 = ?V1 V2

molarityinit · volumeinit = molarityfinal · volumefinal

Titration

• Concentration of reactant A in buret is known accurately• Concentration of reactant B in flask is not known• Add a special pH indicator to reactant B in the flask• Add reactant A to reactant B until the pH indicator changes

color

Practice

What is the concentration of sulfuric acid if 15.00 ml of it react with 18.45 mL of a 0.0973 M NaOH solution?

standard solution

unknown solution

end point equivalence point

pH indicator

stoichiometric analysis

Practice

What is the concentration of sulfuric acid if 15.00 ml of it react with 18.45 mL of a 0.0973 M NaOH solution?

1 mol NaOH 1 mol OH-

1 mol H2SO4 2 mol H+