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MCAT Gen. Chemistry - OLD > Solutions > Flashcards

Flashcards in Solutions Deck (42)
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1
Q

Define:

a solution

A

A solution 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).

2
Q

Define:

a solvent

A

A solvent 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.

3
Q

Define:

a solute

A

A solute 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.

4
Q

Ionic compounds dissolve readily in polar solvents; what ion form do metals usually take?

A

Metals typically become 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.

5
Q

Ionic compounds dissolve readily in polar solvents; what ion form do nonmetals usually take?

A

Nonmetals typically become 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.

6
Q

What charge do Li, K, and Na usually take when forming ions in solution?

A

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.

7
Q

What charge do Br, Cl, and F usually take when forming ions in solution?

A

The halogens form anions with a single negative charge, -1.

Br, Cl, and F are all halogens from column 7 of the periodic table.

8
Q

Give the molecular formula and charge for these common ions:

  1. ammonium
  2. chloride
  3. dichromate
  4. mercury (II)
  5. silver
A
  1. ammonium, NH4+, +1 charge
  2. chloride, Cl-, -1 charge
  3. dichromate, Cr2O72-, -2 charge
  4. mercury(II), Hg2+, +2 charge
  5. silver, Ag+, +1 charge
9
Q

Give the molecular formula and charge for these common ions:

  1. hydroxide
  2. barium
  3. sodium
  4. permanganate
  5. sulfite
A
  1. hydroxide, OH-, -1 charge
  2. barium, Ba2+, +2 charge
  3. sodium, Na+, +1 charge
  4. permanganate, MnO4-, -1 charge
  5. sulfite, SO32-, -2 charge
10
Q

Give the molecular formula and charge for these common ions:

  1. hydrogen phosphate
  2. magnesium
  3. calcium
  4. bromide
  5. copper (I)
A
  1. hydrogen phosphate, HPO42-, -2 charge
  2. magnesium, Mg2+, +2 charge
  3. calcium, Ca2+, +2 charge
  4. bromide, Br-, -1 charge
  5. copper (I), Cu+, +1 charge
11
Q

Give the molecular formula and charge for these common ions:

  1. sulfate
  2. nitrate
  3. peroxide
  4. hydronium
  5. iron (II)
A
  1. sulfate, SO42-, -2 charge
  2. nitrate, NO3-, -1 charge
  3. peroxide, O22-, -2 charge
  4. hydronium, H3O+, +1 charge
  5. iron (II), Fe2+, +2 charge
12
Q

Define:

solvation

A

Solvation 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.

13
Q

Define:

hydration

This term has multiple uses, but assume it refers to solutions here.

A

Hydration 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.

14
Q

Explain the “like dissolves like” rule.

A

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.

15
Q

Explain why the MCAT will rarely refer to an H+ ion in aqueous solution. What ion will be used instead?

A

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).

16
Q

Define:

solubility

A

Solubility is a measure of how much solute can be dissolved in a given solvent at a specific temperature and pressure.

17
Q

In general, are salts of the following ions soluble or insoluble in water?

  1. Nitrates (NO3-)
  2. Sulfites (SO3)
  3. Acetates (CH3COO-)
  4. Chlorides (Cl-)
  5. Bromides (Br-)
A
  1. Nitrates are always soluble
  2. Sulfites are insoluble (except in Group I and ammonium compounds)
  3. Acetates are soluble (except in silver compounds)
  4. Chlorides are soluble (except in silver compounds)
  5. Bromides are soluble (except in silver, lead, copper, or mercury compounds)
18
Q

Describe what is meant by a saturated solution.

A

A saturated solution 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).

19
Q

A solute is being added to a solvent, and the solute is readily dissolving. During that time, what do we call the solution?

A

The solution is unsaturated

A solution containing less solute than needed for saturation is said to be unsaturated, and not yet at saturation equilibrium.

20
Q

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?

A

The solution is 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.

21
Q

Define:

precipitation

A

Precipitation is the reverse reaction of dissolution. Previously dissolved (solvated) salt ions bond together to form the original salt (solid).

Precipitation indicates that saturation has been exceeded at that temperature and pressure.

22
Q

Define:

molarity (M)

A

Molarity is a measure of the concentration of a solution, given in units of moles of solute dissolved per liter of solution.

The equation for calculating a solution’s molarity is:

23
Q

How many moles of sodium chloride would 2 liters of a 5.0 M solution contain?

A

10 mol NaCl

Molarity = mol/L

5 mol/L = x mol / 2L
x = 10 moles

24
Q

Define:

molality (m)

A

Molality is a measure of the concentration of a solution, given in units of moles of solute dissolved per kilogram of solvent.

The equation for calculating a solution’s molality is:

25
Q

58.5 grams of NaCl are dissolved in 2.0 kg of water at room temperature. What is the molality of the solution?

A

The solution is 0.5 m.

molality = mol/kg
The molecular weight of NaCl is 58.5 g, so 1 mole is present.

molality = 1mol / 2kg = 0.5m

26
Q

Define:

mole fraction (x)

A

Mole fraction is a measure of the concentration of a solution, given as a ratio of moles of one component to total moles of all components.

The equation for calculating the mole fraction of a solute is:

27
Q

Define:

percent composition by mass (%)

A

Percent composition by mass is a measure of the amount of one component of a compound in grams compared to the total number of grams of all components.

The equation for calculating the percent composition of a particular solute in a solution is:

28
Q

Define:

parts per million (ppm)

A

Parts per million is a measure of the concentration of one component of a mixture in kilograms compared to the total number of kilograms of all components of the mixture.

The equation for calculating parts per million of a particular solute in a solution is:

29
Q

What equation can be used to calculate the change in molarity of a diluted solution?

A

M1V1 = M2V2

Where:

  • M1 = initial molarity (mol/L)
  • V1 = initial volume (L)
  • M2 = final molarity (mol/L)
  • V2 = final volume (L)
30
Q

Define:

normality (N)

A

Normality is the concentration of the reactive species in mole-equivalents compared to the volume of solvent in liters.

Normality (N) = Molarity * (equivalents of reactive species per mole)

Normality is generally used to quantify acidic or basic equivalents.

31
Q

What is the acid normality (N) of a 1 M solution of sulfuric acid?

A

The sollution is 2N.

Normality (N) = Molarity * (equivalents of reactive species per mole)

In this case, equivalents of acid are being asked for. For H2SO4, two H+ ions dissociate in solution for every one H2SO4. 1M * (2 equivalents) = 2N.

32
Q

Define:

solubility product constant (Ksp)

A

The solubility product constant (Ksp) is the equilibrium constant for the solvation reaction of a salt.

Ksp = [anions]a[cations]c

Just like other equilibrium constants, Ksp is equal to the concentration of products over reactants raised to the power of their coefficients. However, since pure liquids and solids are omitted from equilibrium calculations, the equation simplifies to just the concentration of ions in solution, raised to the power of their coefficients.

33
Q

If the temperature of a solution is increased, what happens to the solute’s Ksp?

A

Ksp increases.

A warmer solvent can dissolve more solutes. This leads to a higher concentration of ions in solution, so Ksp increases.

34
Q

What is the solubility product constant formula for AgCl, which dissociates as written below?

AgCl(s) ⇔ Ag+(aq) + Cl-(aq)

A

Ksp = [Ag+][Cl-]

The coefficient in front of both silver and chloride is 1, so the exponents will also be 1.

35
Q

What is the solubility product constant formula for BaF2, which dissociates as written below?

BaF2(s) ⇔ Ba++ (aq) + 2F-(aq)

A

Ksp= [Ba++][F-]2

The coefficient in front of barium is 1, so the exponent will be 1 for [Ba2+]. The coefficient in front of fluoride is 2, so the exponent will be 2 for [F-].

36
Q

Define:

common ion effect

A

The common ion effect states that a salt will be less soluble in a solution containing product ions than it is in pure solvent. A solution containing ions that a salt would separate into when dissolved is referred to as having common ions.

For example, AgCl will be less soluble in chlorinated water (with Cl- ions present) than in pure water. The added Cl- ion (product) shifts equilibrium to the side containing the solid salt (reactant).

37
Q

What will happen if NaCl is added to a saturated aqueous solution of AgCl?

Note that NaCl has a higher solubility than AgCl.

A
  • NaCl will still dissolve.

The presence of Cl- ions from AgCl will not prevent the highly soluble NaCl from dissociating.

  • AgCl will precipitate.

The additional Cl- ions now present exceed the saturation levels for AgCl in solution, and will force those ions back into solid AgCl salt form.

38
Q

A given solution contains PbCl2 and PbSO4. How could lead (II) chloride be selectively harvested from the solution?

A

If a highly soluble chloride salt such as NaCl were added to the solution, the common ion effect of additional Cl- ions would lead to the selective precipitation of lead (II) chloride.

39
Q

Define:

complex (or coordination complex)

A

A complex is formed when a metallic atom or ion is bonded to a surrounding array of molecules or anions; these are usually written with brackets as [complex].

Complexes cannot be solvated back into their original atoms or ions.

For example, cisplatin [PtCl2(NH3)2] is formed when the Pt2+ cation is bonded to two Cl- anions and two NH3 groups.

40
Q

How is the solubility of a salt affected if:

  1. common ions are present?
  2. complexes form from the product ions?
A
  1. Solubility decreases.
    Common ions prevent salts from fully dissociating, since the solution already contains some concentration of product ions.
  2. Solubility increases.
    Complexes formed from product ions effectively remove those ions from solution, allowing more salt to dissociate and replace the lost product ions.
41
Q

Suppose that a solution is saturated with both K2[PtCl4] and NH3. What must be true of the solubility of both species if the following reaction occurs?

PtCl42- + NH3 ⇒ PtCl2(NH3)2 + 2 Cl-

A

Solubility for both will be increased.

The solution will accept more K2[PtCl4] and NH3, since the [PtCl2(NH3)2] complex that forms effectively removes both PtCl42- ions and NH3 molecules from solution.

According to Le Chatelier’s principle, the system then shifts towards the product ions, increasing the solubility of both species.

42
Q

Explain why:

  1. acids are more soluble in an aqueous base than neutral water.
  2. bases are more soluble in an aqueous acid than neutral water.
A

Base molecules already present in the basic solution will bond to the H+ ions, removing them from solution, shifting equilibrium to the right, and increasing the solubility of the acid (HA).

HA + H2O ⇔ H+ + A-

Acid molecules already present in the acidic solution will bond to the OH- ions, removing them from solution, shifting equilibrium to the right, and increasing the solubility of the base (B-).

B- + H2O ⇔ HB + OH-