Download Solutions, Solubility Rules, and Molarity File

Solutions, Solubility Rules, and
Molarity
Solutions
• Solutions are defined as
homogeneous mixtures of two or
more pure substances.
• Aqueous solution – solution in which
water is the dissolving medium
• The solvent is present in greatest
abundance.
• All other substances are solutes;
they are dissolved in the solvent.
– Example: NaCl dissolved in water:
NaCl = solute
water = solvent
• Water can dissolve many ionic or
molecular compounds – “universal
solvent”
2
Dissociation
• When an ionic substance
dissolves in water, the
solvent separates the
individual ions from the
crystal.
• This process is called
dissociation.
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Dissociation
• This occurs because water
is POLAR – it has poles: a
partial positive (δ+) end and
a partial negative (δ-) end
δO
H
δ+
H
δ+
O
H
•
Solvation – the surrounding of ions
by H2O molecules to help stabilize
the ions
- Keeps the ions from
recombining
NaCl(aq) 
Na+(aq)
+
Cl-(aq)
H
H
O
H
H
Na+
O
H
O
H
H
H
H
Cl-
O
H
H
O
H
H
H
H
O
4
O
Electrolytes
• An electrolyte is a substance that
dissociates into ions when
dissolved in water.
– May be strong or weak
• Strong electrolyte - completely
dissociates into ions
– Solutions conduct electricity well
H+(aq) + Cl-(aq)
Ex: HCl(aq)
• Weak electrolyte – partially dissociates
into ions, but some molecules remain
intact
– Solutions conduct electricity poorly
Ex: CH3COOH(aq)
CH3COO-(aq) + H+(aq)
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Electrolytes
• A nonelectrolyte may
dissolve in water, but it
does not dissociate into
ions when it does so.
– Solutions do not conduct
electricity
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Identifying Strong & Weak
Electrolytes & Nonelectrolytes
Strong electrolytes
1) Strong acids
2) Strong bases
3) Ionic compounds
These are
on the list
below
Weak electrolytes Any other
1) Weak acids
acid or
base NOT
2) Weak bases
on the list
below
Nonelectrolytes
Molecular
compounds, except
acids and bases
Memorize!
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Solution Chemistry
• Pay attention to exactly what species are present in a
reaction mixture (i.e., solid, liquid, gas, aqueous
solution).
• If we are to understand reactivity, we must be aware of
just what is changing during the course of a reaction…
the driving force!
– The driving force is what makes the reaction react (Ex:
formation of a precipitate, gas, or liquid)
– Without a driving force, the reaction won’t occur
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Precipitation Reactions
When one mixes ions
that form compounds
that are insoluble (as
could be predicted by
the solubility
guidelines), a precipitate
is formed.
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Solubility Rules
Solubility of a substance is the amount that can be dissolved in a given quantity of
solvent at a given temperature.
Insoluble – the attraction between the ions in the solid is too great to separate the
ions to any significant extent; substance doesn’t dissolve
Memorize!
***All compounds of alkali metals (Group 1A) and of NH4+ ion are soluble ***
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Solubility Rules
Classify the following as
soluble or insoluble:
a) CuCO3
b) Mg(OH)2
c) CaCl2
d) SrSO4
e) (NH4)3PO4
f) Fe2S3
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Electrolytic Properties ≠ Solubility
Do not confuse extent of solubility with strong or weak
electrolyte
Examples:
• Acetic acid (CH3COOH or HC2H3O2): very soluble in
water but weak electrolyte because most remains in
form of molecule, not ions, in solution
• BaCl2: not very soluble in water but strong electrolyte
because what does dissolve dissociates completely
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Molarity
• Concentration – tells how much solute is dissolved in a
solvent
• Molarity is one way to measure the concentration of a
solution.
moles of solute
Molarity (M) =
volume of solution in liters
Ex: 2.1 M MgCl2 = 2.1 mol MgCl2/L MgCl2 solution
(read as:) “2.1 molar solution of MgCl2”
• Because the units of molarity are mol/L, we can use molarity as a
conversion factor in stoichiometric calculations to interconvert:
moles of solute
liters of solution
2.1 mol MgCl2
1 L solution
or
1 L solution
2.1 mol MgCl2
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Using Molarities in
Stoichiometric Calculations
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Molarity: Examples
1.
Calculate the molarity of 142 mL of an aqueous solution containing 13.31 g
NaNO3.
1.10 M
2.
Calculate the molarity of a 5.623 g sample of NaHCO3 dissolved in enough
water to make 250 ml of solution.
3.
How many grams of KOH are needed to make 500 ml of a 4.8M solution?
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Dilution: Adding water
If an original solution is diluted, the molarity of the new
solution can be determined from the equation:
M1  V1 = M 2  V2
moles of solute before dilution = moles of solute after dilution
(because all we are doing is adding solvent to the original solution)
where M1 and M2 are the molarity of the initial (concentrated) and
final (dilute) solutions, respectively, and V1 and V2 are the volumes of
the two solutions.
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Dilution: Example
If a stock solution of 25.0 mL of 1.00 M HCl is diluted to a
total of 100.0 mL, what is the final concentration of the HCl
solution?
M1  V1 = M 2  V2
M2 = M1V1
V2
M2 = (1.00 M)(0.0250 L)
0.1000 L
M2 = 0.250 M
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Dilution: Example
What is the molarity of a 500 ml HNO3 solution
that was made from 35 ml of a 20. M stock
solution?