Acids and Bases

1.a) Write the expression for the equilibrium constant, $K_a$, for the reaction. (1 point)

$K_a = \frac{[H_3O^+][HCOO^-]}{[HCOOH]}$

1.b) Calculate the pH of a 0.25 $M$ solution of HCOOH. (3 points)

$K_a = \frac{x^2}{0.25} = 1.8 \times 10^{-4}$ $x^2 = 4.5 \times 10^{-5}$ $x = [H_3O^+] = 6.7 \times 10^{-3}$ $pH = -log(6.7 \times 10^{-3}) = 2.17$

1.c) In the box below, complete the Lewis electron-dot diagram for HCOOH. Show all bonding and nonbonding valence electrons. (2 points)

1.d) In aqueous solution, the compound $H_2NNH_2$ reacts according to the equation above. A 50.0 mL sample of 0.25 $M$ $H_2NNH_2 (aq)$ is combined with a 50.0 mL sample of 0.25 $M$ HCOOH$(aq)$.

1.d.i) Write the balanced net ionic equation for the reaction that occurs when <math-inline>H\_2NNH\_2</math-inline> is combined with HCOOH. (1 point)

<math-inline>H\_2NNH\_2(aq) + HCOOH(aq) \rightarrow H\_2NNH\_3^+(aq) + HCOO^-(aq)</math-inline>

1.d.ii) Is the resulting solution acidic, basic, or neutral? Justify your answer. (2 points)

The resulting solution is basic. <math-inline>K\_a</math-inline> of HCOOH is $1.8 \times 10^{-4}$ and <math-inline>K\_b</math-inline> of <math-inline>H\_2NNH\_2</math-inline> is $1.3 \times 10^{-6}$. Since the <math-inline>K\_a</math-inline> of the acid is greater than the <math-inline>K\_b</math-inline> of the base, the solution is expected to be acidic. However, the reaction is between a weak acid and a weak base, so the solution will be slightly basic because the <math-inline>K\_b</math-inline> of <math-inline>H\_2NNH\_2</math-inline> is larger than the <math-inline>K\_a</math-inline> of HCOOH.

1.e) When a catalyst is added to a solution of HCOOH$(aq)$, the reaction represented by the following equation occurs. Is the reaction a redox reaction? Justify your answer. (1 point)

<math-inline>HCOOH (aq) \rightarrow H\_2(g) + CO\_2(g)</math-inline>

Yes, the reaction is a redox reaction. The oxidation state of carbon in HCOOH is +2, and it changes to +4 in <math-inline>CO\_2</math-inline>. The oxidation state of hydrogen in HCOOH is +1 and it changes to 0 in <math-inline>H\_2</math-inline>. Since there is a change in oxidation states, this is a redox reaction.

1.f) The reaction occurs in a rigid 4.3 L vessel at 25°C, and the total pressure is monitored, as shown in the graph above. The vessel originally did not contain any gas. Calculate the number of moles of $CO_2(g)$ produced in the reaction. (Assume that the amount of $CO_2(g)$ dissolved in the solution is negligible.) (2 points)

![](https://zustagstorage.blob.core.windows.net/community/files_questions/frq/ap24-frq-chemistry_948ed8b8a7fc4d1b82923fd4738dccd1)

<math-inline>P\_{total} = 0.60 atm</math-inline>
<math-inline>P\_{total} = P\_{H\_2} + P\_{CO\_2}</math-inline> and <math-inline>P\_{H\_2} = P\_{CO\_2}</math-inline>
<math-inline>P\_{CO\_2} = 0.30 atm</math-inline>
<math-inline>PV = nRT</math-inline>
<math-inline>n = \frac{PV}{RT} = \frac{(0.30 atm)(4.3 L)}{(0.08206 L atm/mol K)(298 K)} = 0.053 mol</math-inline>

1.g) After the reaction has proceeded for several minutes, does the amount of catalyst increase, decrease, or remain the same? Justify your answer. (1 point)

The amount of catalyst remains the same. A catalyst is not consumed in a chemical reaction.

1.a) The structural formula of lactic acid is shown in the following diagram. Circle the hydrogen atom that most readily participates in the chemical reaction with sodium hydroxide.

1.b) The student begins the experiment by dissolving 10.22 g of sodium hydroxide (molar mass 40.00 g / mol) in enough water to produce 500. mL of solution. Calculate the molarity of the sodium hydroxide solution.

1.c) The student uses the sodium hydroxide solution from part (b), a buret, a pH meter, and a 100 mL Erlenmeyer flask to titrate a 25.0 mL sample of lactic acid solution. The student’s data are shown in the following graph. Use the information in the graph to determine the approximate p$K_a$ of lactic acid.

1.d) The preceding diagram represents the relative amounts of major species in a sample of the solution in the flask at one point during the titration. (Note that water molecules are omitted.)

1.d.i) Draw an X on the preceding titration curve at a point in the titration where the reaction mixture would be represented by this diagram.

1.d.ii) Justify your answer.

1.e) The student repeats the experiment but uses a solution of $NaOH(aq)$ with twice the concentration, as shown in the preceding table. On the following graph, draw the titration curve that would be expected for experiment 2. |Experiment |Mass of NaOH(s) (grams) | Volume of Solution (mL) | Titration Curve | | - | - | - | - | |1 |10.22 | 500. | Already shown on graph| |2 |20.44 | 500. | ?|

1.f) In a third experiment, the student investigates the enthalpy of the reaction between lactic acid and sodium hydroxide. The student combines 100.0 mL of a 0.500 $M$ lactic acid solution at 20.0°C with 100.0 mL of a 0.500 $M$ $NaOH$ solution at 20.0°C in a calorimeter. The final temperature of the resulting combined solution is 23.2°C. Assume that the density of each solution before combining is 1.00 g / mL and that the specific heat capacity of the combined solution is 4.2 J / (g · ° C).

1.f.i) Calculate the quantity of heat produced in the reaction, in J.

1.f.ii) Calculate the molar enthalpy of reaction, in kJ / mol$_{rxn}$. Include the sign in your answer.

1.f.iii) The student claims that if heat is lost from the calorimeter to the surrounding air during the reaction, then the experimental value of the molar enthalpy of reaction will be smaller in magnitude than the actual value. Do you agree or disagree with the student’s claim? Justify your answer.

4.a) Calculate the mass of CH$_3$NH$_3$Cl($s$) that contains 0.00250 mol of CH$_3$NH$_3$Cl.

4.b) The following table contains a partial procedure for making the buffer solution. Fill in steps 1 and 4 to complete the procedure using only materials and equipment selected from the choices given. (Not all materials listed will be used. Assume that all appropriate safety measures are already in place.)

4.c) The student prepares a second buffer solution. The student uses 25.00 mL of 0.050 M CH$_3$NH$_2$($aq$) instead of 25.00 mL of 0.100 M CH$_3$NH$_2$($aq$), and half the mass of CH$_3$NH$_3$Cl($s$) that was used in the first buffer. Is the pH of the second buffer greater than, less than, or equal to the pH of the first buffer? Justify your answer.

5.a. Two particulate representations of the ionization of HF molecules in the 0.0350 M $HF(aq)$ solution are shown below in Figure 1 and Figure 2. Water molecules are not shown. Explain why the representation of the ionization of HF molecules in water in Figure 1 is more accurate than the representation in Figure 2. (The key below identifies the particles in the representations.)

5.b. Use the percent ionization data above to calculate the value of $K_a$ for HF.

5.c. If 50.0 mL of distilled water is added to 50.0 mL of 0.035 M $HF(aq)$, will the percent ionization of $HF(aq)$ in the solution increase, decrease, or remain the same? Justify your answer with an explanation or calculation.