Question
A student prepared a saturated aqueous solution of \(Mg(OH)_2\) and measured its pH , as shown in Figure 1 above. Then the student added a few drops of an unknown solution to the test tube and observed cloudiness in the solutions as shown in Figure 2. On the basis of this information and the equilibrium represented above, which of the following is most likely the identity of the reagent added from the dropper?
A Distilled water
B \(NaNO_3\)(aq)
C \(HCl\)(aq)
D \(KOH\)(aq)
▶️Answer/Explanation
Ans:D
Adding \(KOH\)(aq) increases the concentration of \(OH^−\) ions, which disturbs the equilibrium. When establishing a new equilibrium, the system favors the reverse reaction to consume the \(OH^−\) ions, resulting in the precipitation of some of the dissolved \(Mg(OH)_2\).
Question
\(CaF_2\)(s) dissolves in water according to the equation \(CaF_2\)(s)⇄\(Ca^{2+}(aq)+2 F^{−}(aq)\). The value of \(K_{sp}\) for the dissolution is \(3.5\times 10^{−11}\). A student measures the concentration of Ca2+ ions in a saturated solution of \(CaF_2\) at various pH values and uses those values to generate the graph above. Based on the data, which of the following observations about the solubility of \(CaF_2\) is most valid?
A It does not depend on \(pH\) because \([Ca^{2+}]\) does not change between \(pH_4\) and \(pH_{12}\) .
B It does not depend on \(pH\) because \(K_{sp}=[Ca^{2+}][F^−]^2\), so as \([Ca^{2+}]\) decreases, \([F^−]\) increases to compensate, keeping \(K_{sp}\) constant.
C It is higher at a lower \(pH\) ; there are more \(H^+\) ions in solution at low pH, so HF forms and shifts the equilibrium reaction above to the right.
D It is lower at a higher \(pH\) ; there are more \(H^+\) ions in solution at high \(pH\), so \(HF\) forms and shifts the equilibrium reaction above to the right.
▶️Answer/Explanation
Ans:C
As pH decreases, \(H^+\) increases, shifting the equilibrium of the reaction \(H^+(aq)+F^−(aq)\)⇄\(HF(aq)\) to the right, thereby decreasing \([F^−]\) and allowing more \(CaF_2\)(s) to dissolve.
Question
\(Cu^{2+}\)(aq) absorbs a certain frequency of visible light. Absorbance was measured at this frequency for three saturated solutions of \(Cu(OH)_2\), each at a different pH at 298K. Based on the data recorded in the table above, which of the following conclusions about the effect of pH on the solubility of \(Cu(OH)_2\) can be made?
A When the concentration of \(H^+\) ions is increased, the solubility of \(Cu(OH)_2\) increases.
B When the concentration of \(H^+\) ions is decreased, the solubility of \(Cu(OH)_2\) increases.
C The solubility of \(Cu(OH)_2\) is independent of pH.
D \(Cu(OH)_2\) is soluble only at a pH of 7.00.
▶️Answer/Explanation
Ans:A
The data show an increase in the absorbance at lower pH values, which indicates that more \(Cu(OH)_2\) dissolved at higher concentrations of \(H^+\) ions, increasing the concentration of \(Cu^{2+}\) ions. As pH decreases, the concentration of \(H^+\) ions increases.
Question
Three saturated solutions (X, Y, and Z) are prepared at 25°C. Based on the information in the table above, which of the following lists the solutions in order of increasing [\(Ag^{+}\)]?
(A) X <Z< Y
(B) Y < X < Z
(C) Z < Y <X
▶️Answer/Explanation
Ans:C
Question
The value of K w at 40°C is \(3.0 \times 10^{-14}\). What is the pH of pure water at \(40^{\circ}\)C?
(A) 3.0
(B) 6.8
(C) 7.0
(D) 7.2
▶️Answer/Explanation
Ans:D