Solutions Flashcards
Having trouble remembering the difference between molarity and molality, or whether PbBr is soluble? Use these cards to master solution chemistry as tested on the Chemical & Physical Foundations section of the MCAT.
Define:
a solution
It is a homogeneous mixture of two or more substances in a single phase.
For example, NaCl dissolved into water creates a solution of Na+ ions, Cl- ions, and H2O all in one phase (liquid).
Define:
a solvent
It is the substance whose phase remains after solvation (or the substance in excess).
For example, dissolving a small amount of solid NaCl into liquid water produces a liquid solution. In this case, water is the solvent.
Define:
a solute
It is the substance whose phase is lost after solvation (or the substance of which less is present).
For example, dissolving a small amount of solid NaCl into liquid water produces a liquid solution. In this case, NaCl is the solute.
Ionic compounds dissolve readily in polar solvents; what ion form do metals usually take?
cations in solution
Metals, which are found on the left side of the periodic table, mostly form cations by losing electrons to a nonmetal so that both can form stable octets.
Ionic compounds dissolve readily in polar solvents; what ion form do nonmetals usually take?
anions in solution
Nonmetals, which are found on the right side of the periodic table, mostly form anions by gaining electrons from a metal so that both can form stable octets.
What charge do Li, K, and Na usually take when forming ions in solution?
The alkali metals form cations with a single positive charge, +1.
Li, Na, and K are all alkali metals from column 1 of the periodic table.
What charge do Br, Cl, and F usually take when forming ions in solution?
The halogens form anions with a single negative charge, -1.
Br, Cl, and F are all halogens from column 7 of the periodic table.
Give the molecular formula and charge for these common ions:
- ammonium
- chloride
- dichromate
- mercury (II)
- silver
- ammonium, NH4+, +1 charge
- chloride, Cl-, -1 charge
- dichromate, Cr2O72-, -2 charge
- mercury(II), Hg2+, +2 charge
- silver, Ag+, +1 charge
Give the molecular formula and charge for these common ions:
- hydroxide
- barium
- sodium
- permanganate
- sulfite
- hydroxide, OH-, -1 charge
- barium, Ba2+, +2 charge
- sodium, Na+, +1 charge
- permanganate, MnO4-, -1 charge
- sulfite, SO32-, -2 charge
Give the molecular formula and charge for these common ions:
- hydrogen phosphate
- magnesium
- calcium
- bromide
- copper (I)
- hydrogen phosphate, HPO42-, -2 charge
- magnesium, Mg2+, +2 charge
- calcium, Ca2+, +2 charge
- bromide, Br-, -1 charge
- copper (I), Cu+, +1 charge
Give the molecular formula and charge for these common ions:
- sulfate
- nitrate
- peroxide
- hydronium
- iron (II)
- sulfate, SO42-, -2 charge
- nitrate, NO3-, -1 charge
- peroxide, O22-, -2 charge
- hydronium, H3O+, +1 charge
- iron (II), Fe2+, +2 charge
Define:
solvation
It occurs when oppositely charged ends of polar solvent molecules surround solute ions.
For example, water solvates the Na+ ion in the image below, creating a “solvation shell” around it.
Define:
hydration
This term has multiple uses, but assume it refers to solutions here.
It is the process of solvation, specifically when water is used as the solvent.
For example, because water is being used as the solvent in the image below, this can also be called a “hydration shell” of water molecules surrounding the Na+ ion.
Explain the “like dissolves like” rule.
Polar solvents readily dissolve polar solutes, while nonpolar solvents readily dissolve nonpolar solutes.
For example, a nonpolar solute such as naphthalene is insoluble in water, slightly soluble in methanol, and highly soluble in nonpolar benzene.
Explain why the MCAT will rarely refer to an H+ ion in aqueous solution. What ion will be used instead?
Because H+ is simply a proton in solution, it represents a very strong positive ion around which water will form a hydration shell.
More commonly, the hydronium ion (H3O+) is used to represent the fact that a water molecule has bound to the free proton. Rarely seen, but also possible, is H5O2+ (two water molecules sharing one proton) and H7O3+ (three water molecules).
Define:
solubility
It is a measure of how much solute can be dissolved in a given solvent at a specific temperature and pressure.
In general, are salts of the following ions soluble or insoluble in water?
- Nitrates (NO3-)
- Sulfites (SO3–)
- Acetates (CH3COO-)
- Chlorides (Cl-)
- Bromides (Br-)
- Nitrates are always soluble
- Sulfites are insoluble (except in Group I and ammonium compounds)
- Acetates are soluble (except in silver compounds)
- Chlorides are soluble (except in silver compounds)
- Bromides are soluble (except in silver, lead, copper, or mercury compounds)
Describe what is meant by a saturated solution.
It contains the maximum amount of solute that can be dissolved into the solvent at a particular temperature and pressure.
The solution is at equilibrium when fully saturated, so if more solute is added it will not dissolve (or a precipitate will form).
A solute is being added to a solvent, and the solute is readily dissolving. During that time, what do we call the solution?
unsaturated
A solution containing less solute than needed for saturation is said to be unsaturated, and not yet at saturation equilibrium.
A solution is somehow formed that contains more solute particles per volume of solvent than should be possible at that temperature and pressure. What is that solution termed?
supersaturated
A solution containing more solute than needed for saturation is said to be supersaturated; it has exceeded the saturation equilibrium. This is often accomplished by heating a solution, dissolving more solvent, and then cooling it quickly.
Define:
precipitation
It indicates that saturation has been exceeded at that temperature and pressure.
It is the reverse reaction of dissolution. Previously dissolved (solvated) salt ions bond together to form the original salt (solid).
Define:
molarity
(M)
It is a measure of the concentration of a solution, given in units of moles of solute dissolved per liter of solution.
Shown above is the equation for calculating a solution’s molarity.
How many moles of sodium chloride would 2 liters of a 5.0 M solution contain?
10 mol NaCl
Molarity = mol/L
5 mol/L = x mol / 2L
x = 10 moles
Define:
molality
(m)
It is a measure of the concentration of a solution, given in units of moles of solute dissolved per kilogram of solvent.
Shown above is the equation for calculating a solution’s molality.