Chemistry Lab/Aqueous Solutions

Aqueous Solutions was a topic for the event Chemistry Lab in 2010 and 2011. This topic focused on the concentrations of solutions and how chemical properties of solutions are changed when mixed. This topic is very broad, considering that many substances used in chemistry are in aqueous form, including Acids and Bases, which are a slightly different focus.

Some General Concepts

Moles

No, not the animal. A mole (abbreviate mol) is a number. Just like a dozen refers to the number 12, a mole refers to the number 6.022E23.

The way scientists came up with this seemingly random number originated with elements and their molar masses. The mole is designed so that amu is the same thing as g/mol. This is useful if you want to figure out the number of atoms in a gram.

Equilibrium

Equilibrium is where the amount of reactant and the amount of product are not changing. For more info on equilibrium see Chem Lab/Equilibrium

Mixtures

Solutions

Solutions are homogeneous (uniform) mixtures. For example: salt water.

Suspensions

Suspensions are heterogeneous (non uniform) mixtures. For example, dirt mixed in water. It is not uniform and the dirt will eventually settle to the bottom of the mixture.

Colloids

Colloids are heterogeneous (non uniform) mixtures. For example, milk. It you take a close look at milk, you'll find it is composed of oil domains and water domains. The difference between colloids and suspensions is that colloids do not eventually settle out (with dirt in water the dirt will eventually settle to the bottom, but with milk the oil stays mixed with the water).

Components of a Solution

Solute

The solute is what is being dissolved. For example, when dissolving salt in water, the salt is the solute.

Solvent

The solvent is what is dissolving the solute. For example, when dissolving salt in water, the water is the solvent.

Solution

A solution is the combination of a solute and a solvent.

Because of the fact that in most solutions the amount of solute is relatively small compared to the amount of solvent, you may usually assume that the volume of solution is the same as the volume of solvent.

Solution Concentration

There are a number of ways to determine solute concentration.

Concentration is typically expressed as [solute]. This expression means the concentration of the solute, usual expressed in terms of Molarity.

Molarity

Molarity or Molar concentration (abbreviated M) is equal to moles of solute over liters of solution.

Mass Concentration

Mass Concentration is equal to mass(g) of solute over volume(L) of solution. It is in some ways similar to density, which is why it is abbreviated [math]\displaystyle{ \rho }[/math].

Mole Fraction

Mole Fraction is equal to the moles of solute over the moles of solution.

Molality

Molality is equal to the moles of solute over kg of the solvent (not the solution).

Mass Fraction

Mass Fraction is similar to the Mole Fraction. It is the mass of the solute over the mass of the solution.

Mass Percentage

Mass Percentage is Mass Fraction times 100.

Parts Per Million (ppm)

Parts Per Million (abbreviated ppm) is Mass Fraction times 1,000,000.

Parts Per Billion (ppb)

Parts Per Billion (abbreviate ppb) is Mass Fraction time 1,000,000,000.

Conversion Between Units

Molarity -> Molality

Multiply by Liters of solution, divide by kilograms of solvent (approximately equal for dilute solutions).

Molality -> Mass Percentage

Multiply by mass of solute, then divide by moles of solute, then multiply by kilograms of solvent, and divide by kilograms of solution (can be approximated by multiplying by molar mass).

Solubility Rules

There are certain rules that dictate which substances dissolve in water and which ones precipitate out.

Always dissolve
- Nitrate ([math]\displaystyle{ NO_3^- }[/math])
- Acetate ([math]\displaystyle{ CH_3COO^- }[/math])
- Cations of alkali metals (e.g. sodium, potassium, etc.)
Sometimes dissolve
- Sulfate ([math]\displaystyle{ {SO_4}^{2-} }[/math])
  - Precipitates with barium, calcium, lead, silver, strontium and mercury (I)
- Halides (except fluoride)
  - Precipitate with silver, lead, and mercury (I)
- Sulfides
Rarely dissolve
- Carbonates ([math]\displaystyle{ {CO_3}^{2-} }[/math])
- Hydroxides ([math]\displaystyle{ OH^- }[/math])
- Phosphates ([math]\displaystyle{ {PO_4}^{3-} }[/math])

There are some exceptions to these rules, including:

[math]\displaystyle{ AgF }[/math] is soluble
[math]\displaystyle{ PbCl_2 }[/math], [math]\displaystyle{ PbBr_2 }[/math], [math]\displaystyle{ PbI_2 }[/math] are mildly soluble at high temperatures
[math]\displaystyle{ Ag_2SO_4 }[/math] is mildly soluble
[math]\displaystyle{ Li_2CO_3 }[/math], [math]\displaystyle{ Li_3PO_4 }[/math], [math]\displaystyle{ LiF }[/math] are insoluble

Solubility

Equilibrium Constant

Take this reaction:

[math]\displaystyle{ aA + bB \to cC + dD }[/math]

The equilibrium constant is equal to:

[math]\displaystyle{ k=\frac{[C]^c * [D]^d}{[A]^a * [B]^b} }[/math]

Where all of the concentrations are the concentrations at equilibrium and where solids are excluded.

For more info on the equilibrium constant, see Chem Lab/Equilibrium.

Solubility Product Equilibrium Constant

The solubility product equilibrium constant (Ksp) is equal to

[math]\displaystyle{ [A^+][B^-] }[/math]

for the following reaction:

[math]\displaystyle{ AB \to A^+ + B^- }[/math]

Unsaturated

Unsaturated solutions are solutions where the Ksp has not yet been reached.

Saturated

Saturated solutions are solutions where the Ksp has been reached.

Super Saturated

Super Saturated solutions are where the Ksp has been reached and gone over. These solutions can be achieved by heating up a solvent (heat causes the Ksp to increase), adding a solute, and then letting the solution cool.

Miscible

Two things are considered miscible when they can be mixed uniformly in any quantities.

Sample Questions

Questions in the aqueous solutions section might involve...

Solution Concentration (Molarity, Molality, Mass Percentage, Parts Per Million)
Conversion Between Units (at state and national levels)
Determining Concentration using Density, Beer's Law or Titration
Freezing Point Depression and Boiling Point Elevation
Factors Affecting Solution Formation
Solubility