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Chemistry 112 – Elementary Inorganic Chemistry
Morehouse College – Problem Set
DUE: Monday, February 25, 2008 in class
Answer all questions in the space provided, neatly written. Thanks.--BML
- (1 pt. each) For each of the following questions, circle the BEST answer (no explanation is required for full credit).
a. If 5.17 mL of 3.0 M NaOH(aq) is diluted to a final volume of 2.60 L, what is the pH of the resultant solution?
0.05 2.22 5.17 8.83 11.
b. Consider the following reaction:HCl(aq) + H 2 O(liq) H 3 O+(liq) + Cl-(aq) where Keq >>> 1. Which of the following statements are true at equilibrium?
H 2 O is a H 2 O is a Cl-^ is a HCl is a none of these stronger acid stronger acid stronger base stronger acid than HCl than H 3 O+^ H 2 O than H 3 O+
c. If 150 mg KOH(aq) are dissolved in water to give a final volume of 110 mL, what is the pOH of the resultant solution?
0.26 1.1 2.6 12.4 none of these
d. Consider the following reaction: CH 3 CO 2 H + NH 3 NH 4 +^ + CH 3 CO 2 - Which of the following statements is true?
K eq = 1 the reaction will the reaction will none of these favor product favor reactant formation formation
e. Suppose 50 mL of a 0.10 M solution of H 2 S(aq) is prepared. What will be the pH at equilibrium in this solution if the acid dissociation constant Keq = 1.0 x 10-7?
0.00 4.00 7.00 10.0 none of these
- (5 pts) Water that has been saturated with magnesia (MgO) at 25 °C has a pH equal to 10.16. Write a balanced chemical equation for the equilibrium between MgO(s) and the ions it furnishes in aqueous solution then calculate the equilibrium constant for your reaction. Lastly, determine the solubility (in M) of MgO in water. Report your answer in the space provided with the correct number of significant figures. Show your work. A correct response without any justification (work) will receive no credit.
To begin, we write a balance chemical equation. Since O^2 - (aq) is the conjugate base of OH-(aq), a strong base, it would be likely that O2-(aq) will react completely with water in solution to generate two hydroxide ions. That is…
MgO(s) Mg2+(aq) + O2-(aq)
O2-(aq) + H 2 O(liq) 2OH-(aq)
Taken together, these reactions become...
MgO(s) + H 2 O(liq) Mg2+(aq) + 2OH-(aq)
Now that we have a chemical equation, we can take to the work of finding the equilibrium constant. First, we’ll write the expression for K eq for the last of the above equations…
[Mg ][OH] (Question:Whyisthedenominator"one?" ) 1
[Mg ][O ] [Reactants]
[Products] 2 +^2 - 2 + 2 Keq = = =
To solve for K , we need to find the concentrations of magnesium and hydroxide ions. Since the pH is given, we can first determine the [H+].
-pH -10.16 11 pH log 10 [H ] orequally[H ] 10 10 6.918x 10 =− + + = = = −
From the relationship between [H+], [OH-], and K w, we can determine [OH-]…
11 4 M
14 w
w
1.445x 6.218x 10
1.000x 10 [H ]
[OH ]
[H ][OH ]
− −
−
−
K
K
To determine the concentration of Mg2+^ ions, we use the reaction stoichiometry…
MgO(s) + H 2 O(liq) Mg2+(aq) + 2OH-(aq)
From this relationship, we deduce that [Mg2+] = 0.5 [OH-] = 7.227 x 10-5^ M. This is also the solubility of MgO(s) in solution. Lastly, we can write an equilibrium expression for the production of ions in solution. This is just K eq for the same equation written above…
[Mg^2 ][OH]^2 (7.227x 10 -^5 )(1.445x 10 -^4 )^2 1.509x 1012 [Reactants]
[Products] (^) + − Keq = = = =
