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Ch. 7: Solutions

Chem. 20

El Camino College

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Terminology

The solute is dissolved in the solvent. The solute is usually in smaller amount, and the

solvent is usually in larger amount A concentrated solution contains a relatively large

amount of solute A dilute solution contains a relatively small amount

of solute Solubility measures how much of a solute can

dissolve in a solvent at a given temperature.

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Terminology An unsaturated soln can hold more of a solute A saturated soln cannot hold more of a solute. If more

solute is added (without the temp changing), it won’t dissolve

A supersaturated soln is very unstable. It’s made by creating a saturated soln at high temp, and then cooling slowly. Adding a crystal will cause the excess solute to crystallize out

Miscible refers to liquids that are soluble in each other Immiscible liquids are not soluble, like oil and water.

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Terminology “Like dissolves like” means

Polar molecules dissolve polar (and many ionic) substances

Nonpolar molecules dissolve nonpolar substances

Water is a very polar molecule. Which will dissolve in water: NaCl,

sugar, benzene (a nonpolar molecule) Is sugar polar or nonpolar?

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Solution Formation

When water dissolves salt crystals, the + end of water is attracted to the Cl- ions, and the - end is attracted to the Na+ ions

The ions become hydrated, or surrounded by water molecules.

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Electrolytes Substances can be classified as strong-, weak-,

or non-electrolytes in water Strong electrolytes contain a lot of ions that

conduct electricity Weak electrolytes contain only a few ions, and

they conduct electricity weakly Nonelectrolytes do not contain ions. They don’t

conduct electricity

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Strong Electrolytes When a strong electrolyte is placed in water, it

dissolves and forms ions

Write the eqn for Ca(NO3)2(s) dissolving in H2O.

Ca(NO3)2(s) Ca2+(aq) + 2 NO3-(aq)

ions only

Write the eqn for NaCl(s) dissolving in H2O.

NaCl(s) Na+(aq) + Cl-(aq)

ions only

H2O

H2O

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Weak Electrolytes Some substances, such as weak acids, only

form a few ions when placed in water For weak acids, H+ pops off a small % of

molecules Since there are only a few ions in the soln, weak

electrolytes conduct electricity weakly

HF(g) HF(aq) (minor: H+(aq) + F-(aq))

mostly molecules, few ions

H2O

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Nonelectrolytes Many molecules do not form ions when they

dissolve

C12H22O11(s) C12H22O11(aq)

molecules only

H2O

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Classify TheseBased on the given information, decide whether ions

only, few ions, or molecules only form in water. Are they strong, weak, or nonelectrolytes?

H2OLiCl(s) Li+(aq) + Cl-(aq)note: LiCl is water soluble

H2OCH3OH(l) CH3OH(aq)note: CH3OH is a molecule that dissolves, but doesn’t form ions

ions only,strong electrolyte

few ions,weak electrolyte

molecules only,nonelectrolyte

H2OHNO2(g) H+(aq) + NO2-(aq)

note: HNO2 is a weak acid

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Factors Affecting Solubility

Pressure More gas dissolves at higher pressure. Pressure has

no effect on solubility of solids or liquids

Temperature More solid dissolves at higher temps More gas dissolves at lower temps.

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Fig. 16-5, p. 439

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Solution Concentration

(g solute+g solvent)mass % = g soluteg solution

x 100 g solute x 100mass % =

Ex. What is the % concentration of 250.0 g solution which contains 8.75 g solid?

% by mass = g soluteg solution

x 100 = 8.75 g250.0 g

x 100 = 3.50 %

Ex. What is the % conc of a soln which contains 5.33 g NaCl and 10.99 g water?

g solute + g solventg solute x 100

mass % = 5.33 g + 10.99 g5.33 g x 100

mass % =

= 32.7 %

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Molarity Molarity is (moles solute)/(liters solution) M=mol/L Ex. Calculate the molarity of 3.5 L of solution that contains

0.50 mol hydrogen chloride.

0.50 mol3.5 L = .14 M

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Using Molarity as a Conversion Factor

Note, if molarity is given in the problem, use it as a conversion factor with the units mol/L

A solution is listed as 5.60 M. Write a conversion factor with mol on top. Write a conversion factor with mol on the bottom.

5.60 mol1 L

1 L5.60 mol

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Given Molarity, Find Moles

Ex. How many moles of sodium chloride are in 400. mL of a 1.45 M solution?

400. mL 1000 mL

1 L = 0.580 mol 1.45 mol 1 L

Ex. How many moles of HBr are in 755 mL of a 3.50 M solution?

755. mL 1000 mL

1 L = 2.64 mol 3.50 mol 1 L

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Given Molarity, Find Grams

Ex. How many g of sodium chloride are in 50. mL of a 4.0 M solution?

Ex. How many g of I2 are in 2.00 L of a 0.75 M soln?

50. mL 1000 mL

1 L = 12 g 4.0 mol 1 L 1 mol

58.44 g

2.00 L = 381 g 0.75 mol 1 L 1 mol

253.8 g

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Given Molarity, Find Volume

Ex. How many mL of 5.60 M sucrose soln contain 2.50 mol sucrose?

2.50 mol 5.60 mol

1 L = 446 mL 1000 mL 1 L

Ex. How many mL of 12.0 M HCl soln contain 0.330 mol HCl?

0.330 mol 12.0 mol

1 L = 27.5 mL 1000 mL 1 L

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Colligative Properties When a solute is dissolved in a pure liquid, the

freezing point (fp) and boiling point (bp) of the soln will change.

FP depression means that a solute will lower the fp of the soln compared to the pure solvent

BP elevation means that a solute will raise the bp of a soln compared to the pure solvent

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Colligative Properties Colligative Properties are only determined by the

number of solute particles in a quantity of solvent Molality m=(mol solute)/(kg solvent) Freezing Point Depression Tf = Kfm

Tf is the change in temperature

Subtract Tf from the freezing pt (for H2O, the freezing pt is 0 oC)

Kf is the molal freezing pt depression constant

For H2O, Kf = 1.86 oC/m

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Colligative Properties

Boiling Point Elevation Tb = Kbm Tb is the change in temperature

Add Tb to the boiling pt (for H2O, the bp is 100 oC)

Kb is the molal boiling pt elevation constant

For H2O, Kb = 0.52 oC/m

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Ex. Determine the Tf of 3.0 kg of water when 5.0 mol antifreeze is added.

Tf=Kfm = 1.86 oCm

5.0 mol solute3.0 kg H2O

= 3.1 oC

fp = 0 - Tf = - 3.1 oC

Calculate the freezing pt of the above soln.

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Ex. Determine Tb of 3.0 kg of water when 5.0 mol antifreeze is added.

Tb=Kbm = 0.52 oCm

5.0 mol solute3.0 kg H2O

= 0.87 oC

bp = 100 + Tb = 100.87 oC

Determine the bp of the above soln.

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Ex. Determine the freezing pt of 1.53 kg of water when 6.2 mol formic acid (HCO2H) is added.

Tf=Kfm = 1.86 oCm

6.2 mol HCO2H1.53 kg H2O

= 7.5 oC

fp = -7.5 oC