NAME Chemistry 162 SFCC Win 2010 Solutions

name _______________________________ chemistry 162 sfcc win 2010 _______ solutions in this learning activity, you will examine, on the m

NAME _______________________________ Chemistry 162 SFCC Win 2010
_______
Solutions
In this learning activity, you will examine, on the molecular level,
why some substances mix well together and some do not. You will
compare molecular characteristics and relate these characteristics to
solubility. You will study mixtures and the dissolving process. When
different substances evenly disperse in each other with a clear and
transparent appearance, the mixture is called a solution. A solution
is specified by the concentration of each substance present in it. You
will learn several common ways used to express solution concentration
in modern society.
After completing this learning activity packet, you should be able to
do the following:
(1) Describe the process of dissolving on a molecular level; explain
how molecular characteristics influence this process.
(2) State the like-dissolves-like rule and give actual examples.
(3) Define each of these terms in relation to a solution: solvent,
solute and concentration.
(4) Explain and compare different concentration units: percentage,
mg/L, ppm, ppb and molarity.
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I. Why Substances Dissolve
Both salt and sugar dissolve in water, yet neither oil nor gasoline
do. Why do some substances mix well together and others do not? The
answer has to do with the interactions among different molecules or
ions. The diagrams in Figure 1 illustrate dissolving on a molecular
level: B molecules are pulled apart by the attractive forces between
molecules A and B (indicated by the dotted lines) to disperse evenly
among A.
Figure 1.
Dissolving Process On The Molecular Level

A. The Like - Dissolves - Like Rule
Numerous experiments have verified that compounds with compatible
characteristics of molecular structure and polarity mix well with each
other. This fact is called the like-dissolves-like rule. The
Like-dissolves-like rule is basic to our understanding of the
dissolving process on the molecular level. Similar structural features
and polarity cause molecules of one compound to attract those of
another compound. If this intermolecular attractive force is stronger
than the existing attractive forces among molecules of the same type,
the two compounds will dissolve in each other.
For example, as discussed in LAP FC.4, water is a polar compound. The
water molecule, H2O, has two O-H bonds with a partial charge
separation between the H atoms (+ pole) and the oxygen atom (-
pole). Sugar molecule (sucrose, C12H22O11) has a similar structural
feature of multiple O-H bonds with - oxygen and + hydrogen atoms.
This is why sugar and water mix well together. Sugar is one of the
hydrophilic or "water-loving" substances (hydro- means water, -philic
is derived from the Greek word philos, meaning, loving).
Q

Figure 2. Structure of Sucrose
1. (a) Circle and identify all the O-H bonds in the structure of
sucrose in Figure 2.
(b) How many OH bonds are present in this structure? _____
Similar to sugar, table salt is also a hydrophilic substance. Table
salt (sodium chloride, NaCl) is composed of sodium cations (Na+1) and
chloride anions (Cl-1). The + and - charges are comparable to the +
and - polarity in water. This is why salt is very soluble in water.
In addition, ions dissociate in water. Refer to LAP FC.4, Water and
Its Properties, section V, for a more detailed discussion concerning
dissociation of ions in water.
Oil and gasoline are examples of hydrophobic or "water-fearing"
substances (-phobic is derived from the Greek word phobos, meaning, a
fear). Every time you use oil-and-vinegar dressing on your salad, you
have to first shake it up, because the dressing’s oil and water
naturally stay separated. Gasoline also stays separate from water
instead of dissolving in it. On the other hand, the two hydrophobic
substances, gasoline and oil, mix very well with each other. All of
these phenomena show that like-dissolves-like rule is at work!
Let us examine the structures of two hydrophobic compounds. Figure
3(a) shows the structure of heptane, a typical molecule found in
gasoline. Heptane has a structural feature of repeating -CH2- groups.
This feature is also found in Figure 3(b), which shows a typical
structure of corn oil molecules.
Figure 3 (a) Typical Molecule Found In Gasoline Oil
H H H H H H H
│ │ │ │ │ │ │ This structure can be written in a condensed way
H C  C  C  C  C  C  C  H as: CH3CH2CH2 CH2CH2CH2CH3
│ │ │ │ │ │ │
H H H H H H H
O H H
  
CH2-O-CCH2CH2CH2CH2CH2CH2CH2C=CCH2CH2CH2CH2CH2CH2CH2CH3
CH-O-COCH2 CH2CH2CH2CH2 CH2CH2CH2CH2CH2CH2CH2CH2 CH2CH3
CH2-O-C-CH2CH2CH2CH2CH2CH2CH2C=CCH2CH2CH2CH2 CH2CH2CH2CH3
 
O H H
Figure 3 (b) Typical Molecule Found In Corn Oil (written in the
condensed way.)



Q2. The structures of molecules in gasoline and oil are dominated by
many C-C and C-H bonds.
(a) Is the C-C bond polar or nonpolar? __________ (b) Is the C-H bond
polar or nonpolar? ___________
(c) Explain why gasoline and oil do not mix well with water on the
molecular level.
______________________________________________________________________________________
B. Applications Of The Like - Dissolves - Like Rule
Perform the following hands-on exercise to examine how different
compounds mix with water.
Put On Safety Goggles and Protective Clothing !!
Obtain a designated test tube tray marked for solubility test. There
are five test tubes in this tray, each containing a different
compound: test tube #1, methanol (CH3OH), #2 acetone (CH3COCH3) , #3
methylene chloride (CH2Cl2), #4 hexane (C6H14), and #5 sodium iodide
(NaI­). The compounds in test tubes #1 through #4 are volatile, which
means they vaporize easily at room temperature. Avoid inhaling any
vapor by keeping the test tubes stoppered. Open a stopper only briefly
as directed. Never direct the mouth of a test tube toward any persons
while shaking. If skin contact occurs, wash with soap and plenty of
tap water.
With a plastic transfer pipette, deliver 1 mL distilled water into
each test tube. Stopper and gently invert each test tube to mix the
contents. Due to possible vapor build-up, vent the test tube after
mixing by opening the stopper briefly.
Q3. (a) Record in Table 1 whether the compound in each test tube mixes
well with water.
(b) Write the formula for each compound in Table 1.
(c) Calculate the difference in electronegativity of each of the
following types of chemical bond. These bonds are found in the
compounds given in Table 1.
C-H _______ C-O ______ O-H _______ C-C _______ C-Cl _______ Na - I
_______
(d) Label the polar (electronegativity difference > 0.5) and ionic
(electronegativity difference > 1.7) bonds in each model structure in
Table 1 as shown for the structure of methanol.
Table 1. Solubility of Different Compounds in Water

Test tube #
1
methanol
2
acetone
3
methylene chloride
4
hexane
5
sodium iodide
Formula
O-H
C-O
Model
Structure
Label the polar and ionic bond(s) in each structure.





Does this compound mix well with water?
Q4. Does the solubility of each compound in Table 1 correlate with its
molecular characteristics according to the like-dissolves-like rule?
________ Explain.
_______________________________________________________________________________________
The next solubility tests involve iodine (I2). Complete the following.
Q5. (a) Is iodine, I2, polar or nonpolar? _______________
(b) Based on the like-dissolves-like rule, predict whether I2 will mix
better with water or methylene chloride (CH2Cl2). _______ Explain why.
_______________________________________________________________________________________
Obtain a grain of I2 crystal from its reagent vial with a pair of
tweezers. Drop it into test tube #3 (containing water and CH2Cl2).
Stopper and gently invert the test tube to mix its contents.
(c) Report your observations.
_______________________________________________________________
(d) Did the observation in (c) verify your prediction in (b)? _______
Explain.
_______________________________________________________________________________________
Q6. (a) Predict whether iodine will mix better with hexane (C6H14) or
water. __________ Explain.
_______________________________________________________________________________________
Drop a grain of I2 crystal into test tube #4 (containing water and
hexane). Stopper and gently invert the test tube to mix its contents.
(b) Report your observations.
_______________________________________________________________
(c) Did the observation in (b) verify your prediction in (a)? _______
Explain why.
_______________________________________________________________________________________
II. Solutions are Homogeneous Mixtures
If two or more substances are put together and the mixture does not
have a uniform appearance, it is called a heterogeneous mixture. If
the mixture has a uniform, clear and transparent appearance, it is
called a solution. A solution is a homogeneous mixture of two or more
elements and /or compounds. Usually the major component in a solution
is labeled as the solvent and all other components are each called a
solute.
A solution and a pure compound differ in terms of composition. A pure
compound has a definite composition; the elements in the compound
combine in definite ratios, specified by its chemical formula. A
solution, on the other hand, has variable composition; the components
in a solution do not have any definite combining ratios. This is why a
solution may be diluted or concentrated. To specify a particular
solution, we need to tell what chemicals are in it, as well as the
concentration of each compound or element present.
Q7. Examine the rack of test tubes labeled P through W to identify the
type in each tube as either a solution or mixture. (Do not open the
stopper on any test tube.) Complete Table 2. Each test tube contains
either a solution or a heterogeneous mixture. If a tube contains a
solution, tell which component is the solvent.
Table 2. Test Tubes of Mixtures
Name
Content
mixture / solution
solvent used
P
Cobalt nitrate & water
H2O (15 mL), Co(NO3)2 (0.1 g)
Q
Copper sulfate & water
H2O (15 mL), CuSO4 (0.1 g)
R
Cobalt chloride & water
H2O (15 mL), CoCl2 (0.1 g)
S
Isopropyl alcohol and water (Rubbing alcohol)
C3H8O (7 mL), H2O (3 mL)
T
Acetic acid and water (vinegar)
H2O (9.5 g) , C2H4O2 (0.5 g)
U
Hexane and water
hexane (4 mL), water (4 mL)
V
Biphenyl and water
water (10 mL) , biphenyl (0.1 g)
W
Orange juice
water and orange pulp
Note that most of the solutions in Table 2 use water, these are
aqueous solutions, which are the most common type.
III. Solution Concentrations
Solution concentration specifies the amount of solute present in
certain standard quantity of solvent. There are many different ways to
express the concentration of a solution. Commonly used concentration
units include percentage (%), parts-per-million (ppm),
parts-per-billion (ppb) and molarity (M). These are summarized in
Table 3. Each concentration unit can be seen as a ratio of the amount
of solute over a certain amount of solvent or solution. In this
section, we will discuss the practical significance of each
concentration unit, and how to convert or relate one concentration
unit to another.
Table 3. Commonly Used Concentration Units
Unit of concentration
Molarity
Percentage or Parts per hundred
Parts per million
Parts per billion
Expression
M
%
ppm
ppb
solute in unit of:
mole
part
part
part
amount of solution
1 Liter
100 parts
1,000,000 parts
1,000,000,000 parts
Ratio format
x mole

1 L
x part

100 parts
x part

1,000,000 parts
x part

1,000,000,000 parts
common usage
In chemistry laboratories
relatively high concentrations
relatively low concentrations
extremely low concentrations
Example
Stomach juice contains 0.15 M HCl.
Rubbing alcohol is a 70% solution of alcohol and water.
Danger level for carbon monoxide (CO) in air is 9 ppm in 8
hour-period.
Maximum contamination level of lead (Pb) in drinking water is 50 ppb.
A. Molarity (M)
Molarity (M) is the concentration unit most frequently used in
chemistry. Molarity specifies the number of moles of a solute present
in one liter (L) of a solution. Each mole equals 6.02  1023
individual units. For examples: every mole of acetone contains 6.02 
1023 molecules of acetone, C3H6O. Every mole of sodium chloride, NaCl,
contains 6.02  1023 pairs of the ions sodium (Na+1) and chloride (Cl-1).
An aqueous solution of 1 M sucrose, for example, contains 1 mole of C12H22O11
per liter of this solution.
Q8. Calculate the mole(s) of solute present in solutions (a) through
(d).
(a) A 0.05 M KCl solution contains ______________ mole(s) of KCl in
one liter of the solution.
(b) A 1.0 M HCl solution contains ___________ mole(s) of HCl in two
liters of the solution.
(c) A 0.15 M NaOH solution contains _____________ mole(s) of NaOH in
0.5 liter of the solution.
(d) A 0.83 M HC2H3O2 solution contains ____________ mole(s) of HC2H3O2
in 2.6 liters of the solution.
(e) The common arithmetic procedure you used to find the answers to
(a) through (d) is:
_______________________________________________________________________________________
B. Percentage (%), Parts Per Million (ppm) and Parts Per Billion (ppb)
Solution concentrations are also expressed in three other related
units: percentage (%), parts per million (ppm) and parts per billion
(ppb). These are related because they all designate concentration by
the parts of a solute in a specified parts of the solution.
*
Percentage (%) concentration tells the amount of solute present in
100 parts of the solution.
% = (amount of solute  amount of solution )  102
*
Parts per million (ppm) measures the parts of solute in one
million parts of the solution.
ppm = (amount of solute  amount of solution )  106
*
Parts per billion (ppb) measures the parts of the particular
solute in one billion parts of the solution.
ppb= (amount of solute  amount of solution )  109
In calculating concentrations using any of the above three units, the
“amount” of solute and that of solution should be given in the same
units. For example, if the amount of solute is given in grams (g), the
amount of solution is also given in grams. If the amount of solute is
given in milliliters (mL), that of the solution is also given in mL.
However, since 1 g = 1 mL for most aqueous solutions, in some cases,
solute is given in grams and solution is given in mL, or vice versa.
Q9. (a) A solution contains 2 L of a solute in 1 L of the solution.
What is its concentration in ppm? Explain.
______________________________________________________________________________________________
(b) A solution contains 0.5 mL of a solute in 1 L of the solution.
What is its concentration in ppm? Show work.
_______________________________________________________________________________________
(c) A 1% acetone solution contains _______ L acetone in 10 mL of the
solution. Show work.
_______________________________________________________________________________________
Q10. (a) A solution contains 5 g of a solute in 1 Kg of the solution.
What is its concentration in ppb? Show work
_______________________________________________________________________________________
(b) A solution contains 3.5 mg per Kg. What is its concentration?
Explain or show work.
_______________________________________________________________________________________
FURTHER STUDIES
Further Application Of The Like-Dissolves-Like Rule
You have learned that some molecules are hydrophobic; they do not mix
with water. Other molecules are hydrophilic; they mix with water well.
A third type of molecules have double nature: a part of such a
molecule is hydrophobic, and the other part of it is hydrophilic. An
example of this type of molecules is soap. Soap is a chemical you use
frequently to clean greasy or oily hands. Without using soap, the more
you try to rinse grease off of your hands with water, the more grease
sticks to your hands. This is because grease, like oil, is
hydrophobic. When soap is present, grease is easily rinsed off with
running water. Soap acts as a middle agent to bring grease and water
together. Soap does this by having a molecular structure that contains
both a polar segment and a nonpolar segment. The structure of a
typical soap molecule is shown in Figure 4.
Figure 4. A Typical Soap Molecule

Q11. (a) Which section in the molecule is polar, the head or the tail?
_______
(b) When coming in contact with grease, which part of soap will mix
with grease, head or tail? _________
(c) Which part of soap will stay in water? _______
As soap molecules pull grease together with it to water, they carry
grease away with them in rinse water.
Examples of Aqueous Solutions
Q12. Almost all of the liquid solutions we encounter in everyday life
are aqueous solutions, with water as the solvent. Look around your
house or consider what you drink each day, to list a few examples of
these aqueous solutions in Table 4. Check product labels, if
necessary, to find the names of solutes in each solution. These you
create on your own.
Table 4. Common Aqueous Solutions
Solution type
solvent
possible solute(s)
water
water
water
water
Solution Concentrations
Q13. Complete the following:
a.
A normal saline solution of 0.017 M NaCl contains ____________
mole(s) of NaCl in 2.5 liters of the solution. Show work.
____________________________________________________________________________________
b.
There are 5 gram of acetic acid in every 100 grams of vinegar. The
percentage of this solution is ______
c.
There are 7 g of minerals in every 1000 grams of milk. The
percentage of minerals in milk is _________
Show work.
__________________________________________________________________________
d.
There are 75 g of potassium iodide (KI) in every gram of iodized
table salt (NaCl).
The concentration of KI in this iodized salt is __________________
ppm.
e.
The maximum contamination level of arsenic allowed in drinking
water is 0.05 milligrams per liter (mg/L). In water and most
aqueous solutions, 1 L = 1 Kg.
This concentration of 0.05 mg/L is equivalent to ________________ ppm,
and ________________ ppb.
Show work.
____________________________________________________________________________________
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