Test Prep MCAT Test Exam
Medical College Admission Test: Verbal Reasoning, Biological Sciences, Physical Sciences, Writing Sample (Page 66 )

The automobile airbag was designed to inflate upon impact and decrease the risk of injury to drivers and passengers. Among the challenges to its development was the need to find a reliable inflation mechanism that was sufficiently rapid, controllable, and nontoxic. Prototypes employing compressed gases failed to meet these criteriA. Researchers thus turned their attention to chemical alternatives.

The ideal inflatant requires a chemical reaction in which the reactants are stable and relatively dense in the condensed phase while the products are mostly or completely gaseous at ambient temperature and pressure. Additionally, the ideal chemical reaction would require a low activation energy and have a high kinetic rate constant, without the large exothermicity characteristic of most such reactions. Traditional explosives such as nitroglycerin, C3H5N3O9(l), were rejected almost immediately because of the extremely exothermic nature of their conversion. Benign solids such as calcium carbonate, CaCO3 , were similarly rejected, because of their large activation requirements.
The desired attributes were finally found in sodium azide, NaN3, a stable, dense, ionic solid which rapidly decomposes into elemental sodium and nitrogen gas when ignited by an electrical impulse.
2NaN3 2Na + 3N2
Reaction 1
The gas generating mixture includes excess KNO3 which reacts with the sodium metal from Reaction 1 to produce additional N2 and potassium and sodium oxides (Reactions 2 and 3). These oxides react with SiO2 to produce a non-toxic and stable alkaline silica (glass).
10Na + 2KNO3 K2O + 5Na2O + N2
Reaction 2
K2O + Na2O + SiO2 glass
Reaction 3

Decomposition of which of the following transition metal complexes would produce the highest theoretical yield of carbon monoxide per gram of reactant?

Many nutrients required by plants exist in soil as basic cations: Mg2+, Mn2, and Ca2. A soil's cation-exchange capacity is a measure of its ability to adsorb these basic cations as well as exchangeable hydrogen and aluminum ions. The cation-exchange capacity of soil is derived from two sources: small clay particles called micelles consisting of alternating layers of alumina and silica crystals, and organic colloids.
Replacement of A13+ and Si4+ by other cations of lower valence creates a net negative charge within the inner layers of the micelles. This is called the soil's permanent charge. For example, replacement of an atom of aluminum by calcium within a section where the net charge was previously zero, as shown below, produces a net charge of -1, to which other cations can become adsorbed.

Figure 1
A pH-dependent charge develops when hydrogen dissociates from hydroxyl moieties on the outer surfaces of

the clay micelles. This leaves negatively-charged oxygen atoms to which basic cations may adsorb. Likewise, a large pH-dependent charge develops when hydrogen dissociates from carboxylic acids and phenols in organic matter.
In most clays, permanent charges brought about by substitution account for anywhere from half to nearly all of the total cation-exchange capacity. Soils very high in organic matter contain primarily pH-dependent charges.
In a research study, three samples of soil were leached with a 1 N solution of neutral KCl, and the displaced A13+ and basic cations measured. The sample was then leached again with a buffered solution of BaCl2 and triethanolamine at pH 8.2, and the displaced H+ measured. Table 1 gives results for three soils tested by this method.
Table 1

Due to the buffering effect of the soil's cation exchange capacity, just measuring the soil solution's pH will not indicate how much base is needed to change the soil pH. In another experiment, measured amounts of acid and base were added to 10-gram samples of well-mixed soil that had been collected from various locations in a field. The volumes of the samples were equalized by adding water. The results were recorded in Figure 2.

Figure 2
What percentage of the cation exchange capacity of Sample I is base-saturated?

Many nutrients required by plants exist in soil as basic cations: Mg2+, Mn2, and Ca2. A soil's cation-exchange capacity is a measure of its ability to adsorb these basic cations as well as exchangeable hydrogen and aluminum ions. The cation-exchange capacity of soil is derived from two sources: small clay particles called micelles consisting of alternating layers of alumina and silica crystals, and organic colloids.
Replacement of A13+ and Si4+ by other cations of lower valence creates a net negative charge within the inner layers of the micelles. This is called the soil's permanent charge. For example, replacement of an atom of aluminum by calcium within a section where the net charge was previously zero, as shown below, produces a net charge of -1, to which other cations can become adsorbed.

Figure 1
A pH-dependent charge develops when hydrogen dissociates from hydroxyl moieties on the outer surfaces of

the clay micelles. This leaves negatively-charged oxygen atoms to which basic cations may adsorb. Likewise, a large pH-dependent charge develops when hydrogen dissociates from carboxylic acids and phenols in organic matter.
In most clays, permanent charges brought about by substitution account for anywhere from half to nearly all of the total cation-exchange capacity. Soils very high in organic matter contain primarily pH-dependent charges.
In a research study, three samples of soil were leached with a 1 N solution of neutral KCl, and the displaced A13+ and basic cations measured. The sample was then leached again with a buffered solution of BaCl2 and triethanolamine at pH 8.2, and the displaced H+ measured. Table 1 gives results for three soils tested by this method.
Table 1

Due to the buffering effect of the soil's cation exchange capacity, just measuring the soil solution's pH will not indicate how much base is needed to change the soil pH. In another experiment, measured amounts of acid and base were added to 10-gram samples of well-mixed soil that had been collected from various locations in a field. The volumes of the samples were equalized by adding water. The results were recorded in Figure 2.

Figure 2
What would be the effect of leaching the three soil samples in Table 1 with a buffered BaCl2 solution at pH 9.5 instead of 8.3?

A ski jump is an inclined track from which a ski jumper takes off through the air. After traveling down the track, the skier takes off from a ramp at the bottom of the track. The skier lands farther down on the slope.
Figure 1 shows a ski jump, in which the ramp at the lower end of the track makes an angle of 30° to the horizontal. The track is inclined at an angle of to the horizontal and the slope is inclined at an angle of 45° to the horizontal. A ski jumper is stationary at the top of the track. Once the skier pushes off, she accelerates down the track, and then takes off from the ramp. The vertical height difference between the top of the track and its lowest point is 50 m, and the vertical height difference between the top of the ramp and its lowest point is 10 m.

Figure 1
The distance traveled by the skier between leaving the ski jump ramp and making contact with the slope is called the jump distance. In some cases, in order to increase the jump distance a skier will jump slightly upon leaving the ramp, thereby increasing the vertical velocity. Unless otherwise stated, assume that friction between the skis and the slope is negligible, and ignore the effects of air resistance.
If a skier uses skis of greater surface area, which of the following would occur?

Suppose an -particle starting from rest is accelerated through a 5 megavolt potential difference. What is the final kinetic energy of the -particle? (Note: Assume that e = 1.6 × 10-19 C.)