Kaplan MCAT General Chemistry Review

Gases are compressible, because they travel freely with large amounts of space between molecules. Because gas particles are far apart from each other and in rapid motion, they tend to take up the volume of their container. Many gases that exist as diatomic molecules (i.e., O₂, H₂, N₂), but this is not a property that characterizes all gases.

12. B

We will use V₁/T₁ = V₂/T₂. First, we must convert the temperature to Kelvin by adding 273 to get 300 K as the initial temperature. Plugging into the equation and solving for T₂ gives 450 K. Subtracting the initial temperature, 300 K, gives an increase of 150 K. They are measured by the same increments, so an increase of 150 K corresponds to an increase of 150°C.

13. B

The partial pressure of each gas is found by multiplying the total pressure by the mole fraction of the gas. Because 80 percent of the molecules are nitrogen, the mole fraction of nitrogen gas is equal to 0.80. Similarly, for helium, the mole fraction is 0.20, because helium comprises 20 percent of the gas molecules. To find the pressure exerted by nitrogen, multiply the total pressure (150 torr) by 0.80 to obtain 120 torr of nitrogen. To find the pressure exerted by helium, multiply the total pressure by 0.20 to get 30 torr of helium.

14. C

Both a change in temperature and a change in volume can affect the gas’s pressure. So if one of those two variables is kept constant (i.e., (A) and (B)), we’ll definitely be able to predict which way the pressure will change. At a constant volume, heating the gas will increase its pressure, and cooling the gas will decrease it. What about when both temperature and volume are changing? If they have the same effect on pressure, then we can still predict which way it will change. This is the case in (D). Cooling the gas and increasing its volume both decrease pressure. (C), on the other hand, presents us with too vague a scenario for us to predict definitively the change in pressure. Heating the gas would amplify the pressure, while increasing the volume would decrease it. Without knowing the magnitude of each influence, it’s impossible to say whether the pressure would increase, decrease, or stay the same.

Chapter 8: Phases and Phase Changes

Thwak! Thwak! “Stupid ketchup bottle! Why . . . won’t you . . . come . . . out?!” Few things are more frustrating than ketchup that just refuses to budge. It usually happens with newly opened bottles, and the “stuck ketchup” phenomenon is so common that there’s almost a ritualized nature to the process of getting it “unstuck”: Unscrew the top, turn the bottle on its side, and give it a few gentle shakes. If it doesn’t come out right away, the next step is the administration of a few sturdy whacks with the palm of the hand against the bottom of the bottle. This is usually accompanied by some mild epithet suggestive of ketchup’s alleged lack of intelligence (see quote above). More often than not, the physical abuse imposed upon the bottle only results in soft-tissue bruising and perhaps an alarming increase in the violent nature of the anti-ketchup epithets. Finally the ritual ends in an act of stabbing, and the evidence to convict is the spilled pool of red and the seemingly blood-coated butter knife that rests along the edge of the plate next to the pile of crispy french fries.

You can’t really blame the ketchup, though. It doesn’t know any better. In fact, it’s only acting according to its nature as a member of a unique class of liquids called Bingham fluids. Bingham fluids do not begin to flow immediately upon application of shear stress. Unlike Newtonian fluids, such as water or vegetable oil, which begin to flow as soon as a finite amount of shear stress is applied, Bingham fluids will only begin to flow when a minimum force value called the yield value is exceeded. Essentially, Bingham fluids behave like solids under static conditions and flow, as fluids, only when a shear stress at least equal to the yield value is applied. The sharp blows that you apply to the bottle of ketchup are usually strong enough to exceed ketchup’s yield value. Unfortunately for the diner who ends up with a plate (and probably lap) flooded with ketchup, ketchup also belongs to a class of liquids known as pseudoplastic fluids, which demonstrate the property of shear thinning. Shear-thinning liquids display reducing viscosity with increasing shear rate (related to fluid velocity). That ketchup is stuck, like a solid, in that bottle, until it isn’t (because the yield value has been exceeded), at which point it begins to flow and the faster it flows, the more it “thins” and becomes less viscous. The result is the mess on your plate and lap.