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Chapter 7: Solutions

A solution is a homogeneous mixture that consists of the solute and the solvent.

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Liquid Solutions

The solute can NOT be separated from the solvent by filtration.

It can ONLY be separated by evaporation of the solvent.

Solute particles are not visible, but can produce a colored solution.

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Types of Solutions

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Water as a Solvent

Water is one of the most common (and important!) solvents as it can dissolve a variety of substances.

Water is highly polar (Ch. 4) with hydrogen bonding intermolecular forces.

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Water as a Solvent

Water dissolves most ionic compounds. When NaCl dissolves in water, sodium ions at

the surface are attracted to the oxygen atoms in water and the chloride ions at the surface are attracted to the hydrogen atoms in water.

Each ion is surrounded by six water molecules referred to as a hydration sphere.

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Water as a

Solvent

Video

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Water as a Solvent

Water also dissolves highly polar molecules like formaldehyde (CH2O) and molecules with hydrogen bonding like methanol.

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CCl4 – a Non-polar solvent

Carbon tetrachloride is a tetrahedral molecule, but each C-Cl bond pulls in an equal and opposite direction making it non-polar.

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CCl4 – a Non-polar solvent

Only non-polar solutes will dissolve in CCl4.

Ex) I2, C10H8, and CS2. What do these have in common?

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Summary of Solubility

“Like dissolves like”

Water

CCl4

Distinct boundary layer

I2 dissolves in non-polar layer

Ni(NO3)2

dissolves in polar layer

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Learning Check

Would the following substances be soluble in Water or CCl4?– Glyceraldehyde (a simple sugar)– Hexane– CaCl2– Br2

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Electrolytes

Pure water is a poor conductor of electricity.

When ions are present, though, water becomes an excellent conductor.

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Strong, Weak, and Non-electrolytes

Strong electrolytes are substances that completely dissociate into ions when in water.

Most ionic compounds are strong electrolytes.

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Strong, Weak, and Non-electrolytes

Weak electrolytes are substances that partially dissociate into ions when in water.

Weak acids and weak bases are weak electrolytes.

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Strong, Weak, and Non-electrolytes

Non-electrolytes are substances that dissolve in water, but produce no ions.

These are molecular compounds like the alcohols and sugars.

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Formation of Ions

Split the following into ions.– NaCl– CaCl2– Na2SO4

– Al(NO3)3

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Equivalent (Eq)

An equivalent is the amount of an electrolyte that produces one mole of charge.

Does NOT depend on the type of charge (positive or negative), just the magnitude of the charge.

1 mole Na+ = 1 Eq 1 mole Ca+2 = 2 Eq 1 mole PO4

-3 = 3 Eq

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Equivalent

Determine the number of equivalents present in:– 0.25 moles of Mg+2

– 0.23g of Na+1

– 4.50g of CO3-2

Convert Eq to grams– A solution contains 0.23 Equivalents of

HPO4-2. What mass is present?

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Electrolytes in Body Fluids

In replacement solutions for body fluids, the electrolytes are given in milliequivalents per liter (mEq/L).

Ringer’s Solution (Saline)

Na+ 147 mEq/L Cl− 155 mEq/L

K+ 4 mEq/L

Ca2+ 4 mEq/L Note that the mEq/L of cations must equal the

mEq/L of anions.

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Solubility

Solubility is the maximum amount of solvent that can be dissolved in a specific amount of solvent.

Usually expressed as the mass of solute per 100. grams of solvent.

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Unsaturated Solution

When a solution could dissolve more of the solute, it is said to be unsaturated.

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Saturated Solution

Contains the maximum amount of solute allowed. If you added more solid, it will not dissolve.

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Solubility and Temperature

Most solid solutes will increase in solubility with an increase in the temperature.

Can produce a super-saturated solution!

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Sodium Acetate

Start with 160g of NaC2H3O2 in 100mL of water at 20oC.

Begin to warm to 100oC. Then cool – no solid forms!

Temperature Solubility Limit20o C 46g / 100mL60o C 139g / 100mL

100o C 170g / 100mL

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Sodium Acetate

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Solubility and Temperature

The solubility of a gas in a liquid decreases with an increase in temperature.

Fish need >5ppm to thrive.

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Concentration of Solutions

The quantity of solute in a solution can be defined as a percentage.

Used commonly in the health field. Mass / Mass Percentage (m/m or w/w).

= mass of solute (g) x 100 mass of solution (g)

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Concentration of Solution

Volume / Volume Percentage (v/v).= volume of solute (mL) x 100 volume of solution (mL)

Mass / Volume Percentage (m/v or w/v).= mass of solute (g) x 100 volume of solution (mL)

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Using a Percentage

A percentage can be written as a fraction and then used as a conversion factor.

Thus, a 5.0% NaCl (m/m) can be written as:

5.0 g of NaCl100.g of solution

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Molarity

Chemists use this method for concentration.

M = moles of solute / Liters of solution. Problems may involve mass that must

be converted to moles.

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Dilution

Stock solutions are often diluted to use in a laboratory.

The moles of solute remain the same! C1 x V1 = C2 x V2 C’s can be either a percentage or a

molarity. V’s can be either milliliters or liters, but

both must be the same unit.

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Solution Stoichiometry

Molarity and volumes can be used to calculate quantities for an aqueous reaction.

Remember that M = Moles / Liters. Will require a mole-to-mole conversion

in second step from balanced reaction.

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Solutions

A solution:– is transparent (may be colored).– contains tiny solute particles, which are not

visible.– does not settle out over time.– can not be filtered.

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Colloids

A colloid contains larger solute particles that causes the “Tyndall effect.”

A collloid:– will be opaque.– can not be filtered.– does not settle out over time.– can be separated by a semi-permeable

membrane.

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Types of Colloids

Aerosol = a liquid or solid in a gas.

Foam = a gas in a liquid or solid.

Emulsion = a liquid or solid in a liquid.

Sol = a liquid or solid in a solid.

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Suspensions

A suspension contains very large solute particles.

A suspension:– is opaque.– will settle out over time.– can be filtered.

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Comparison

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Learning Check

Classify as a solution, colloid, or suspension.– The CaCO3 made a few weeks ago in the

lab (Chemical Reaction exercise).– Mixing a Crystal lightTR packet with water.– Orange juice (no pulp).

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Osmosis

When a semi-permeable membrane separates two solutions, only the solvent molecules can move across the barrier.

If two solutions of different concentrations are placed on either side of the barrier, then the solvent will flow from the lower solute side to the higher solute side.

To prevent this, an external pressure would be needed to stop the net flow.

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Osmosis

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Osmotic Pressure

The pressure necessary to prevent the net flow is proportional to the molarity of the solution.

Higher molarity = Higher osmotic pressure.

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Learning Check

A semipermeable membrane separates a 10% sucrose solution A from a 5% sucrose solution B. If sucrose is a colloid, fill in the blanks in the statements below.1. Solution ____ has the greater osmotic pressure.2. Water initially flows from ___ into ___.3. The level of solution ____will be lower.

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Importance of Osmosis

Our cells have semi-permeable membranes to contain the cell materials inside.

Our blood plasma has an osmotic pressure equal to that of our red blood cells.

Any external fluid administered like an I.V. must be isotonic.

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Isotonic Solutions

A saline solution (NaCl) will have a concentration of 0.90% (m/v).

A glucose (aka dextrose) will have a concentration of 5.0% (m/v).

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Hypotonic Solutions

If a solution of lower concentration comes into contact with a red blood cell, that solution is said to be hypotonic.

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Hypertonic Solutions

If a solution of higher concentration comes into contact with a red blood cell, that solution is said to be hypertonic.

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Learning Check

When placed in each of the following, indicate if a red blood cell will1) not change, 2) hemolyze, or 3) crenate.A.____ 5% glucose solution B.____ 1% glucose solution C.____ 0.5% NaCl solution D.____ 2% NaCl solution