Ch. 17 Quiz
What are the main groups in the general qualitative analysis scheme described in this chapter? Describe the steps and reagents necessary to identify each group.
A general qualitative analysis scheme involves separating a mixture of the common ions into five groups by sequentially adding five different precipitating agents. After each precipitating agent is added, the mixture is put in a centrifuge to separate the solid from the liquid. Group 1: insoluble chlorides Group 2: acid-insoluble sulfides Group 3: base-insoluble sulfides and hydroxides Group 4: Insoluble carbonate, metal ions Group 5: Alkali metals and NH4+
What is an indicator? How can an indicator signal the equivalence point of a titration?
An indicator (HIn) is itself a weak organic acid that has a different color than its conjugate base (In-). The color of a solution containing an indicator depends on the relative concentrations of HIn and In-. As the [H3O+] changes during the titration, the above relative concentrations of HIn and In- change accordingly. At low pH, the [H3O+] is high, the equilibrium favors the acid species, and the color is that of the acid species. As the titration proceeds, the [H3O+] decreases, shifting toward higher concentrations of the conjugate base, and the color changes. Because the color of an indicator is intense, only a small amount is required- an amount that will not affect the pH of the solution or the equivalence point of the neutralization reaction.
How does a common ion affect the solubility of a compound? More specifically, how is the solubility of a compound with the general formula AX difference in a solution containing one of the common ions (A+ or X-) than it is in pure water? Explain.
In accordance with Le Chatlier's principle, the presence of a common ion in solution causes the equilibrium to shift to the left compared to its position with pure water as the solvent, which means that less of the ionic compound dissolved. Thus, the solubility of an ionic compound is lower in a solution containing a common ion than in pure water. The exact value of the solubility can be calculated by working an equilibrium problem in which the concentration of the common ion is accounted for in the initial conditions. The molar solubility of a compound, AmXn, in a solution with an initial concentration of A^n+ = [A^n+]0 can be computed directly from Ksp by solving for S in the expression Ksp = ([A^n+]0 + mS)^m([X^m-]0 + nS)^n
Describe acid-base titration. What is the equivalence point?
In an acid base titration, a basic or acidic solution of unknown concentration is reacted with an acidic or basic solution of known concentration. The known solution is slowly added to the unknown solution while the pH is monitored with a pH meter or an indicator. As the acid and base combine, the neutralize each other. The equivalence point is the point in the titration when the number of moles of base is stoichiometrically equal to the number of moles of acid, and the titration is complete. When the equivalence point is reached, neither the reactant is in excess.
How is the solubility of an ionic compound with a basic anion affected by pH? Explain.
In general, the solubility of an ionic compound with a strongly basic or weakly basic anion increases with increasing acidity (decreasing pH). If the anion is neutralized by reaction with H+, to form the conjugate acid of the basic anion, Le Chatleir's principle says that the solubility equilibrium will shift to the right, allowing more solid to dissolve. If the anion is converted to a different species (such as the conjugate acid), the concentration of the anion drops, allowing more solid to dissolve.
For a given solution containing an ionic compound, what is the relationship between Q, Ksp, and the relative saturation of the solution?
Q is the reaction quotient, the product of the concentrations of the ionic compounds raised to their stoichiometric coefficients, and K is the product of the concentrations of the ionic compounds components raised to their stoichiometric coefficients at equilibrium. A solution contains an ionic compound: if Q < Ksp the solution is unsaturated. More of the solid ionic compound can dissolve in the solution if Q=Ksp, the solution is saturated, the solution is holding the equilibrium amount of the dissolved ions, and additional solid will not dissolve in the solution. If Q > Ksp the solution is supersaturated and under most circumstances, the excess solid will precipitate out.
What is qualitative analysis? How does qualitative analysis differ from quantitative analysis?
Qualitative analysis is a systematic way to determine the metal ion present in an unknown solution by the selective precipitation of ions present in the solution. The word qualitative means "involving quality or kind". So qualitative analysis involves finding the kind of ions present in the solution. Quantitative analysis is concerned with quantity, or the amounts of substance in a solution or mixture.
What is selective precipitation? Under which conditions does selective precipitation occur?
Selective precipitation is a process for separating metal cations from a solution containing several different dissolved metal cations. In the process, a metal cation can often be separated by the addition of a reagent that forms a precipitate with one of the dissolved cations but not the others. The appropriate regent must form compounds with both metal cations with sufficiently different Ksp values (a difference of a factor of at least 10^3), so that one compound remains dissolved while the other compound significantly precipitates.
What is the difference between the endpoint and the equivalence point in a titration?
The end point is the point when the indicator changes color in an acid-base titration. The equivalence point is when stoichiometrically equivalent amounts of acid and base have reactanted. With the correct indicator, the end point of the titration will occur at the equivalence point.
What is molar solubility? How can you obtain molar solubility of a compound from Ksp?
The molar solubility, S, is simply the solubility in units of moles per liter (mol/L). The molar solubility of a compound, AmXn, can be computed directly from Ksp by solving for S in the expression Ksp = (mS)^m(nS)^n = (m^m)(n^n)(S^m+n)
What is the solubility product constant? Write a general expression for the solubility constant of a compound with the general formula AmXn.
The solubility constant (Ksp) is the equilibrium expression for a chemical equation representing the dissolution of an ionic compound. The expression of the solubility product constant of AmXn is Ksp = [An+]m[Xm-]n.
The pH at the equivalence point of the titration of a strong acid with a strong base is 7. However, the pH at the equivalence point of the titration of a weak acid with a strong base is above 7. Explain.
The titration of weak acid by a strong base always has a basic equivalence point because at the equivalence point, all of the acid has been converted into its conjugate base, resulting in a weakly basic solution.
The volume required to reach the equivalence point of an acid-base titration depends on the volume and the concentration of the acid or base to be titrated and on the concentration of the acid or base used to do the titration. It does not, however, depend on whether or not the acid or base being titrated is strong or weak. Explain.
The volume required to get to the equivalence point is only dependent on the concentration and volume of acid or base to be titrated and the base or acid used to do the titration because the equivalence point is dependent on the stoichiometry of the balanced reaction of the acid and base. The stoichiometry only considers the number of moles involved, not the strength of the reactants involved.
In the titration of a polyprotic acid, the volume required to reach the first equivalence point is identical to the volume required to reach the second one. Why?
The volume required to get to the first equivalence point is identical to the volume between the first and second equivalence points because the equivalence point is dependent on the stoichiometry of the balanced reaction of the acid and base. The stoichiometry only considers the number of moles involved, not the strength of the reactants involved. There are the same number of moles of the first acidic proton and the second acidic proton.
The titration of a polyprotic acid with sufficiently different pKas displays two equivalence points. Why?
When a polyprotic acid is titrated with a strong base and if Ka1 and Ka2 are sufficiently different, the pH curve will have two equivalence points because the two acidic protons will be titrated sequentially. The titration of the first acid proton will be completed before the titration of the second acidic proton.
In the titration of a weak acid with a strong base, how do you calculate these quantities? a. initial pH b. pH before the equivalence point c. Halfway to the equivalence point d. pH at the equivalence point e. pH beyond the equivalence point
a. The initial pH is that of the weak acid solution to be titrated by working through an equilibrium problem with an ICE table. b. Between the initial pH and the equivalence point, the solution becomes a buffer. Use the reaction stoichiometry to compute the amounts of each buffer component then use the Henderson-Hasselbalch equation to compute the pH. c. Halfway to the equivalence point, the buffer components are exactly equal and pH = pKa. d. At the equivalence point, the acid has all been converted to its conjugate base. Calculate the pH by working an equilibrium problem for the ionization of water by the ion acting as a weak base. e. Beyond the equivalence point, OH- is in excess. You can ignore the weak base and calculate the [OH-] by subtracting the initial number of moles of H3O+ from the number of moles OH_ and dividing by the total volume, then converting to pH using -log[H3O+].
In the titration of a strong acid with a strong base, how do you calculate these quantities? a. initial pH b. pH before the equivalence point c. pH at the equivalence point d. pH beyond the equivalence point
a. The initial pH of the solution is simply the pH of the strong acid. Because strong acids completely dissociate, the concentration of H3O+ is the concentration of the strong acid and pH = -log[H3O+] b. Before the equivalence point, H3O+ is in excess. Calculate the [H3O+] by subtracting the number of moles of added OH- from the initial number of moles of H3O+ and dividing by the total volume. Then convert to pH using -log[H3O+] c. At the equivalence point, neither reactant is in excess and the pH = 7.00 d. Beyond the equivalence point, OH- is in excess. Calculate the [OH-] by subtracting the initial number of moles of H3O+ from the number of moles added OH- and dividing by the total volume. Then convert to pH using -log[H3O+].
