1) CH3NH2 + H2O <===> CH3NH3+ + OH¯
Methylamine, CH3NH2, is a weak base that reacts according to the equation above. The value of the ionization constant, Kb, is 5.25 x 10¯4. Methylamine forms salts such as methylammonium nitrate, (CH3NH3+) (NO3¯).
(a) Calculate the hydroxide ion concentration, [OH¯], of a 0.225-molar solution of methylamine.
(b) Calculate the pH of a solution made by adding 0.0100 mole of a solid methylammonium nitrate to 120.0 milliliters of a 0.225-molar solution of methylamine. Assume that no volume change occurs.
(c) How many moles of either NaOH or HCl (state clearly which you choose) should be added to the solution in (b) to produce a solution that has a pH of 11.00? Assume that no volume change occurs.
(d) A volume of 100. milliters of distilled water is added to the solution in (c). How is the pH of the solution affected? Explain.
2) Elemental analysis of an unknown pure substance indicates that the percent composition by mass is as follows:
Carbon - 49.02%
Hydrogen - 2.743%
Chlorine - 48.23%
A solution that is prepared by dissolving 3.150 grams of the substance in 25.00 grams of benzene, C6H6, has a freezing point of 1.12°C. (The normal freezing point of benzene is 5.50°C and the molal freezing-point depression constant, Kf, for benzene is 5.12 C°/molal.)
(a) Determine the empirical formula of the unknown substance.
(b) Using the data gathered from the freezing point depression method, calculate the molar mass of the unknown substance.
(c) Calculate the mole fraction of benzene in the solution described above.
(d) The vapor pressure of pure benzene at 35°C is 150. millimeters of Hg. Calculate the vapor pressure of benzene over the solution described above at 35°C.
Here is a look-alike question I happened to think of.
I. 2Mn2+ + 4OH¯ + O2(g) ---> 2MnO2(s) + 2H2O
II. MnO2(s) + 2I¯ + 4H+ ---> Mn2+ + I2(aq) + 2H2O
III. 2S2O32¯ + I2(aq) ---> S4O62¯ + 2I¯
The amount of oxygen, O2, dissolved in water can be determined by titration. First, MnSO4 and NaOH are added to a sample of water to convert all of the dissolved O2 to MnO2, as shown in equation I above. Then H2SO4 and KI are added and the reaction represented by equation II proceeds. Finally, the I2 that is formed is titrated with standard sodium thiosulfate, Na2S2O3, according to equation III.
(a) According to the equations above, how many moles of S2O32¯ are required for analyzing 1.00 mole of O2 dissolved in water?
(b) A student found that a 50.0 milliliter sample of water required 4.86 milliliters of 0.0112 molar Na2S2O3 to reach the equivalence point. Calculate the number of moles of O2 dissolved in this sample.
(c) How would the results in (b) be affected if some I2 were lost before the S2O32¯ was added? Explain.
(d) Name an appropriate indicator for the reaction shown in equation III and describe the change you would observe at the end point of the titration.
4) Give the formulas to show the reactants and the products for FIVE of the following chemical reactions. Each of the reactions occurs in aqueous solution unless otherwise indicated. Represent substances in solution as ions if the substance is extensively ionized. Omit formulas for any ions or molecules that are unchanged by the reaction. In all cases a reaction occurs. You need not balance.
Example: A strip of magnesium is added to a solution of silver nitrate.
Mg + Ag+ ---> Mg2+ + Ag
(a) A strip of copper is immersed in dilute nitric acid.
(b) Potassium permanganate solution is added to an acidic solution of hydrogen peroxide.
(c) Concentrated hydrochloric acid is added to solid manganese(II) sulfide.
(d) Excess chlorine has is passed over hot iron filings.
(e) Water is added to a sample of solid magnesium nitride.
(f) Excess sulfur dioxide gas is bubbled through a dilute solution of potassium hydroxide.
(g) Excess concentrated ammonia solution is added to a suspension of silver chloride.
(h) Solutions of tri-potassium phosphate and zinc nitrate are mixed.
5) The following observations are made about reactions of sulfuric acid, H2SO4. Discuss the chemical processes involved in each case. Use principles from acid-base theory, oxidation-reduction, and bonding and/or intermolecular forces to support your answers.
(a) When zinc metal is added to a solution of dilute H2SO4, bubbles of gas are formed and the zinc disappears.
(b) As concentrated H2SO4 is added to water, the temperature of the resulting mixture rises.
(c) When a solution of Ba(OH)2 is added to a dilute H2SO4 solution. the electrical conductivity decreases and a white precipitate forms.
(d) When about 10 milliliters of 0.10 molar H2SO4 is added to 40 milliliters of 0.10 molar NaOH the pH changes only by about 0.5 unit. After 10 more milliliters of 0.10 molar H2SO4 is added, the pH changes by about 6 units.
6) Account for each of the following in terms of principles of atomic structure, including the number, properties, and arrangements of subatomic particles.
(a) The second ionization energy of sodium is about three time greater than the second ionization energy of magnesium.
(b) The difference between the atomic radii of Na and K is relatively large compared to the difference between the atomic radii of Rb and Cs.
(c) A sample of solid nickel chloride is attracted into a magnetic field, whereas a sample of solid zinc chloride is not.
(d) Phosphorus forms the fluorides PF3 and PF5, whereas nitrogen forms only NF3.
7) A galvanic cell is constructed using a chromium electrode in a 1.00 molar solution of Cr(NO3)3 and a copper electrode in a 1.00 molar solution of Cu(NO3)2. Both solutions are at 25°C.
(a) Write a balanced net ionic equation for the spontaneous reaction that occurs as the cell operates. Identify the oxidizing agent and the reducing agent.
(b) A partial diagram of the cell is shown below.
(i) Which metal is the cathode?
(ii) What additional component is necessary to make the cell operate?
(iii) What function does the component in (ii) serve?
(c) How does the potential of this cell change if the concentration of Cr(NO3)3 is changed to 3.00 molar at 25 °C? Explain.
8) 2 C4H10(g) + 13 O2(g) ------> 8 CO2(g) + 10 H2O(l)
The reaction represented above is spontaneous at 25 °C. Assume that all reactants and products are in their standard states.
(a) Predict the sign of DS° for the reaction and justify your prediction.
(b) What is the sign of DG° for the reaction? How would the sign and magnitude of DG° be affected by an increase in temperature to 50 °C? Explain your answer.
(c) What must be the sign of DH° for the reaction at 25°C? How does the total bond energy of the reactants compare to that of the products?
(d) When the reactants are placed together in a container, no change is observed even though the reaction is known to be spontaneous. Explain this observation.
9) Observations about real gases can be explained at the molecular level according to the kinetic molecular theory of gases and ideas about intermolecular forces. Explain how each of the following observations can be interpreted according to these concepts, including how the observation supports the correctness of these theories.
(a) When a gas-filled balloon is cooled, if shrinks in volume; this occurs no matter what gas is originally placed in the balloon.
(b) When the balloon described in part (a) is cooled further, the volume does not become zero; rather, the gas becomes a liquid or solid.
(c) When NH3 gas is introduced at one end of a long tube while HCl gas is introduced simultaneously at the other end, a ring of white ammonium chloride is observed to form in the tube after a few minutes. This ring is closer to the HCl end of the tube than the NH3 end.
(d) A flag waves in the wind.
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