Question 1

Compounds containing a hydroxyl group attached to a benzene ring are called phenols. Derivatives of phenols,

such as naphthols and phenanthrols, have chemical properties similar to those of phenols, as do most of the

many naturally-occurring substituted phenols. Like other alcohols, phenols have higher boiling points than

hydrocarbons of similar molecular weight. Like carboxylic acids, phenols are more acidic than their alcohol

counterparts. Phenols undergo a number of different reactions; both their hydroxyl groups and their benzene

rings are highly reactive. A number of chemical tests can be used to distinguish phenols from alcohols and

carboxylic acids.

Thymol, a naturally occurring phenol, is an effective disinfectant that is obtained from thyme oil. Thymol can

also be synthesized from m-cresol, as shown in Reaction A below. Thymol can then be converted to menthol,

another naturally-occurring organic compound; this conversion is shown in Reaction B.

Reaction A

Reaction B

Comparing the Ka values for cyclohexanol (Ka = 10−13) and phenol (Ka = 1.3 × 10−10) reveals that phenol is more

acidic than cyclohexanol. Which of the following explain(s) the acidity of phenol?

I. The exceptionally strong hydrogen bonding possible with phenol facilitates the loss of a proton, making it

more acidic than cyclohexanol.

II. Phenol’s conjugate base, phenoxide, is stabilized by resonance to a greater extent than phenol itself.

III. The negative charge of the oxygen atom on the phenoxide ion is delocalized over the benzene ring.

Section: Biological Sciences

Options :

A : I only

B : II only

C : II and III only

D :

I, II, and III

Answer: C

Question 2

Yeast can be used to covert simple sugars into:

Section: Physical Sciences

Options :

A : ethanoic acid and oxygen.

B : ethanol and oxygen.

C : ethanol and carbon dioxide.

D : starch and carbon dioxide.

Answer: C

Question 3

Nitric oxide, NO, has recently been found to have widespread physiological effects, acting as a major regulator

in the nervous, cardiovascular, and immune systems. The production of NO in the body is regulated by specific

NOS enzymes which exist in at least three different isoforms – bNOS, eNOS, and macNOS. Each of these

isoforms differ in location and function and serve to mediate different physiological responses to NO. Some

physiological roles of NO have been demonstrated as follows:

I. In the central nervous system, NO production is regulated by bNOS. Calcium ion concentrations of 200-400

nM in the central nervous system activate bNOS to catalyze the formation of NO. NO exerts definite effects on

brain function although its specific roles are not well established. bNOS inhibitors have been found to block the

release of neurotransmitter from presynaptic neurons. Excess levels of NO are also thought to contribute to

neurodegenerative disorders such as Alzheimer’s disease.

II. In the blood vessels, NO is produced by eNOS which is activated by Ca2+ concentrations of 200-400 nM. NO

acts as the major endogenous vasodilator in blood vessels. It diffuses into smooth muscle cells and leads to

muscle relaxation by stimulating cGMP formation through activation of guanylyl cyclase. In addition, NO

regulates the vascular system by inhibiting platelet aggregation and adhesion.

III. The role of NO in the immune system is regulated by macNOS through a pathway that is not Ca2+

dependent. Rather, exposure to cytokines, including interleukin-1 and interferon- γ, leads to synthesis of large

amounts of NO by activation of macNOS in response to inflammatory stimuli. The NO produced plays a

definitive role in the mediation of the activities of macrophages and neutrophils. NO also acts to inhibit the

mechanism of viral replication.

A “knock out” mouse with a mutant bNOS protein was generated by recombination techniques. The mutant

protein was identical to the wild-type protein except for the identity of amino acid 675; the mutated bNOS has

Tryptophan instead of Cysteine at position 675. Which of the following is responsible for the mutant protein?

Section: Biological Sciences

Options :

A :

Frame shift mutation

B :

Single base-pair deletion

C : Point mutation

D : Nonsense mutation

Answer: C

Question 4

The equation of state of an ideal gas is given by the ideal gas law: PV = nRT where P is the pressure, V is the volume, n is the number of moles of gas, R is the ideal gas constant, and T is the temperature of the gas. The gas particles in a container are constantly moving at various speeds. These speeds are characterized by the Maxwell shown in the figure below.

If two particles collide, their velocities change. However, if the gas is in thermal equilibrium, the velocity

distribution of the gas as a whole will remain unchanged by the collision.

The average kinetic energy (E) of a gas particle is given by:

Equation 1

where m is the mass of one particle and u is the root mean square speed (rms speed) of the gas particles:

where N is the number of gas particles; this is different from the average speed. For an ideal gas, the kinetic

energy of all the particles is:

Equation 2

where n is the number of moles of gas. Combining these equations gives:

Equation 3 where M is the molar mass of the gas particles. The average distance a particle travels between collisions is known as the mean free path l. Intuitively, the mean free path (mfp) could be expected to be larger for gases at low pressure, since there is a lot of space between particles. Similarly, the mfp should be larger when the gas particles are small. The following expression for the mfp shows this to be correct.

Equation 4

In this equation, s is the atomic diameter (typically on the order of 10-8), k is the Boltzmann constant, and P is

the pressure.

In addition to colliding with one another, gas particles also collide with the walls of their container. If the

container wall has a pinhole that is small compared to the mfp of the gas, and a pressure differential exists

across the wall, the particles will effuse (or escape) through this pinhole without disturbing the Maxwellian

distribution of the particles. The rate of effusion can be described by:

Equation 5

Where neff is the number of moles of effusing particles, A is the area of the pinhole, P and P1 are the pressures

on the inside and outside of the container wall respectively, and P > P1.

Which of the following will have the smallest root mean square speed at 298K?

Section: Physical Sciences

Options :

A :

B :

C :

D :

Answer: A

Question 5

Band theory explains the conductivity of certain solids by stating that the atomic orbitals of the individual atoms

in the solid merge to produce a series of atomic orbitals comprising the entire solid. The closely-spaced energy

levels of the orbitals form bands. The band corresponding to the outermost occupied subshell of the original

atoms is called the valence band. If partially full, as in metals, it serves as a conduction band through which

electrons can move freely. If the valence band is full, then electrons must be raised to a higher band for

conduction to occur. The greater the band gap between the separate valence and conduction bands, the poorer

the material’s conductivity. Figure 1 shows the valence and conduction bands of a semiconductor, which is

intermediate in conductivity between conductors and insulators.

Figure 1

When silicon, a semiconductor with tetrahedral covalent bonds, is heated, a few electrons escape into the

conduction band. Doping the silicon with a few phosphorus atoms provides unbonded electrons that escape

more easily, increasing conductivity. Doping with boron produces holes in the bonding structure, which may be

filled by movement of nearby electrons within the lattice. When a semiconductor in an electric circuit has

excess electrons on one side and holes on the other, electron flow occurs more easily from the side with

excess electrons to the side with holes than in the reverse direction.

Figure 2

How does heat increase the conductivity of a semiconductor?

I) By reducing collisions between moving electrons

II) By breaking covalent bonds

III) By raising electrons to a higher energy level

Section: Physical Sciences

Options :

A : I only

B : III only

C : I and III only

D : II and III only

Answer: D

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