8.2 Solubility and Concentration Fresh lemonade is a solution of water, lemon juice, and sugar....

8.2 Solubility and Concentration

Fresh lemonade is a solution of water, lemon juice, and sugar.

There is a limit to the amount of sugar that can dissolve in a given amount of water. Once that limit is reached, you cannot make the solution taste any sweeter.


How are solutions with different amounts of solute described?

Solubility

Solutions are described as saturated, unsaturated, or supersaturated, depending on the amount of solute in solution.


The maximum amount of a solute that dissolves in a given amount of solvent at a constant temperature is called solubility.

Solubilities are usually expressed in grams of solute per 100 grams of solvent at a specified temperature.

Solubility


At a given temperature, different substances have different solubilities in water.

Solubility


Saturated Solutions

A saturated solution is one that contains as much solute as the solvent can hold at a given temperature.

At 20°C, 203.9 grams of table sugar will dissolve in 100 grams of water. If you add more sugar, it will not dissolve.

Solubility


Unsaturated Solutions

A solution that has less than the maximum amount of solute that can be dissolved is called an unsaturated solution.

As long as the amount of solute is less than the solubility at that temperature, the solution is unsaturated.

Solubility


Supersaturated Solutions

A supersaturated solution is one that contains more solute than it can normally hold at a given temperature.

A solvent can sometimes dissolve more solute than you might expect, based on its solubility.

If you heat a solution, more solute may dissolve. Carefully cool the solution without jarring it and the extra solute in solution remains.

Solubility


Supersaturated solutions are very unstable. When a single crystal of sodium acetate is added to a supersaturated solution of sodium acetate in water, the excess solute rapidly crystallizes.

Solubility


What factors determine the solubility of a solute?

Factors Affecting Solubility

Three factors that affect the solubility of a solute are the polarity of the solvent, temperature, and pressure.


Polar and Nonpolar Solvents

Oil does not dissolve in water because oil molecules are nonpolar and water molecules are polar.

Solutions are more likely to form when the solute and solvent are either both polar or both nonpolar.

A soap molecule has a polar end, which attracts water molecules, and a nonpolar end, which attracts oil.

Solubility


Generally, a solute is more likely to dissolve in a “like” solvent than an “unlike” solvent.

Solubility


Soaps and detergents are used to remove grease and oil stains.

Solubility


Temperature

In general, the solubility of solids increases as the solvent temperature increases.

In general, gases become less soluble as the temperature of the solvent increases.

Solubility


Pressure

Increasing the pressure on a gas increases its solubility in a liquid.

The pressure of CO2 in a sealed 12-ounce

can of soda at room temperature can be two to three times atmospheric pressure.

Solubility


What are three ways to measure the concentration of a solution?

The concentration of a solution is the amount of solute dissolved in a specified amount of solution.

Concentration of Solutions

Concentration can be expressed as percent by volume, percent by mass, and molarity.


Percent by Volume

Most bottled or canned juices are made by adding water to natural fruit juices. These solutions are less concentrated than natural fruit juices.

Percent by volume is a way to measure the concentration of one liquid dissolved in another.



Use the following equation to calculate concentration as a percent by volume.



Percent by Mass

Concentration expressed as a percent by mass is more useful when the solute is a solid.

Percent by mass is the percent of a solution’s total mass that is accounted for by a solute.



Molarity

To compare the number of solute particles in solutions, chemists often use moles to measure concentration.

Molarity is the number of moles of a solute dissolved per liter of solution.



To make a 1-molar (1M) solution of sodium chloride in water, first calculate the molar mass of the solute. Sodium chloride, NaCl, has a molar mass of 58.5 grams.

If 58.5 grams of sodium chloride is mixed with enough water to make one liter of solution, the resulting solution is 1-molar.



One liter of a 1M NaCl solution contains 58.5 grams of NaCl.



Assessment Questions

1. Compound X has a solubility of 20 g in 100 g of water at 20°C. What is the minimum amount of water needed to dissolve 50 g of compound X? a. 250 g

b. 100 g

c. 500 g

d. 200 g



1. Compound X has a solubility of 20 g in 100 g of water at 20°C. What is the minimum amount of water needed to dissolve 50 g of compound X? a. 250 g

b. 100 g

c. 500 g

d. 200 g

ANS: A



2. A saturated solution of carbon dioxide in water suddenly experiences an increase in pressure. What happens to the solution?a. Bubbles form as the carbon dioxide comes out of the

water.b. It becomes supersaturated.c. It remains saturated.d. More carbon dioxide can now be dissolved in the water.



2. A saturated solution of carbon dioxide in water suddenly experiences an increase in pressure. What happens to the solution?a. Bubbles form as the carbon dioxide comes out of the

water.b. It becomes supersaturated.c. It remains saturated.d. More carbon dioxide can now be dissolved in the water.

ANS: D



3. How does the number of particles in a 1 M solution of LiOH (molar mass = 24) compare to the number of particles in a 1 M solution of NaCl (molar mass = 58.5)?

a. There are more particles of solute in the LiOH solution.

b. There are more particles of solute in the NaCl solution.

c. The number of particles of solute is the same in both solutions.

d. More information is necessary in order to compare the solutions.



3. How does the number of particles in a 1 M solution of LiOH (molar mass = 24) compare to the number of particles in a 1 M solution of NaCl (molar mass = 58.5)?

a. There are more particles of solute in the LiOH solution.

b. There are more particles of solute in the NaCl solution.

c. The number of particles of solute is the same in both solutions.

d. More information is necessary in order to compare the solutions.

ANS: B

8.2 Solubility and Concentration Fresh lemonade is a solution of water, lemon juice, and sugar....

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