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Solutions
- A solution is a homogeneous mixture composed of a solvent and one or more solutes.
- A solute is a substance dissolved in the solvent.
- The solvent is the substance that dissolves the solute or solutes.
Compounds and Ions in solution
Ionic compounds often dissociate into their ions when dissolved in water. Compounds that undergo complete (100%) dissociation (like NaCl) are called strong electrolytes because their solutions are good electrical conductors. Some compounds only partially dissociate in water (like H 3 PO 4 ) and are called weak electrolytes , because their solutions are poor electrical conductors.
Dissolution of Sodium Chloride in Water
Attraction of water molecules to ions because of
the ion-dipole force.
Covalent compounds (like glucose, C 6 H 12 O 6 ) do
NOT dissociate in water and are called
nonelectrolytes, because their solutions do not
conduct electricity.
Note: Pure water (which we rarely actually
have) is a very poor electrical conductor.
Compounds and Ions in solution
Sugar Cube Dissolving in Water
Molarity
Concentration is the measure of the amount of solute in a solution (part / whole). Molar concentration, or molarity is a measure of the moles of a solute in one liter of solution. Brackets around a formula indicate the concentration of the substance is being discussed: [NaCl] means “the molarity of NaCl”
M
Lsolution
molsolute
volumeofsolution
molesofsolute
V
Molarity n
Calculating Concentration
Example #1: What is the concentration of a
solution found to contain 0.00834 mol of BaCl 2
in a 20.0 mL sample of solution?
Example #2: What is the concentration of
sodium hypochlorite solution prepared by
dissolving 5.66 g of NaOCl in enough water to
make 250.0 mL of solution?
Preparing a Solution
To prepare a solution of known concentration from a solid substance soluble in water: Determine the mass required to make the desired volume of the solution. Dissolve that quantity of solid in the appropriate volumetric flask. Example: An experiment calls for 250.0 mL of 0.2000 M solution of CuSO 4. How could we prepare this solution starting with solid CuSO 4 •5H 2 O?
The calculated amount of the solid compound is
massed out on an analytical balance.
The copper (II) sulfate pentahydrate is then
transferred carefully to the volumetric flask.
Dissociation of Ionic Compounds
When an ionic compound like MgBr 2 dissociates in water, one magnesium ion and two bromide ions are released into solution for each formula unit: What are the concentrations of iron and phosphate ions in a 0.10 M Fe 3 (PO 4 ) 2 solution?
^
(aq) 2 2(aq) (aq) HO
MgBr2(s) MgBr Mg 2 Br
2
Mass Percent
Calculate the percent by mass of magnesium chloride in a solution if 18.3 g dissolved in 250.0 mL of pure water. (DH2O=1.00 g/mL) What mass of aluminum nitrate is in 500.0 mL of a solution that is 7.85% aluminum nitrate? The density of the solution is 1.093 g/mL.
masssolution
masssolute
Mass %
Parts per million (ppm)
masssolution
masssolute
ppm
Calculate the ppm concentration of a solution
that contains 265 mg of mercury ions in 8.00 L
of solution (D = 1.00 g/mL)
Express the above ppm concentration as a
percent by mass AND in units of molarity.
N - Nitrosodimethylamine (NDMA), a by-product of drinking water chlorination, is a chemical that is highly toxic and is a suspected human carcinogen. The EPA has determined that the maximum admissible concentration of NDMA in drinking water is 7 ng/L. Tap water analysis in Upper San Leandro/Oakland Area has found concentrations of NDMA as high as 0.021 ppb (parts per billion). Is this within the EPA limit of 7 ng/L? Assume the density of tap water is 1.00 g/mL. Recall: nano = 10-^9
