Chapter 4
H2O, The Universal Solvent
Much of chemistry that affects each of us occurs
among substances dissolved in water. Virtually all
chemistry that makes life possible occurs in an
aqueous environment.
Aqueous Solution: a solution in which water is the
dissolving medium or solvent.
Solvent: the dissolving medium in a solution.
Solute: a substance dissolved in a liquid to form a
solution.
H2O, The Universal Solvent
Waters polar characteristics: Electrons shared
spend more of their time with the oxygen atom
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http://www.chem1.com/acad/webtext/chembond/cb05.html
H2O, The Universal Solvent
Hydration: hydration of ions tends to cause salts
to “fall apart” or dissolve.
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acl2.gif
H2O, The Universal Solvent
It is very important to recognize that when an ionic
substance dissolves in water they break up into the
individual cations and anions.
Example: Ammonium Nitrate in
water
NH4NO3 (s)
H2O (l)
NH4+ (aq) + NO3- (aq)
When cations and anions have been dissolved in
solution they move around independently
H2O, The Universal Solvent
Solubility: the amount of substance that dissolves in a
given volume of solvent at a given temperature
The solubility of substances in water varies
greatly and for ionic compounds it depends on the
relative attraction between the ions.
“Like dissolves Like”, in general polar or ionic
compounds are expected to be soluble in water
than non polar substances
Strong and Weak
Electrical Conductivity: the ability to conduct
electricity
Strong Electrolytes:
solution that conducts
electricity efficiently
Weak Electrolytes:
solution that conducts
electricity inefficiently
Non electrolytes: solution
that prevents the flow of
electricity
Strong and Weak
Strong Electrolytes completely ionize when dissolved in
water
HCl
HNO3
H2SO4
H2O (l)
H2O (l)
H2O (l)
H+ (aq) + Cl- (aq)
H+ (aq) + NO3 - (aq)
H+ (aq) + HSO4 - (aq)
Strong Acids: a substance that produces H+ ions
(protons) when dissolved in water (virtually every
molecule ionizes)
Strong and Weak
Strong Electrolytes completely ionize when dissolved in
water
NaOH (s)
KOH
H2O (l)
H2O (l)
Na+ (aq) + OH- (aq)
K+ (aq) + OH- (aq)
Strong Bases: a substance that produces OH- ions
(hydroxide) when dissolved in water (virtually every
molecule ionizes)
Strong and Weak
Weak Electrolytes do not completely ionize when
dissolved in water
HC2H3O2
H2O (l)
H+ (aq) + C2H3O2- (aq)
Acetic Acid is very different from strong acids, for
every 100 molecules of acetic acid only 1 will ionize
(approximately 1%)
Weak Acid: any acid that dissociates only to a slight
extent in aqueous solutions.
Strong and Weak
Weak Electrolytes do not completely ionize when
dissolved in water
NH3 + H2O
NH4+ (aq) + OH- (aq)
Ammonia is the most common weak base. It also is very
different from strong bases, for every 100 molecules of
ammonia only 1 will ionize (approximately 1%)
Weak Base: any base that dissociates only to a slight
extent in aqueous solutions.
Molarity
Whenever chemical reactions take place when two
solutions mix and we want to perform Stoichiometry
calculations we must know two things:
1. The nature of the reaction (the chemicals involved)
2. The amounts of chemical involved
Moles of Solute
M = Molarity =
Liters of solution
Molarity
Calculate the molarity of a solution prepared by
dissolving 11.5 grams of Sodium Hydroxide in
enough water to make 1.50 Liters of solution.
First calculate the number of moles of solute:
11.5 grams NaOH x 1 mol NaOH
1
40.00 g NaOH
= 0.288 mol NaOH
Then divide the number of moles by the volume of solution:
0.288 mol NaOH
1.50 L
= 0.192 M NaOH
Molarity
Calculate the molarity of a solution prepared by dissolving
1.56 g of gaseous HCl in enough water to make 26.8 ml of
solution.
First calculate the number of moles of solute:
1.56 grams HCl x 1 mol HCl
1
36.46 g HCl
= 4.28 x 10-2 mol HCl
Then divide the number of moles by the volume of solution:
26.8 ml x
1L .
= 2.68 x 10-2 L
1
1000 ml
4.28 x 10-2 mol HCl = 1.60 M HCl
2.68 x 10-2 L
Molarity
Calculate the number of each type of Ion in a 0.5 M
solution of Fe(ClO4)3.
First determine the balanced reaction:
Fe(ClO4)3 (s)
H2O (l)
Fe3+ (aq)
+ 3ClO4- (aq)
Then multiply the molarity by the coefficients of the
products:
3 ClO3- ions x 0.5 mol
= 1.5 mol ClO4- ions
1 Fe3+ ions x 0.5 mol
= 0.5 mol Fe3+ ions
Molarity
Typical blood serum is 0.14 M NaCl. What volume of blood
would contain 1.0 mg of NaCl?
First determine the number of moles represented by
1.0 mg NaCl:
1.0 mg NaCl x
1
1g NaCl x 1 mol NaCl = 1.71109 x 10-5 mol NaCl
1000 mg 58.44207g
Next determine what volume of 0.14 M contains 1.71109 x
10-5 mol NaCl:
V x 0.14 M NaCl =1.7 x 10-5 mol NaCl
1.7 x 10-5 mol NaCl = 1.2 x 10-4 L
V =
0.14 M NaCl/
Dilution & Molarity
To save time and space solutions are often stored
in a concentrated form. To achieve the molarity
desired you can dilute a known concentration to
produce a new solution.
The key is to understand that we ARE NOT
adding any solute only solvent.
M1V1 = M2V2
Dilution & Molarity
What volume of 16 M sulfuric acid must be used to
prepare 1.5 L of a 0.10 M solution? (M1V1 = M2V2)
First, you know the volume of the final solution and the
two molarity values.
Plug in the values and solve for the unknown
(16 M) V1 = (0.10 M) (1.5 L)
V1 = (0.10 M) (1.5 L)
(16 M)
V1 = 0.0094 L