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The University of Tokyo 2013 Entrance Examination—Chemistry (Partial)



Question 2

Answer the following questions I and II. Use the following value if necessary.
Atomic mass of Cu: 63.5


Read the following passage and answer the questions AF.

    Rare and precious metals are used in various products, and due to their scarcity and importance, many techniques to recycle them are being developed. Typically, a product consists of different metals, and to separate out the desired metal, multiple chemical techniques are employed, such as dissolution, precipitation, solvent extraction, and electrolytic refining.
    As an example of dissolution and precipitation, we consider the recovery of gold and silver from a solid (solid A). We first add concentrated nitric acid to the solid and filter the solution. When a small amount of ammonia solution is added to the filtrate, one of our desired metals can be collected as (     a     ), a brown precipitate. As for the remaining solid material after the filtration process, we can add to it two different acids, (     b     ) and (     c     ), and perform another filtration to obtain a filtrate containing the other desired metal.
    Solvent extraction can be used to increase the purity of a metal. In this technique, two immiscible liquids, such as water and a low-polarity organic solvent, are shaken together to extract the desired compound into one of the liquid phases. For the extraction of metals, a chelating agent is used. A chelating agent is a reagent that contains two or more atoms that can coordinate with a metal ion within a molecule.
    We consider the extraction of indium ions (In3+) using 8-hydroxyquinoline (HQ), a chelating agent, as an example. HQ dissolves in both water and organic solvents, and under certain pH conditions, acts as a monoprotic acid in aqueous solutions, dissociating into Q and H+ as shown in Figure 2-1.

Equilibrium constant (electrolytic dissociation constant): K1

Figure 2-1    Electrolytic dissociation equilibrium of HQ

    The distribution constant K2 can be expressed as K2 = [HQ]organic/[HQ]aqueous, where [HQ]organic is the concentration of HQ in the organic phase, and [HQ]aqueous is the concentration of HQ in the aqueous phase. Under the above conditions, HQ dissolved in the organic phase transfers into the aqueous phase and (1)forms a coordination complex with In3+. The complex is charge-neutral and is extracted back into the organic phase.  There are six atoms coordinated with In3+ in the complex.

    Electrolytic refining can also be used for purification. Here, we consider the electrolytic refining of copper. The anode consists of low-purity copper (containing aluminum, silver, and iron), and the cathode consists of pure copper. The electrolysis is performed by immersing the anode and cathode in 2.0 L of a copper(II) sulfate solution and applying 0.30 V. As a result, (2)the mass of the anode decreased by 112.0 g, and the mass of the cathode increased by 110.0 g. The concentration of copper(II) sulfate in the aqueous solution decreased by 0.020 mol·L-1. (3)The electrolysis produced a precipitate called anode slime under the anode.

[Questions]
Question A: Write down the chemical formula that corresponds to (     a     ) and the names of the substances that correspond to (     b     ) and (     c     ) .

Question B: When excess ammonia solution is added to the precipitate (     a     ), it dissolves as an ion.  What is the chemical formula of this ion?

Question C: The distribution ratio D describes the extent to which HQ is distributed into the organic phase, and is defined as [HQ]organic/([HQ]aqueous+[Q-]aqueous).  Write down the expression for D when there are no In3+ ions present using K1, K2, and [H+]aqueous, which is the concentration of H+ in the aqueous phase.

Question D: Draw the structure of the coordination complex corresponding to the underlined phrase (1).  In drawing the structure, the coordination bonds between In3+ and the appropriate atoms must be shown with dotted lines. The drawing does not need to show the spatial structure of the compound.

Question E: In the underlined sentence (3), what metal element is contained in the anode slime, and why did it precipitate out?

Question F: Regarding the underlined phrase (2), how many grams out of the 112.0 g that was lost from the anode are due to elements other than copper?  Give your answer in two significant figures. Assume that the change in volume of the solution is negligible.

   
II 
Read the following passage on the elements and answer the questions GL.

The five most common elements found in the solar system, in order of abundance, are hydrogen, (     d     ), oxygen, carbon, and nitrogen. (     d     ) is a member of group (     e     ), and has the largest ionization energy among all elements. In contrast, oxygen has an ionization energy that is close to the average ionization energy of the period 2 elements. Ozone is an allotrope of oxygen, and it absorbs ultraviolet radiation from the Sun in the stratosphere, protecting life on Earth from the harmful effects of ultraviolet radiation. Of the isotopes of carbon, which is one of the main constituent elements of living organisms and fossil fuel, the isotope with mass (     f     ) is radioactive with a half-life (the time required for half of the radioactive isotopes to decay into another isotope) of 5730 years, and is used for absolute dating in archaeology. The ratio of radioactive to non-radioactive carbon contained in carbon dioxide in the atmosphere has been fairly consistent, but with (4)changes in the cosmic ray intensity that reaches the Earth, the use of fossil fuel, and nuclear testing since 1945, the ratio is changing. Nitrogen constitutes about 80% of the air we breathe, and is found in many biological substances such as amino acids; it is one of the most important elements that support life.

[Questions]

Question G: Write down the name of the element that corresponds to  (     d     ) and the numerical values that correspond to (     e     ) and  (     f     ).

Question H: The element in group  (     e     ) and period 3 forms a crystal with a face-centered cubic structure under 80 K. The length of an edge of the unit cell is 0.526 nm. Find the atomic spacing (nearest-neighbor distance) to two significant figures. There are 4 atoms in the unit cell. If necessary, use the following values: √ 2 = 1.41,√ 3 = 1.73,√ 5  = 2.24.

Question I: As mentioned in Question H, the element in group (     e     ) and period 3 only crystallizes at low temperatures, but KCl, which contains another element in period 3, chlorine, forms a crystal that is stable at room temperature. Briefly explain, in 30 words or more and 50 words or fewer, the reason for this, focusing on the nature of chemical bonds.

Question J: When 44.8 L of air (containing 0.20 mol fraction of oxygen) under standard temperature and pressure (STP) was irradiated with ultraviolet light, ozone was produced. The volume of the gas decreased by 1.4 L under STP after the reaction. Obtain the molar fraction of ozone in the gas after the reaction to two significant figures. Show all your calculations. Assume that the only reaction that occurs with ultraviolet irradiation is the production of ozone.

Question K: As an effect of the events in the underlined phrase (4), the ratio of radioactive carbon in carbon dioxide found in the atmosphere is changing. Will the ratio increase or decrease with the increase in cosmic ray intensity?  How about with the use of fossil fuel?

Question L: In general, the shape of a molecule is determined by the electrostatic forces between covalent electron pairs, non-covalent electron pairs, and unpaired electrons. For example, in a water molecule, there is electrostatic repulsion between the two non-covalent electron pairs and the two covalent electron pairs in the two O—H bonds. This causes the non-covalent electron pairs and the H atoms to form the corners of a regular tetrahedron, resulting in a bent molecular geometry. With this in mind, identify which of the following molecules or ions (1)(4) are polar, and write down their numbers.

       (1)  nitrogen dioxide        (2)  dinitrogen tetroxide
(3)  nitrogen trifluoride    (4)  ammonium ion

(End of the problems)

*The answers are available and please confirm if necessary.