Chemistry - Solutions

[Concept] What is a solution?

A homogeneous mixture of two or more pure substances in which each component retains its chemical identity.

[Concept] Solvent versus solute

The solvent is present in the greatest amount and does the dissolving; solutes are present in smaller amounts and are dissolved.

[Rule] Key properties of a true solution

Variable composition, uniformly mixed particles, no settling, and components separable by physical means.

[Concept] What is solubility?

The maximum amount of solute that dissolves in a given amount of solvent under stated conditions.

[Rule] Temperature effect on most solid solutes

Increasing temperature usually increases the solubility of a solid in a liquid.

[Rule] Temperature effect on gases dissolved in liquids

Increasing temperature decreases gas solubility.

[Rule] Henry's law concept

Gas solubility in a liquid is directly proportional to the gas's partial pressure above the liquid.

[Practice] If a gas's partial pressure doubles at constant temperature, what happens to its solubility?

It approximately doubles.

[Practice] Why does warm soda lose carbonation faster?

Gases are less soluble at higher temperature, so CO2 escapes more readily.

[Concept] What is a saturated solution?

A solution containing the maximum amount of dissolved solute under the current conditions.

[Concept] What is an unsaturated solution?

A solution containing less than the maximum amount, so more solute can dissolve.

[Concept] What is a supersaturated solution?

An unstable solution containing more dissolved solute than the normal maximum under the current conditions.

[Rule] Concentrated versus saturated

Concentrated means relatively much solute; saturated means the solubility limit has been reached. They are not the same.

[Memory] Aqueous versus nonaqueous

Aqueous means water is the solvent; nonaqueous means another substance is the solvent.

[Concept] Solubility versus dissolution rate

Solubility is how much can dissolve at equilibrium; dissolution rate is how fast dissolving occurs.

[Rule] Factors that increase the rate of dissolving a solid

Smaller particle size, stirring/agitation, and usually higher temperature.

[Trap] Do crushing and stirring normally increase the final solubility limit?

No. They mainly make the system reach the limit faster.

[Rule] “Like dissolves like”

Substances with similar polarity tend to dissolve each other; polar dissolves polar and nonpolar dissolves nonpolar.

[Practice] Why does water dissolve many ionic compounds?

Water is polar and can surround and stabilize separated ions.

[Practice] Which end of water points toward a cation?

The partially negative oxygen end.

[Practice] Which end of water points toward an anion?

The partially positive hydrogen ends.

[Concept] What is concentration?

The amount of solute present in a specified amount of solution.

[Rule] General percent-concentration equation

Percent = amount of solute / amount of solution x 100%.

[Rule] Percent by mass

% (m/m) = grams solute / grams solution x 100%.

[Rule] Percent by volume

% (v/v) = mL solute / mL solution x 100%.

[Rule] Mass-volume percent

% (m/v) = grams solute / mL solution x 100%.

[Practice] Interpret 15% (m/m) NaCl.

15 g NaCl per 100 g solution.

[Practice] Interpret 6% (v/v) ethanol.

6 mL ethanol per 100 mL solution.

[Practice] Interpret 25% (m/v) glucose.

25 g glucose per 100 mL solution.

[Practice] Find the KCl mass needed for 250.0 g of 4.000% (m/m) solution.

250.0 x 4.000/100 = 10.00 g KCl.

[Practice] How much solvent is in that 250.0 g solution?

250.0 - 10.00 = 240.0 g solvent.

[Practice] A solution contains 8.0 g salt in 200.0 g solution. Find % (m/m).

8.0/200.0 x 100% = 4.0% (m/m).

[Practice] How many mL ethanol are in 350 mL of 12% (v/v) solution?

350 x 12/100 = 42 mL ethanol.

[Practice] What solution volume contains 18 g glucose in a 6.0% (m/v) solution?

18 x 100/6.0 = 300 mL solution.

[Concept] What is molarity?

Moles of solute per liter of final solution: M = mol/L.

[Rule] Molarity rearrangements

moles = M x L; liters = moles/M.

[Trap] What volume must be used in a molarity calculation?

Liters of the final solution, not liters of solvent.

[Practice] Find M when 15.0 g KOH makes 750.0 mL solution.

15.0/56.11 = 0.267 mol; 0.267/0.7500 = 0.356 M.

[Practice] Find M for 0.250 mol NaCl in 500.0 mL solution.

0.250/0.5000 = 0.500 M.

[Practice] How many moles are in 125.0 mL of 2.00 M HCl?

2.00 x 0.1250 = 0.250 mol HCl.

[Practice] How many grams NaOH are in 250.0 mL of 0.400 M NaOH?

0.400 x 0.2500 = 0.100 mol; 0.100 x 40.00 = 4.00 g.

[Practice] A patient needs 2.00 g glucose from 0.25 M glucose. Find solution volume.

2.00/180.16 = 0.0111 mol; V = 0.0111/0.25 = 0.044 L = 44 mL.

[Concept] What is dilution?

Adding solvent so volume increases and concentration decreases while the amount of solute remains constant.

[Rule] Dilution equation

C1V1 = C2V2, also written M1V1 = M2V2.

[Trap] In a dilution, what does V2 mean?

The final total solution volume, not the amount of solvent added.

[Practice] Dilute 125.0 mL of 6.0 M HCl to 1.000 M. Find V2.

V2 = 6.0 x 125.0/1.000 = 7.5 x 10^2 mL.

[Practice] What volume of 12.0 M stock makes 500.0 mL of 1.00 M solution?

V1 = 1.00 x 500.0/12.0 = 41.7 mL.

[Practice] 25.0 mL of 2.00 M solution is diluted to 100.0 mL. Find final M.

M2 = 2.00 x 25.0/100.0 = 0.500 M.

[Concept] What are colligative properties?

Solution properties that depend on the number of dissolved particles rather than their identity.

[Memory] Colligative effects emphasized in the slides

Solute lowers vapor pressure, raises boiling point, and lowers freezing point.

[Rule] Why does a nonvolatile solute change these properties?

It helps hold solvent particles in the liquid phase and reduces their ability to escape.

[Concept] What is osmosis?

Movement of solvent through a semipermeable membrane from the more dilute side toward the more concentrated side.

[Concept] Isotonic solution relative to a cell

Equal effective solute concentration; no net water movement and the cell keeps its size.

[Concept] Hypotonic solution relative to a cell

Lower outside solute concentration; water enters and the cell swells or may burst.

[Concept] Hypertonic solution relative to a cell

Higher outside solute concentration; water leaves and the cell shrinks.

[Practice] A cell is placed in a solution more concentrated than its cytoplasm. Predict the result.

Water moves out by osmosis and the cell shrinks.

[Practice] A cell is placed in a solution less concentrated than its cytoplasm. Predict the result.

Water moves in and the cell swells.

[Trap] Three concentration mistakes to avoid

Using solvent mass instead of solution mass, leaving mL unconverted in molarity, and using water-added volume instead of final volume in dilution.