7 physical science questions from AAMC 5

[2010premed]

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1.. .An object that is totally immersed in benzene (specific gravity = 0.7) is subject to a buoyancy force of 5N. When the same object is totally immersed in an unknown liquid, the buoyancy force is 12 N. What is the approximate specific gravity of the unknown liquid?
A: 1.7 N
Ok so I get that since the object is immersed, then the buoyant force equals density of liquid times volume of object time gravity, but after that I don’t know what to do. Why do they do 12/5 * 0.7?

2.. .In a nearsighted individual, the image of a distant object is focused:
A: in front of the retina, requiring divergent lens correction.
don’t even know where to start with this question

3.. .Which of the following expressions gives the amound of energy lost to friction by the toboggan and rider between points A and B?
A.. .60N (l)
B.. .60kg (g) (l)
What’s the difference between these 2 answers? The correct answer was A
4.. .When would a hybridization of a molecule in a compound be sp3d2 or s2p2 etc..?

5.. .According to Equation1, the concentration of the polymer with respect to [HPO4 2-] is:

A: (1/n) [HPO4 2-]
Equation 1: nCDP à (CP)n + n HPO4 2-
I’m not seeing the math here, thought the answer would be n [HPO4 2-]

6.. .When a strip of Cu is placed in H2O (l), no change is observed. However, when a strip of Cu is placed into a solution of HNO3(aq), a gas evolves. What is the most likely identity of the gas?
A.. .NO
B.. .CO2
C.. .H2
D.. .O3
A: NO
How exactly should I come to this answer?

7.. .From reactions 3-5, what can be determined about the relative concentrations of CFCs and O2?
A: A catalytic amount of CFC can produce much O2
Rxn3: CF3CLàCF3 + Cl
Rex 4: Cl + O3 à CLO + O2
Rexn 5: CLO + Oà CL + O2
How can we say “a catalytic amount of CFC” if CF3Cl is not being recycled??

 

[vsl5]

1.An object that is totally immersed in benzene (specific gravity = 0.7) is subject to a buoyancy force of 5N. When the same object is totally immersed in an unknown liquid, the buoyancy force is 12 N. What is the approximate specific gravity of the unknown liquid?
A: 1.7 N
Ok so I get that since the object is immersed, then the buoyant force equals density of liquid times volume of object time gravity, but after that I don't know what to do. Why do they do 12/5 * 0.7?

Fb (benzene) = 5 = 0.7Vo*g ---> 5/7 = Vo
Fb(unknown) =12N = pVog ---> p = 12/g*(5/7) = 1.7
5*7 is simply the volume of the object (5N/0.7*g)

2.In a nearsighted individual, the image of a distant object is focused:
A: in front of the retina, requiring divergent lens correction.
don't even know where to start with this question

Near sighted means light focuses IN FRONT of the retina, therefore you want to make it diverge at the lens and focus at the retina so you use a diverging lens. Far sighted means it focuses BEHIND the retina and you use a converging lens

3.Which of the following expressions gives the amound of energy lost to friction by the toboggan and rider between points A and B?
A.60N (l)
B.60kg (g) (l)
What's the difference between these 2 answers? The correct answer was A
4.When would a hybridization of a molecule in a compound be sp3d2 or s2p2 etc..?

Work done by friction = Force (friction) x distance = 60N * l
Its not B because it never says force = 60kg * g. Friction force = Fk= umgcos(theta), in this case the normal force is NOT 90 degrees and also they never tell you what the friction coefficient is (but you could figure it out).

6.When a strip of Cu is placed in H2O (l), no change is observed. However, when a strip of Cu is placed into a solution of HNO3(aq), a gas evolves. What is the most likely identity of the gas?
A.NO
B.CO2
C.H2
D.O3
A: NO
How exactly should I come to this answer?

The answer has something to do with oxidation/reduction potentials, can't remember for sure tho.
EDIT: Not 100% sure about this explanation but here goes. You can eliminate O3 and CO2 right off the bat. The reaction probably has to do with Cu oxidizing somehow, so if it oxidizes and does not react with H2O, then you would guess that H2 would not be formed since H2 would be oxidized along with Cu being oxidized (eg. nothing being reduced). So by process of elimination, A. I know this is a ****ty answer but the best I can do so far.

7.From reactions 3-5, what can be determined about the relative concentrations of CFCs and O2?
A: A catalytic amount of CFC can produce much O2
Rxn3: CF3CLàCF3 + Cl
Rex 4: Cl + O3 à CLO + O2
Rexn 5: CLO + Oà CL + O2
How can we say "a catalytic amount of CFC" if CF3Cl is not being recycled??

Cl* is being recycled, which is the free radical and can react further. (See chain propagation in free radicals)

 

[cwfergus]

1.. .An object that is totally immersed in benzene (specific gravity = 0.7) is subject to a buoyancy force of 5N. When the same object is totally immersed in an unknown liquid, the buoyancy force is 12 N. What is the approximate specific gravity of the unknown liquid?
A: 1.7 N
Ok so I get that since the object is immersed, then the buoyant force equals density of liquid times volume of object time gravity, but after that I don’t know what to do. Why do they do 12/5 * 0.7?

2.. .In a nearsighted individual, the image of a distant object is focused:
A: in front of the retina, requiring divergent lens correction.
don’t even know where to start with this question

3.. .Which of the following expressions gives the amound of energy lost to friction by the toboggan and rider between points A and B?
A.. .60N (l)
B.. .60kg (g) (l)
What’s the difference between these 2 answers? The correct answer was A
4.. .When would a hybridization of a molecule in a compound be sp3d2 or s2p2 etc..?

5.. .According to Equation1, the concentration of the polymer with respect to [HPO4 2-] is:

A: (1/n) [HPO4 2-]
Equation 1: nCDP à (CP)n + n HPO4 2-
I’m not seeing the math here, thought the answer would be n [HPO4 2-]

6.. .When a strip of Cu is placed in H2O (l), no change is observed. However, when a strip of Cu is placed into a solution of HNO3(aq), a gas evolves. What is the most likely identity of the gas?
A.. .NO
B.. .CO2
C.. .H2
D.. .O3
A: NO
How exactly should I come to this answer?

7.. .From reactions 3-5, what can be determined about the relative concentrations of CFCs and O2?
A: A catalytic amount of CFC can produce much O2
Rxn3: CF3CLàCF3 + Cl
Rex 4: Cl + O3 à CLO + O2
Rexn 5: CLO + Oà CL + O2
How can we say “a catalytic amount of CFC” if CF3Cl is not being recycled??

1. Im sure there are other ways to do it, but this is how i did it:

Fb(benzene)=p(rho fluid)V(sub)g
5=0.7(Vsub)(10)
Vsub=5/7

Vsub will be the same in each fluid, therefore.

Fb(new fluid)=p(rho new fluid)Vsub(g)
12=p(rho new fluid)(5/7)(10)
p(rho fluid) = 1.7

2. For Myopia (nearsightedness), the image is focussed in front of the retina --> memorization. Therefore, you need a lens that diverges light so that it will focus further back (subsequently, on the retina). Umm.. google a picture of myopia, and you'll see what I mean.

3. Energy lost by friction is the work done by friction, therefore it is the force multiplied by the distance travelled. In this case it is 60N (force) multiplied by the length of the path, "l" The other answer is just bizarre (Kg . g)?

4. sp3d2 would be if there were 5 'things' around it. AKA 5 bonds to other atoms or a lone pair and 4 bonds or 2 lone pairs and 3 bonds (trigonal bipyramidal, seesaw or tshaped)

 

[phltz]

For 6, you just need to remember a couple of details. I don't think there's a great way to puzzle it out if you don't know the relevant facts.

When things are oxidized in acidic solutions, H+ is usually reduced to H2. We normally see H2 gas bubble out of solution when we're oxidizing something. However, nitric acid is a special case. Nitric acid is an especially strong oxidizing agent. If you really want to oxidize something tough in an inorganic chemistry lab, it will almost certainly be with nitric acid. Thinking about it logically, if most acids evolve H2 when they're oxidizing a metal, and nitric acid has a different strength than all of the others, it sees likely that it'll evolve a different gas. Probably one with N in it.

It's extremely likely that you oxidized things with acid at least once in your lab classes. Hydrogen evolves as a clear gas, and is typical when you're using HCl or H2SO4 or whatever. NO comes from nitric acid, and has a distinctive brownish color.

 

[2010premed]

Thank you very much, this really helps

 

[sangria1986]

For 6, you just need to remember a couple of details. I don't think there's a great way to puzzle it out if you don't know the relevant facts.

When things are oxidized in acidic solutions, H+ is usually reduced to H2. We normally see H2 gas bubble out of solution when we're oxidizing something. However, nitric acid is a special case. Nitric acid is an especially strong oxidizing agent. If you really want to oxidize something tough in an inorganic chemistry lab, it will almost certainly be with nitric acid. Thinking about it logically, if most acids evolve H2 when they're oxidizing a metal, and nitric acid has a different strength than all of the others, it sees likely that it'll evolve a different gas. Probably one with N in it.

It's extremely likely that you oxidized things with acid at least once in your lab classes. Hydrogen evolves as a clear gas, and is typical when you're using HCl or H2SO4 or whatever. NO comes from nitric acid, and has a distinctive brownish color.

I still dont follow....I understand that NO3 is a really strong oxidizing agent...but I dont follow how we can use this broadly though. Is this just a special case?

The way I would have done it was to look at Cu and since its a metal its zero charge. since a gas evolves, that means some reaction is taking place and i'd assume its oxid/reduction since there are ions involved.

Then i'd think...ok so Cu cant be reduced (metals dont usualy get negative charge) so its becoming positive. That means something else is becoming more negative. since I know H in HNO3 is positive I'd assume that its being reduced to H2 gas since thats zero charge.

theres just no way I'd come to the conclusion you did without knowing this special case. thing is i never came across this special case in the text book so is there another way about it?

 

[Rabolisk]

It's just a matter of chemical intuition. You know that the only possible choices are H2 and NO. You have to then decide whether H+ or NO3- is the stronger oxidizing agent. NO3- is the stronger oxidizing agent. If you look at the standard reduction potential, CU would not be oxidized in a simple acidic solution, like HCl.