Solutions Part II - Georgetown ISD



Solutions Part II

IX. Electrolyte VS Nonelectrolyte

1. Electrolyte – compounds that conduct an electric current in an aqueous solution OR in the molten state. An electrolyte solution contains charged particles (ions), which can move. Any salt dissolved in water is an electrolyte: NaCl, KI, etc. Some polar molecules also conduct electricity (most acids are electrolytes because H is the only nonmetal that has a + charge).

Types of Electrolytes

1. Strong electrolytes – a large portion of the solute exists as ions:

a. aqueous solutions of all ionic compounds

a. b. strong acids: have at least 2 oxygens per hydrogen (H2SO4, HNO3)

c. strong bases – these are hydroxides from Group I and II, except Be. (NaOH, CsOH, etc)

2. Weak electrolytes – these are solutions in which only a small portion of the solute exists as ions

a. weak acids:

-all binary acids – HF, H2S, etc

-weak acids that have less than 2 oxygens per hydrogen

b. weak bases – hydroxides of everything else not in Group I or II, including Be(OH)2

2. Non-Electrolytes- compounds that do NOT conduct electricity in either aqueous solution or when melted:

a. distilled water

b. gases

c. molecular compounds (2 nonmetals)

d. organic compounds – alcohols, sugars, etc. (anything containing a Carbon)

Practice Problems: Tell whether each of the following aqueous solutions would be a STRONG, WEAK, or NON electrolyte.

1. NaCl

2. CH3Br (l)

3. HMnO4

4. HC2H3O2

5. LiOH

X. Concentration: the concentration of a solution is a measure of the amount of solute in a given amount of solvent or solution.

1. Molarity: the number of moles of solute in one liter of solution.

 

M = amount of solute (moles)

OR grams of solute/molar mass of solute volume of solution ( liters) L of solution

A bottle labeled 6M HCl is pronounced 6 molar HCl and it was prepared by mixing 6 moles of HCl with enough water to make 1 liter of solution. Molarity is always moles/liter. Ex: 6M is 6 moles/ liter

Molarity is probably the MOST IMPORTANT unit of concentration that we work with in chemistry. Knowing the correct technique for preparing molar solutions is extremely important. A volumetric flask MUST be used. Here is the technique: (Know it)

Example Molarity Problems:

1. Calculate the molarity, M, of a solution prepared by dissolving 11.85 g of potassium permanganate in enough water to make 750. mL of solution.

2. Calculate the mass of NaCl needed to prepare 175 ml of 0.500 M saline solution.

3. Calculate the volume (in mL) needed to prepare a 2.48 M sodium hydroxide solution containing 31.52 g of the dissolved solid.

2. Molarity of Ions in solution – Most ionic solids when dissolved in water, ionize (break apart into ions)

Ex: CaCl2 (aq) ( Ca2+ + 2 Cl-

Na3PO4 ( 3 Na+ + PO43-

Mg(OH)2 ( Mg2+ + 2 OH-

Example Molarity of Ions Problems: Calculate the molarity of the ions in the following solutions:

1. 0.25 M calcium phosphide

2. 2.0 M Chromium (III) chloride

3. 0.25 M barium hydroxide

3. Dilutions – you need to make 800.0 mL of a 0.25 M solution of HCl. The only available HCl is concentrated (12 M). How would you do this? Being able to prepare dilutions is a very common application of chemistry. Our department buys concentrated acids, but normally uses more dilute solutions of these acids in our labs. Therefore, it is important to know how to correctly dilute.

M1V1 = M2V2 where M1 = concentrated solution V1 = volume of concentrated solution M2 = dilute solution V2 = volume of dilute solution

In the above problem, M1 = 12.0 M M2 = 0.25 M Vl = ? V2 = 800.0 ml therefore

(12.0 M) (V1) = (0.25 M)(800 mL) and V1 = 16.67 ml.

This 16.67 mL is the amount of concentrated acid we will take out, but we still have to make 800 mL of 0.25 M. The other 783.3 mL of solution must be water (800 – 16.67). Therefore we would place 16.67 mL of HCl into 783.3 mL of water and we would have our diluted solution.

Practice Dilutions Problems:

l. How would you prepare 485 mL of 0.39 M solution of NaCl when a l.0 M solution of NaCl is all you can find?

2. If 300.0 mL of a 2.5 M solution of nitric acid is added to 500.0 mL of water, what is the molarity of the dilute solution?

4. Percent Solutions (2 types):

1. Percent mass or %(m/m)– used when a solid solute is dissolved in liquid, usually water.

% (m/m) = grams of solute

grams of solution

Example 1: Prepare a 10.00 % NaCl solution using 50.0 g water and solid salt:

0.l0 = x / 50 + x and x = 5.67 g NaCl in 50 grams of water

Example 2: How many grams of water must be added to 25.0 g salt in order to have a 2.00 % (by mass) salt solution?

Example 3. Prepare 600.0 g of a 3.00 % saline solution (NaCl solution).

2. Percent volume or %(v/v) – used when a liquid solute is mixed with a liquid solvent. The units are mL or L, but are worked the same.

% (v/v) = mL solute

mL solution

Example 1: Prepare a 20.00 % alcohol solution using 400.0 mL of water:

0.20 = x / 400 + x and x = 100 mL alcohol + 400 mL water

Example 2: What is the percent (v/v)of ethanol in the final solution when 90.0 mL of it are diluted to a volume of 300. mL with water?

5. Molality: is the concentration of a solution expressed in moles of solute per kilogram of solvent. Molality is represented by a lower case m.

molality = amount of solute (moles) OR molality = grams of solute/molar mass

mass of solvent (kg) kg of solvent

5m NaOH is pronounced 5 molal NaOH solution.

 

5m = 5 moles of NaOH

1 kg of water 5m or 5moles/kg

Example Molality Problems:

1. Calculate the molality of a solution prepared by dissolving 5.0l g sodium sulfate in 700.0 g water.

2. Prepare a solution that is 2.50 molal barium nitrate in 1500. grams of water.

A. Colligative Properties of Solutions - Characteristics of a solution that do NOT depend on size or type of solute particles, but only upon the concentration of particles. The physical properties of solutions are different from those of the pure solvent. Colligative properties depend on the number of ionized particles dissolved in a given mass of solvent:

NaCl ( Na+ + Cl- 2 particles

Na2SO4 ( 2Na+ + SO42- 3 particles

(NH4)3PO4 ( 3NH41+ + PO43- 4 particles

What do you notice about the solutes that contained polyatomic ions?

Organic and molecular molecules do not ionize so they stay as one particle.

Practice Problems: How many particles will the following solutions become?

1. magnesium chloride 2. calcium bromide 3. aluminum sulfate

4. iron (III) nitrate 5. ethanol (C2H5OH) 6. dinitrogen pentoxide

B. Types of Colligative Properties

1. Boiling Point Elevation (BPE) – some solutes increase the point at which a solution boils

Example: salt added to water increases the boiling point of the water

This occurs because in water there are fewer particles to “energize” or boil, once you add salt the number of particles increases and more energy must be added to break the intermolecular forces that hold the molecules together.

Another explanation: Solutions have a lower vapor pressure than the pure solvent. Because the vapor pressure is decreased, more kinetic energy is required to raise the vapor pressure to atmospheric pressure; therefore the boiling point of a solution is higher than the boiling point of the pure solvent.

2. Freezing Point Depression (FPD) – some solutes decrease the point at which a solution freezes

Example: salt added to water decreases the freezing point of the water

Occurs because in water there are fewer particles to freeze, once you add salt the number of particles increases and more energy must be given off (lost) until the solution freezes. (Intermolecular forces are allowed to become stronger as more energy is lost that is why energy is lost during freezing.)

Another explanation: When a solute is present, the orderly pattern that the pure solvent normally takes when freezing is disrupted, so more kinetic energy must be lost from the solution for it to solidify (freeze). This lowers the freezing point of the solution.

Factors that affect the amount which the boiling point or freezing point changes are:

1. Molality of the solution

2. Whether the solution is ionic or molecular (# of particles)

3. The boiling point and freezing point constants (given or on page 438)

Colligative Property Constants

|Solvent |Boiling Point (°C) |Kb (° C/m) |Freezing Point ( °C) |Kf (°C/m) |

|Water |100 |0.5l2 |0 |l.86 |

|Benzene |80.l |2.53 |5.48 |5.12 |

|Acetic Acid |118.l |3.07 |16.6 |3.90 |

|Nitrobenzene |210.88 |5.24 |5.70 |7.00 |

|Phenol |182 |3.56 |43 |7.40 |

C. Formulas for solving boiling point elevation or freezing point depression problems:

1. BPE = (molality)(Kb)(# particles of solute)

2. FPD = (molality)(Kf)(# particles of solute) Both BPE and FPD are always positive.

Molality = moles of solute OR Molality = grams solute/molar mass of solute

kg of solvent kg of solvent

D. Formulas to find freezing point or the boiling point are:

1. BP solution = BP pure solvent +BPE

2. FP solution = FP pure solvent - FPD

Practice Problems:

1. What is the boiling point of a solution in which 75.00 grams of sodium sulfate are dissolved in 800.0 grams of water? Kb = 5.12 °C/m

1st solve for BPE: BPE = grams solute/molar mass of solute (Kb) (# particles)

kg of solvent

BPE = 75.00 g/142 g/mol (5.12°C/m) (3) = 10.1°C

0.8000 kg

2nd solve for the boiling point of the solution: BP solution = BP pure solvent +BPE

BP solution = 100°Ct + 10.1°C = 110.1°C

2. What is the freezing point of a solution in which 35.0 grams of magnesium bromide are dissolved in 400.0 grams of acetic acid? Kf = 3.90°C/m

3. A benzene solution, C6H6, boils at 87.3°C and freezes at -1.12°C. Find the BPE and FPD.

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