MCAT Question of the Day

Correct Answer: D. halved.

The centripetal force of the car is mv²/r, where m is the car’s mass, v is the velocity, and r is the radius of the circular track. If we double the radius while keeping everything else uniform, the centripetal force will be halved because they are inversely proportional.

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Correct Answer: B. Phospholipids

The cell membrane is composed of a phospholipid bilayer that has a hydrophilic head (where the polar phosphate is attached) and a hydrophobic tail (made up of hydrocarbon chains). Starch is a polysaccharide that is generally hydrophilic and can be dissolved in water. Triglycerides, steroids and cholesterol are all entirely hydrophobic not being able to be dissolved in water. The molecule mentioned in the question is both hydrophilic and hydrophobic, indicative of the phospholipid, therefore only answer option [B] is correct.

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Correct Answer: D. The pH will remain close to 5.

Buffers are designed to maintain pH at a certain level, and operate through the complementation of a conjugate acid and base. As long as the ratios of acid and base remain the same in the buffer system, and they will no matter what the dilution, as long as the dilution is not very very small (i.e. 1 mM or less), the pH will remain the same.

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Correct Answer: D. Hydrochloric acid pH = 0; acetic acid pH = 4; water pH = 7; ammonia solution pH = 10; sodium hydroxide solution pH = 14.

Acids have pH values of less than 7, with the stronger the acid the lower the pH. Bases have pH values of greater than 7, with the stronger the base the higher the pH. Typically:

• Strong acids have a pH of 0 or less, hydrochloric acid pH = 0.
• Weak acids between pH 3-6, acetic acid pH = 4.
• Neutral solutions have pH = 7, water pH = 7.
• Weak bases have a pH 8-10, ammonia solution pH = 10.
• Strong bases have a pH of 14 or more, sodium hydroxide solution pH = 14.

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Correct Answer: C. 6000 N-m

Assuming that the water stream is tangent to the wheel, it will be perpendicular to the radius vector at the point of contact. Therefore, the torque of the force of the water stream is Torque = r*F = (20m)(300 N) = 6000 N-m.

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Correct Answer: D. Boiling and condensation

Melting: solid to liquid and Freezing: liquid to solid occur at the Melting/Freezing Point.
Boiling: liquid to gas and Condensation: gas to liquid occur at the Boiling/Condensation Point.
Both evaporation: liquid to gas at the surface of the liquid and sublimation: solid to gas occur at any temperature.

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Correct Answer: D. pH of the titrant

During a titration procedure, a buret is lowered into a clean Erlenmeyer flask. The initial volume of titrant is recorded and a stream of titrant is delivered while constantly mixing the sample solution. Once the pH indicator changes colors we have reached the endpoint. The final volume is recorded and used in conjunction with the initial volume to find the volume of titrant used, and along with the concentration of the titrant to find how many moles of titrant were used. Finally, we can use stoichiometry of the titration reaction to calculate the number of moles of reactant in our solution. The pH of the titrant is not needed.

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Correct Answer: D. 3.17 x 22.4 = 71.0

STP is Standard Temperature and Pressure or 0^oC and 1 atm pressure. At STP 1 mole of an ideal gas occupies 22.4 L. A good approximation of the molecular mass of a gas is obtained by multiplying its density in g.L^-1 by 22.4 L to attain the mass of 1 mole of gas. Hence: 3.17 x 22.4 = 71.0

24.5 L is the molar volume of an ideal gas under LTP or Laboratory Temperature and Pressure, a balmy 25^oC and 1 atm pressure.

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Correct Answer: C. Part of the innate immune system comprised of plasma proteins synthesized primarily by hepatocytes.

The complement system is a clear example of innate immunity, being a form of host defense that is non-specific and does not adapt in response to antigens. However, there is evidence of cross-talk between the complement system and adaptive immunity. Similarly components of the complement system communicate with components of the inflammatory system and hematopoietic system. Nonetheless, complement system components are distinct from leukocytes and the components of the inflammatory system and are primarilysynthesized in the liver.

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Correct Answer: A. The method relies on the fact that solids are more soluble in a cold solvent than they are in the same amount of hot solvent.

This statement is reversed: The process of recrystallization involves dissolution of the solid in appropriate solvent at a high temperature and the subsequent re-formation of the crystals upon cooling, so that any impurities remain in solution.

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