Unit 7: Solution ChemistryChapter 13

Chemistry CPAApril 2014

Water: The “Universal Solvent”

• Water is referred to as the “universal solvent” because it’s really good at dissolving a vast variety of substances.

Water is a Polar Molecule• Water is a polar molecule which means it has a “negative” end and a

“positive” end• The “charged” ends of water help “dissociate” (pull apart) other

molecules through attractive forces

Hydrogen Bonding

• Hydrogen bonding: helps water dissociate covalent compounds• When atoms of hydrogen are

strongly attracted to electronegative atoms on another molecule (specifically oxygen, nitrogen, and fluorine) they make a connection to one another that acts as a bond.

Solution Formation

• A solution consists of a solute and a solvent• Solute: lesser amount of particles that gets dissolved• Solvent: greater amount of particles that “do” the dissolving.

Examples include:•Sugar and tea– sugar is the solute, water/tea the solvent•Medicine and your blood stream – medicine is the solute, blood is the solvent

What affects the rate of a solution’s formation?• Solution formation is chemical term for “dissolving” 1. Stirring – aka “agitation”

Stirring continually brings solutes into contact with solvent, speeding up solution formation.

2. TemperatureHigher temperature means higher kinetic energy – particles move more quickly and collide with one another to speed up solution formation

3. Particle SizeSmaller particle size = quicker solution formation

Solubility – How does it work?

• Dissolving all happens due to the kinetic energy of ALL particles involved in the solution.• Solvent particles are constantly “bumping into” solute particles, breaking

their bonds, and ultimately separating the solute into smaller pieces.

• Solubility – amount of solute that dissolves in a given quantity of a solvent at a specified temperature and pressure. Solubility is expressed in grams of solute per 100g of solvent.

Concentration

• Is the ratio of solute to solvent• If something is very concentrated, there is much more solute than

solvent.• If something is very un-concentrated (i.e., “watered down” kool-aid),

there is much more solvent than solute.

Three expressions of solution concentration:1. Saturated2. Unsaturated3. Supersaturated

Saturated Solution

• contains the maximum amount of solute for a given quantity of solvent at standard temperature and pressure.

Unsaturated

Contains less solute than a saturated solution at standard temperature and pressure.

Supersaturated

• Contains more solute than a solvent can theoretically hold. • You will have solid particles left over at the bottom of the container

that won’t dissolve.

How can you make a saturated solution unsaturated?

Add more solvent; Add more water; dilute the solution

How can you make a saturated solution supersaturated?

• Add more solute

Miscible vs. Immiscible

• Miscible • two liquids that dissolve in each other in all proportions.• “Like dissolves Like” – polar and polar dissolve one another; nonpolar and

nonpolar dissolve on another. • Example – ethanol and water, both polar molecules

• Immiscible • liquids that are not soluble. • Polar and nonpolar substances repel one another and do not dissolve• Example – oil (nonpolar) and water (polar), oil (nonpolar) and vinegar (polar)

Miscible vs. immiscible

Solubility Curve• x-y graph showing solubility vs. temperature• Solubility is on the y-axis, expressed in

grams/100g. of solvent• Temperature is on the x-axis, expressed in

°C• On the curve = saturated solution.• Under the curve = unsaturated solution.• Above the curve – supersaturated

solution.

Solubility Curve Calculations

• Solubility is grams of solute/100 g of solvent. Solubility = g. solute

100 g. of solvent

What if a problem asked you to find how many grams of a solute you would need to dissolve into 200 g of solvent? 300 g of solvent?

Solubility Curve

1. How many grams of KCl can be dissolved at 40°C?

Solubility Curve

2. How many grams of KCl can be dissolved at 80°C?

Solubility Curve

3. At 10°C, 20 grams of KCl is added to 100 grams of water. Is this solution saturated, unsaturated, supersaturated?

Solubility Curve

4. At 10°C, 30 grams of KCl is added to 100 grams of water. Is this solution saturated, unsaturated, supersaturated?

Solubility Curve

5. At 10°C, 50 grams of KCl is added to 100 grams of water. Is this solution saturated, unsaturated, supersaturated?

Solubility Curve

6. Using your answer from #5, how many grams of KCl will settle to the bottom?

Solubility Curve

7. At 10°C, how many grams of KCl are need to make a saturated solution in 200 grams of water?

Solubility Curve

8. At 10°C, how many grams of KCl are need to make a saturated solution in 300 grams of water?

1. What is the solubility of KNO3 at 20⁰C?2. You dissolve 30 g of KCl in water at a

temperature of 20⁰C. Is the solution unsaturated, saturated, or supersaturated?

3. Suppose you wanted to make a perfectly soluble solution of NaCl in 200 grams of water at 90⁰C. How many grams of NaCl would you need?

4. Which salt shows the least change in solubility from 0 ⁰C to 100 ⁰C?

5. Which salt shows the greatest change in solubility from 0 ⁰C to 100 ⁰C?

6. What general affect does increasing temperature have on the solubility of a substance?

Concentration of Solutions

• Used to describe the ratio of solute to solvent. • Three common ways to express concentration in chemistry:• Molarity – overall, molar mass-based concentration. Used on stock

chemicals.• molality – can use just a scale!• % solution – used on household products; doesn’t use molar masses.

Molarity

• Defined as moles of solute per 1 liter of solution• *Note that the volume is the total volume that results, not the

volume of the solvent alone. • The unit for molarity is M and is read as "molar." (i.e. 3 M = three

molar)• Equation:

M = moles of solute_ Volume (Liters of solution)

Solve for Molarity

• What is the molarity of a 5.00 liter solution that was made with 10.0 moles of KBr ?

Solve for Volume

What would be the volume of a 2.00 M solution made with 6.00 moles of LiF?

Solve for Volume

• # of moles of soluteLiters of solution = -------------------- Molarity

• Given: # of moles of solute = 6.00 moles Molarity = 2.00 M (moles/L)

• Liters of solution = 6.00 moles ----------- 2.00 moles/L

• Answer = 3.00 L of solution

Solve for molesIII. Basic molarity problems where the number of moles/grams of solute is the

unknown.

• How many moles of CaCl2 would be used in the making of 0.500 L of a 5.0M solution?

# of moles of solute = Molarity x Liters of solution

Given: Molarity = 5.0 M (moles/L) Volume = 0.500 L

# of moles of CaCl2 = 5.0 moles/L x 0.500 moles

Answer = 2.5 moles of CaCl2

IV. Given grams instead of molesConvert grams to moles

• mass given# of moles = ----------------- Molar mass

Solve for Volume (and convert to moles)

What is the volume of 3.0 M solution of NaCl made with 526 g of solute?

Convert to molesSolution: First find the molar mass of NaCl.Na = 23.0 g x 1 = 23.0 g

Cl = 35.5 g x 1 = 35.5 g =58.5 g

Convert to moles mass of sample

# of moles = ----------------- Molar mass

526 g

# of moles of NaCl = ------------ 58.5 g

Answer: # of moles of NaCl = 8.99 moles

Example 2. What is the volume of 3.0 M solution of NaCl made with 526g of solute?

Finally, go back to your molarity formula to solve the problem: # of moles of solute

Liters of solution = ------------------------------ Molarity

Given: # of moles of solute = 8.99 moles Molarity of the solution = 3.0 M (moles/L)

8.99 moles# of Liters of solution = ------------- 3.0 moles/L

Final Answer = 3.0 L

Solve for moles and then convert to grams

• How many grams of CaCl2 would be used in the making 0.500 L of a 5.0M solution?

Molarity by Dilution

• Dilution is the process of decreasing the concentration of a stock solution by adding more solvent (usually water).

Dilution Equation

M1V1 = M2V2

Where:

M1 is the Molarity of the stock solution

V1 is the volume of the stock solution

M2 is the Molarity of the diluted solution

V2 is the volume of the diluted solution

• The general purpose of the dilution equation is to find out the volume needed to dilute a stock solution to a desired (lower) concentration.

• But - You will be given any three (3) of the four variables and be asked to solve for the missing one.

Dilution Example #1

A stock solution of 1.00M of NaCl is available. How many milliliters are needed to make a 100.0 mL of 0.750M?

• M1 = ______ M x _____ L = ______M x ______ L• V1 = ______ = ______ x ______• M2 = _____• V2 = ______

Dilution Example #2

• Concentrated HCl is 12M. What volume is needed to make 2 L of a 1 M solution?

• M1 = ______ M x _____ L = ______M x ______ L• V1 = ______ = ______ x ______• M2 = _____• V2 = ______

Dilution Example #3

• Calculate the final concentration of a solution that was made by adding a 6 M/1L solution of NaOH to 2.5 L of water.

• M1 = ______ M x _____ L = ______M x ______ L• V1 = ______ = ______ x ______• M2 = _____• V2 = ______