tbr test 4 qs

[2010premed]

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Which compound is a chemical by-product in Stage II of the firework in Figure 1?
A.. .NO3
B.. .B. K2O
C.. .C. SO3
D.. .D. Cl2
I got this answer from the passage, but is there any other way of doing it? (ie by looking at the oxidation numbers)
2. Which of the following is NOT a propert of an emitter?
A. is highly exothermic when undergoing combustion
B. Has nearly monochromatic emission
C. can absorb the thermal energy needed to excite electrons
D. as energy-level transitions in the visible range of the EM spectrum
The emitter here is referring the transition metal, correct?
3. when question asks, which has the highest molar solubility? It’s just asking for which one has the highest mol/L dissolved in water. However, when do we have to calculate x^2, 4x^3 etc, is this only when we want to know Ksp value? Lastly, to determine which salt is more soluble, we have to calcular the x^2 or 4x^3, and not just the molar solubility, correct?
4. If a compound becomes less soluble as the temperature of the solution is increased, then which of the following statements about the entropy and enthalpy changes associated with its solvation is correct?
A. delta s is negative delta h is negative
B. delta s is positive delta h is negative
What’s the difference between this question and when we say that at positive delta s and delta h, equation is spontaneous only at high temps, and vice versa. I was really surprised to know this answer is B.
5. If a compound becomes less soluble as the temperature of the solution is increased, then which of the following statements about the entropy and enthalpy changes associated with its solvation is correct?
A. Orange (color doesn’t change) with shorter wavelength
So the frequency of light NEVER changes, right?
6. Which of the following should we expect to see over time in a car powered by an aluminum-air battery?
A. The mass of the car will increase, because the aluminum in the battery gains mass through the formation of aluminum hydroxide.
B. The mass of the car will remain the same, because the aluminum in the battery is only being oxidized and does not leave the car.
How would the mass of the car increase? Aren’t all the reactants present in the beg n they just recombine differently, so that the mass is conserved?
7. How can the frequency of a pendulum remain constant as energy changes (passage 6 q 38)?
8. The table below shows the positions of the bob for five consecutive flashes of the strobe light:
A. The period of the strobe light is less than the period of the pendulum, but greater than half the period of the strobe light.
B. The period of the strobe light is less than half the period of the pendulum.

How do you do

this?
9. Which statement violates the junction rule?
Wrong answer: The current continuing along any path beyond a junction is lower than the current entering the junction.
Correct answer: If a junction splits a current into two paths, both paths have currents greater than half of the current entering the junction.
I get why the correct answer is correct, but is there a type in the wrong answer?
10. What change is observed in the force exerted on charged particle P in a uniform electric field when it is moved closer to the plate on the left, so that the distance r separating them is reduced by half?


A.. .The force increases by a factor of 4, regardless of the type of charge.
B.. .The force does not change, regardless of the magnitude or sign of the charge.
So if you’re not supposed to use F=qq/r^2 here, when do we use it?
11. In a galvanic cell, the electrode that builds up a precipitate of metal (known as plating out) is:
A: the cathode, and its surface is rich in electrons.
Are all cathodes (even of other equipment) rich in electrons? Does this mean they’re negatively charged? I always thought they were positively charged.

 

[krnpanda]

tl;dr

i suggest you use the enter key a bit more, the words are too crammed together. at the very least, space out different questions to make it easier to read. and if there is a figure, please post that figure so we can see what you see.

 

[fizzgig]

9. no, it says from A path leaving the node. there could be multiple paths leaving the node

10. i don't think it will change. the force a particle feels wherever it is should be derived from how strong the field is there. this field is the same no matter where the particle moves. if you have one charge in the field of another charge, then the force one feels will depend on the other. and the other's field changes with distance.

11. remember 'cations go to the cathode'. they do this because electrons go there (and current goes opposite, to the anode, and since current is positive anions go to the anode as well). so plating should always involve the reduction, where electrons are being gained, which will be where the cations change to solid metal, at the cathode. the positive *designation* that you write down indicates which end of your battery is at higher potential. this positive designation can mean anode or cathode because you can have your battery discharging (current comes OUT of the higher potential end, galvanic, current out means electrons in, electrons in means cations in, that's your cathode) or you can have your battery charging (forcing current INTO the higher potential end by a bigger battery, positive current in so anions in, this is your anode). draw out some pictures.

 

[Rabolisk]

So if you're not supposed to use F=qq/r^2 here, when do we use it?

When you are given two charges.

 

[BerkReviewTeach]

• Which compound is a chemical by-product in Stage II of the firework in Figure 1?
  A. NO3
  B. K2O
  C. SO3
  D. Cl2

I got this answer from the passage, but is there any other way of doing it? (ie by looking at the oxidation numbers)

Why do you need a different way if you got it from the passage? That's the point of the practice test, to get you to use the passage. The MCAT is about logic, interpretation, incorporation of information, and some background info. The best you could do is a combination of passage and background. You known they are burned in air, so the byproduct must be rich in oxygen (full oxidized). N, K, and S are all fully oxidized, so the only way to decide is to know from experience or read from the passage that they burn sulfur. The smell of fireworks could be a possible clue, but the passage gives the info.

• 2. Which of the following is NOT a propert of an emitter?
  A. is highly exothermic when undergoing combustion
  B. has nearly monochromatic emission
  C. can absorb the thermal energy needed to excite electrons
  D. has energy-level transitions in the visible range of the EM spectrum

The emitter here is referring the transition metal, correct?

Yes, the transition metal is responsible for the color. In order to emit in the visible range, it must have absorption/emission in the visible range. It has to absorb heat from the other reaction to excite the electrons and one color is nice.

3. when question asks, which has the highest molar solubility? It’s just asking for which one has the highest mol/L dissolved in water. However, when do we have to calculate x^2, 4x^3 etc, is this only when we want to know Ksp value? Lastly, to determine which salt is more soluble, we have to calcular the x^2 or 4x^3, and not just the molar solubility, correct?

Most soluble correlates with the greatest amount of salt in solution, which is measured directly by mol/L. Molar solubility is the correct value (and conceptual term). x^2, 4x^3 etc... are Ksp values and only used if the question is a calculation question or ratio question.

• 4. If a compound becomes less soluble as the temperature of the solution is increased, then which of the following statements about the entropy and enthalpy changes associated with its solvation is correct?
  A. delta S is negative delta H is negative
  B. delta S is positive delta H is negative
  C. not given by OP
  D. not given by OP

What’s the difference between this question and when we say that at positive delta S and delta H, equation is spontaneous only at high temps, and vice versa. I was really surprised to know this answer is B.

This question is asking if you recognize that when a salt breaks apart into ions, it becomes more random (hence the positive delta S). All of the extraneous information is serving as a distractor. Getting ready for the MCAT requires that you get used to questions like this, where they overload you with too much information. Keep it simple and visualize everything you can.

• 6. Which of the following should we expect to see over time in a car powered by an aluminum-air battery?
  A. The mass of the car will increase, because the aluminum in the battery gains mass through the formation of aluminum hydroxide.
  B. The mass of the car will remain the same, because the aluminum in the battery is only being oxidized and does not leave the car.
  C. not given by OP
  D. not given by OP

How would the mass of the car increase? Aren’t all the reactants present in the beg n they just recombine differently, so that the mass is conserved?

To clarify, the car is not what is getting heavier, it's the battery in the car. And all the reactants are not present. Where does the O2 come from? It comes from the surrounding air and combines with the battery. The battery starts as pure metal and finishes as Al2O3, a heavier compound. The battery builds up heavier oxide over the course of its usage.

7. How can the frequency of a pendulum remain constant as energy changes (passage 6 q 38)?

Why should it change? Running out of energy means it will move slower and not travel as far with each cycle. The two effects cancel out. Going slower on average but having to travel less distance causes the pendulum to break even and take the same amount of time per cycle. The period is the same, so frequency (f = 1/T) is the same.

• 8. The table below shows the positions of the bob for five consecutive flashes of the strobe light:
  A. The period of the strobe light is less than the period of the pendulum, but greater than half the period of the strobe light.
  B. The period of the strobe light is less than half the period of the pendulum.

Not sure what your question is, but the idea here is that it takes some amount of time to elapse between flashes, so "flashes" is a hidden way of saying "time". A full cycle takes more than 2 flashes of time, so the period is 2.something flashes and thus the frequency is less than half a cycle per flash.

9. Which statement violates the junction rule?
Wrong answer: The current continuing along any path beyond a junction is lower than the current entering the junction.
Correct answer: If a junction splits a current into two paths, both paths have currents greater than half of the current entering the junction.
I get why the correct answer is correct, but is there a typo in the wrong answer?

If current enters a junction, it splits into multiple paths, each with a current less than the entering current in a way where all of the multiple currents sum to equal the entering current.

Fizzgig and Rabolisk have answered your remaining questions. And I have to agree krnpanda that if you posted a mush of eleven questions the way you did, no one wants to read gthe mess, let alone atke the time to decipher it and help you out. Please be considerate and post only a couple questions per thread and in a way where they aren't so cluttered.