Part I: Solubility, Factors Affecting Solubility  

CHM 102

Friday, July 6th

Practice: Solubility and Saturation

• If the solubility of sodium acetate is 65 g per 100 mL H2O at 100oC, then will any sodium acetate remain undissolved if we pour 140 g of sodium acetate into 200 mL of water? What is the condition of the solution (saturated….unsaturated)?

Practice: Solubility and Saturation

• If the solubility of potassium chloride is

42.1 g per 100 mL H2O at 100oC, then will any sodium acetate remain undissolved if we pour 81 g of sodium acetate into

150 mL of water? What is the condition of the solution (saturated….unsaturated)? How much more (if any) could we dissolve into the solution?

Factors Affecting Solubility

• Solute-Solvent Interactions

• Pressure Effects (dissolving gases)

• Temperature Effects (for both gaseous and solid solutes)

Solute-Solvent Interactions• The formation of solute-solvent

interactions is the only exothermic contribution to the heat of solution.

• Exothermic (negative H) values for the solute-solvent interaction term points towards a solution process that is spontaneous.

• So the stronger the solute-solvent interactions, the more exothermic the overall solution process will be.

Solute-Solvent Interactions

• This idea breaks into the classic “like dissolves like” principle.

• Water, the “universal solvent” is really not entirely universal. Water is polar, and will tend to dissolve polar and ionic substances, but will not dissolve nonpolar substances (i.e. oil) readily.

NaCl Dissolves in Water

Example: Water and Alcohol

• Do water and alcohol mix? What’s an example? These two liquids are said to be miscible. If they didn’t, they’d be termed imiscible.

Practice: Alcohols and Solvent-Solute Interactions

• Which is more soluble in water, ethanol (C2H3OH), butanol (C4H9OH), or hexanol

(C6H13OH)? Why?

Pressure Effects

• When dissolving a gas in water, Henry’s Law shows that when pressure of a gas above a solution goes up, its solubility in the solution increases:

S = k P

• S is solubility, k is Henry’s gas constant (which is solute and solvent AND temperature dependent).

Practice: Pressure Effects

• What is the solubility of carbon dioxide in coke if the soda is at 25oC and the pressure of CO2 above the solution is 0.025 atm? (henry’s law constant is 3.1*10-2 mol/L*atm)

S = k P

More Practice: Pressure Effects

• What is the henry’s law constant for CO2 at 298 K when the solubility of carbon dioxide is known to be 0.12 mol/L when the pressure of CO2 above the liquid is 4.0 atm?

S = k P

Temperature Effects-Gases• When dissolving gases, the solubility of a

gas in a substance tends to go DOWN as the temperature goes up.

• Think of a cold soda sitting outside open on a hot day. What happens?

Temperature Effects-Solids• When dissolving solids, the solubility of a

solid in a substance tends to go UP as the temperature goes up.

• Think of making sweet tea or kool-aid. If you want the sugar buzz of your life, you don’t dissolve sugar in cold water, you dissolve it in HOT water. The solubility of the sugar goes up as the temperature gets hotter.

Part II: Ways of Expressing Concentration

CHM 102

Friday, July 6th

Ways of Expressing Concentration

•Mass Percentage

•Parts-per-million (ppm)

•Parts-per-billion (ppb)

•Mole Fraction

•Molarity

•Molality

Mass Percentage

• Mass Percentage is just as it sounds. It is the percent of what component you’re interested in compared to the whole:

• Practice: What is the mass percent sugar in a 2.07 kg solution with 350 g of sugar in it?

%100*'___

'____%

nsolofmasstotal

nsolincomponentofmassMass

Parts-per-million (and –billion)

• Parts-per-million and parts-per-billion are often used for very dilute solutions to express concentration:

• If 16.5 mg of NaCl is dissolved in 1.6 kg of water, what is the concentration of NaCl in ppm? ppb?

910*)(___

)(____

gsolutiontotalofmass

gsolutionincomponentofmassppb

610*)(___

)(____

gsolutiontotalofmass

gsolutionincomponentofmassppm

More Practice: ppm and ppb

• What is the ppm of Na+ if 24.2 mg of NaCl is added to 1.78 kg of water? (hint: use mass percentage first!) ppb of Na+?

910*)(___

)(____

gsolutiontotalofmass

gsolutionincomponentofmassppb

610*)(___

)(____

gsolutiontotalofmass

gsolutionincomponentofmassppm

Mole Fraction

• Mole fraction is also just as it sounds. It is the number of moles of what component you’re interested in compared to the total moles:

Practice: If you’ve got 73 g of HCl in 162 g of water,

what is the mole fraction of HCl in the solution?

molestotal

nsolincomponentofmolesonMoleFracti

_

'____

Molarity and Molality

• Do not get these two mixed up! VERY COMMON MISTAKE!

• The following two definitions for molarity and molality should be memorized!!!

• Molarity is given in moles of solute per liter of solution.

• Molality is given in moles of solute per kilogram of solvent.

Molarity

• What is the concentration of sodium carbonate (MW = 106 g/mol) in molarity when 26.5 g Na2CO3(s) is dissolved to a total volume of 2 L? What is the molarity of sodium ions, K+?

solutionofliters

soluteofmolesMolarity

__

__

Molality

What is the concentration of sodium carbonate (MW = 106 g/mol) in molality when 53 g Na2CO3(s) is dissolved in 1896 g of DI water?

solventofkg

nsolincomponentofmolesMolality

__

'____

Moving Between Concentrations

1. An aqueous solution of NaCl is 10% sodium chloride by mass. What the concentration of NaCl in molarity?

2. 50.0 g of sucrose (MW = ) is dissolved in 1375 g of ethanol (MW = , density = 0.79 g/mL). What is the molarity of this solution? What is the mass percent sucrose in the solution?