Chapter 4 lecture notes - The colony HIGH SCHOOL



Chapter 4: Solution Chemistry

NOTE: Remember a solution is a homogeneous mixture where a solute is dissolved in a solvent.

TERMS

1. Solute is dissolved.

2. Solvent does the dissolving.

3. Aqueous solutions are solutions where the solvent is water.

4. Polar molecule:

a. A molecule that has unequal sharing of electrons, causing dipoles to form.

b. Water is a polar molecule.

5. Hydration: when polar molecule dipoles “pull-apart” a solute compound.

6. Solubility: how much solute can be dissolved in the solvent.

PROPERTIES OF AQUEOUS SOLUTIONS

Electrolytes

a) Solutions that conduct an electric current

b) Two types

i) strong

1) completely dissociate (consult solubility rules)

2) light bulb very bright

ii) weak

1) do not dissociate

2) only about 5% dissociation (ammonia & acetic acid)

3) light bulb very weak, but still emits light

c) Non-electrolytes

i) Solutions where dissolving has occurred, but the solute does not make ions and therefore cannot conduct electricity.

ii) Pure water, sugar, alcohols, antifreeze, and starch.

iii) Light bulb does not come on.

Composition of Solutions

1) Molarity

a) Concentration unit of moles per liter.

i) [NaCl] = 0.75 M means 0.75 moles of salt is contained in 1.00 L of solution.

ii) [square brackets]

1) Indicate whatever is inside is

1. in solution

2. its concentration is expressed in molarity.

iii) Concentration of ions

1) [MgCl2] = 0.5 M means that:

a) [MgCl2] = 0.5 M

b) [Mg2+] = 0.5 M

c) [Cl-1] = 1.0 M

i) since MgCl2 has two Cl- ions,

ii) ( 2 x 0.5 M = 1.0 M

[pic]

PREPARING SOLUTIONS OF KNOWN CONCENTRATIONS

1) use a volumetric flask

2) weigh out the solid as accurately as possible

3) fill the flask about ½ full of liquid

4) add solid to flask

5) add water filling to the mark on the flask

NOTE: You cannot dump the solid into flask that is already filled to the mark. The solid will take up space and overflow the flask (go above the line).

PREPARING SOLUTIONS BY DILUTION

1) Often your solution will need to be diluted.

2) Diluting a more concentrated solution to make a dilute solution

Use the equation:

[pic]

3) Determine the volume needed of the stock solution (the solution that is more concentrated).

4) Use any of the following to measure out the stock solution:

a) Volumetric flask

b) graduated cylinder

c) pipet

i) measuring pipet

ii) volumetric pipet

5) Clean a volumetric flask the same size as the diluted solution that you require.

a) Fill the flask about ½ full (or an amount that will allow you to add the volume of the stock solution to the flask).

b) Add water filling to the mark on the flask.

AP Question 1993 B

I. 2 Mn2+ + 4 OH- + O2(g) ( 2 MnO2(s) + 2 H2O

II. MnO2(s) + 2 I- + 4 H+ ( Mn2+ + I2(aq) + 2 H2O

III. 2 S2O32- + I2(aq) ( S4O62- + 2 I-

The amount of oxygen, O2, dissolved in water can be determined by titration. First, MnSO4 and NaOH are added to a sample of water to convert all of the dissolved O2 to MnO2, as shown in equation I above. Then H2SO4 and KI are added and the reaction represented by equation II proceeds. Finally, the I2 that is formed is titrated with standard sodium thiosulfate, Na2S2O3, according to equation III.

a) According to the equation above, how many moles of S2O32- are required for analyzing 1.00 mole of O2 dissolved in water?

b) A student found that a 50.0-milliliter sample of water required 4.86 milliliters of 0.0112-molar Na2S2O3 to reach the equivalence point. Calculate the number of moles of O2 dissolved in this sample.

c) How would the results in (b) be affected if some I2 were lost before the S2O32- was added? Explain.

d) What volume of dry O2 measured at 25oC and 1.00 atmosphere of pressure would have to be dissolved in 1.00 liter of pure water in order to prepare a solution of the same concentration as that obtained in (b)?

e) Name an appropriate indicator for the reaction shown in equation III and describe the change you would observe at the end point of the titration.

AP Question 1995 B

A sample of dolomitic limestone containing only CaCO3 and MgCO3 was analyzed.

a) When a 0.2800 gram sample of this limestone was decomposed by heating, 75.0 milliliters of CO2 at 750 mm Hg and 20 oC were evolved. How many grams of CO2 were produced?

b) Write equations for the decomposition of both carbonates described above.

c) It was also determined that the initial sample contained 0.0448 grams of calcium. What percent of the limestone by mass is CaCO3?

d) How many grams of the magnesium-containing product were present in the sample in (a) after it had been heated?

2000 AP Question

Answer the following questions about BeC2O4(s) and its hydrate.

(a) Calculate the mass percent of carbon in the hydrated form of the solid that has the formula BeC2O4 · 3 H2O.

(b) When heated to 220°C, BeC2O4 · 3 H2O dehydrates completely as represented below.

BeC2O4 · 3 H2O (s) ( BeC2O4 (s) + 3 H2O (g)

If 3.21 grams of BeC2O4 · 3 H2O is heated to 220°C, calculate

(i) the mass of BeC2O4 (s) formed, and,

(ii) the volume of the H2O(g) released, measured at 220.°C and 735 mm Hg.

(c) A 0.345 gram sample of anhydrous BeC2O4 , which contains an inert impurity, was dissolved in sufficient water to produce 100. mL of solution. A 20.0 mL portion of the solution was titrated with KMnO4 (aq). The balanced equation for the reaction that occurred is as follows.

16 H+ (aq) + 2 MnO4-1 (aq) + 5 C2O42- (aq) ( 2 Mn2+ (aq) + 10 CO2 (g) + 8 H2O (l).

The volume of 0.0150 M KMnO4 (aq) required to reach the equivalence point was 17.80 mL.

(i) Identify the reducing agent in the titration reaction.

(ii) For the titration at the equivalence point, calculate the number of moles of each of the following that reacted.

• MnO4-1(aq)

• C2O42- (aq)

(iii) Calculate the total number of moles of C2O42- (aq) that were present in the 100. mL of

prepared solution.

(iv) Calculate the mass percent of BeC2O4 (s) in the impure 0.345 gram sample.

1992 D AP Question

Four bottles, each containing about 5 grams of finely powdered white substance, are found in a laboratory. Near the bottles are four labels specifying high purity and indicating that the substances are glucose (C6H12O6), sodium chloride (NaCl), aluminum oxide (Al2O3), and zinc sulfate (ZnSO4).

Assume that these labels belong to the bottles and that each bottle contains a single substance. Describe the tests that you could conduct to determine which label belongs to which bottle. Give the results you would expect for each test.

1) 3.67 grams of potassium dichromate is dissolved in enough water to make exactly 250.00 mL of solution. What is the molar concentration of:

i) potassium dichromate

ii) dichromate ion

iii) potassium ion

2) What volume of 0.250 M solution of NaOH contains exactly 20 grams of NaOH?

(This problem is to be done without calculators!).

3) Vitamin C has the formula C6H8O6. Besides being an acid, it is a reducing agent. One method for determining the amount of vitamin C in a sample is to titrate it with a solution of Br2 (an oxidizing agent). A 1.00 gram chewable vitamin C tablet requires 27.85 mL of 0.102 M Br2 to reach the equivalence point. What is (i) the mass of the vitamin C in the tablet, and (ii) what is the percent of vitamin C in the table?

C6H8O6 + Br2 (2 HBr + C6H6O6

4) To analyze an iron-containing compound (example would be an iron ore sample from a mine) you first convert all of the iron to Fe2+ in aqueous solution and then titrate the solution with standardized KMnO4 (in an acidified solution). A 0.598 gram sample of the iron-containing compound requires 22.25 mL of 0.0123 M KMnO4 for the titration to reach the equivalence point. The unbalanced equation is below.

MnO41- + Fe2+ + H3O+ ( Mn2+ + Fe3+ + H2O

Answer the following questions:

a) What is the oxidation number of manganese in permanganate ion, MnO41-?

b) Identify the reducing agent in the reaction?

c) What is reduced in the reaction?

d) How does the oxidation number of iron change in the problem?

e) What is the coefficient for water when the equation is balanced?

f) What is the mass percent of the iron in the sample?

-----------------------

L

M

MOLES

“Litre” is the European spelling of “liter”

This is very common on the multiple choice part of the AP exam. Simple numbers – remember no calculators!!!

NOTE: This problem, in some form, has been on the free-response section of the AP Chemistry test at least TEN times since 1970. That is 25% of the tests had a question like this!!!!

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download