9 questions from AAMC 4 physical sciences

[2010premed]

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• Suppose that CH4 reacts completely with O2 to form CO2 and H2O with a total pressure of 1.2 torr. What is the partial pressure of H2O?

A: 0.8 torr

I know the equation is mole fraction times total pressure, and I balances the equation right. However, how do you know that the mole fraction of H20 is 2/3 and not 2/6? In other words, how do you know that Ptotal = PCO2 + PH2O only?

• Mechanical waves in a medium such as water function to transport:

A: energy only

What are examples of other waves that transfer something other than energy only?

• Table: Depth = 5 cm P = 250

Depth=10 cm P = 450
Depth = 15 cm P = 650
What is the pressure at a depth of 10 cm for a liquid whose density is twice that of the liquid in the table?

A: 850

Is this saying that Patm = 50atm? And therefore from the equation Pt= Patm + pgh we should only double the pgh value?

• An object with 15 g mass is immersed in benzene and suffers an apparent lostt of 5 grams. What is the approximate specific gravity of the object? (Specific gravity of benzene is 0.7)

A: 2.1

Ok I understand that 5 g of benzene are displaced, but after that I’m a bit lost. The explanation says to get the ratio of object mass to fluid mass (15/5 = 3) and multiply by 0.7
How is getting the ratio of object mass to fluid mass the same thing as specific gravity?

• Which of the following expressions gives the mole fraction of KNO3 in he solution prepare by the student?

A: 226g/L/101.1 g/mol KNO3 all over (226.5g/L / 101.1 g/mol) + (906.1 g/L / 18 g/mol water)

My question is, howcome when you cancel out everything there are still Liters and not just moles?

• What is the approximate number of potassium ions in the student-repared solution of KNO3?

A: 10^24

Explanation: molarity of solution is 2.241 M, so solution has 2.241M of K+ (Howcome? I thought the total molarity was 2.241M?) Then, 2 x 6 x 10^23 = 10^24

• A receiver is in a jet flying alongside another jet that is emitting 2 x 10^6 Hz radio waves. If the jets fly at 268 m/s, what is the change in frequency detected at the receiver?

A: 0 Hz

Exp: B/c there is no relative motion between the jets.

What if there was relative motion, how would you calculate the change in frequence?

• An astronomer observes a hydrogen line in the spectrum of a star. The wavelength of hydrogen in the laboratory is 6.565 x 10^-7m, but the wavelength in the star’s light is measured at 6.56186 x 10^-7m. Which of the following explains this discrepancy?

A: The star is approaching Earth.

Exp: the wavelength of the light detected from the star is smaller than the laborator value on Earth.

Does this mean the “source” is greater that the “listener”? I’m a little confused on why it’s no the opposite… I thought that the source would be the measured wavelength in the star’s light.

• If a student did NOT remove all the moisture from the KHP before the titration with NaOH, then the molarity determined for the NaOH would be:

A: too high because the actual number of moles of KHP titrated would be less than the number used in the calculations.

Equation: KHP + NaOh à KNaP + H2O

 

[Rabolisk]

OK, here goes..

1. Total pressure is 1.2 atm, and the equation goes to completion. So there is only H2O and CO2 left. Nothing suggests that there is anything else.

2. Not that I know of. There is vibration of matter, but matter isn't transported. For example, air molecules aren't transported from a source of sound to a detector of sound, only energy is.

3. Yes, that's exactly what they're saying.

4. Since the object is submerged, the volume of the object and the benzene displaced must be equal. The mass of the object is 15g, but the mass of the displaced benzene is only 5g. Going from mobjectg = ρobjectVg = 3 * ρbenzeneVg, ρobject has to be three times ρbenzene. Specific gravity is just density of anything relative to water (1.0 is defined as density of water). So tripling density is same as tripling specific gravity.

5. You get moles of KNO3 / L solution in the numerator, and moles of KNO3 + moles of H2O / L of solution in the denominator. The liters cancel out (they're referring to the same volume), giving you moles of KNO3 / total moles, which is mole fraction.

6. In solution, KNO3 dissociates completely to K+ and NO3-. When we say that there is a 2.241M KNO3 solution, we really mean that the concentration of K+ and NO3- are 2.241M.

7. If there was relative motion, you would use the doppler equation f = f0 (v+vd)/(v+vs). The sign convention is somewhat convoluted, but if you just keep in mind that frequency goes up when two objects are going towards each other, you can figure it out.

8. The wavelength is smaller at the source than at the detector. So the frequency is greater at the source than at the detector.

9. One of the tougher questions. Molarity of NaOH is moles of NaOH/V NaOH soln. The moles of NaOH should equal the moles of KHP when the titration is complete. To get the moles of KHP, the student measured the mass of KHP and divided it by molar mass. However, if the measured mass contained impurities, that means that the actual mass of KHP is less than the measured mass. Conversely, the student thinks he has higher mass of KHP than he actually does, resulting in higher calculation of moles of NaOH, and higher molarity of NaOH.

 

[2010premed]

Wow, thanks so much, this really helps. I have a couple more q's

For number 4, I don't see how you know that ρobjectVg = 3 * ρbenzeneVg.
For number 8, I was confused about how they defined the source vs the detector.

 

[Rabolisk]

For 4, the object has a mass of 15g, but only 5g of benzene is displaced. Since the volumes of the object and the displaced benzene are equal, only the density can vary.

For 8, that was a bad explanation on my part. In the laboratory, hydrogen emits a certain wavelength. In the star, hydrogen also emits the same wavelength. However, as seen from the Earth, this wavelength is greater than the wavelength that one expects to see from hydrogen (the laboratory value). There is red shift, or shifting towards longer wavelength and lower frequency, which means that the star is moving away from the earth.

 

[vsl5]

For 4, the object has a mass of 15g, but only 5g of benzene is displaced. Since the volumes of the object and the displaced benzene are equal, only the density can vary.

For 8, that was a bad explanation on my part. In the laboratory, hydrogen emits a certain wavelength. In the star, hydrogen also emits the same wavelength. However, as seen from the Earth, this wavelength is greater than the wavelength that one expects to see from hydrogen (the laboratory value). There is red shift, or shifting towards longer wavelength and lower frequency, which means that the star is moving away from the earth.

isnt it a blue shift? In the lab, you assume there is nothing moving so no doppler effect. When you measure it from the star, the wavelength is shorter so the source must be moving toward the detector (or vice versa) giving a blue shift.

 

[Rabolisk]

Right. Blue shift. For some reason, I thought the OP said star was going away from the earth (that's also how I remembered AAMC 4). Sorry.

 

[2010premed]

Thank you =)🙂