April MCAT review thread

[Shredder]

I'm starting this thread to avoid forgetting information in these last few weeks. If you will, please post a fact that you think has a good chance of showing up on the April test, and perhaps something that is easily forgettable and needs refreshing. If no one posts, oh well, this is better than studying for my test tomorrow. If they do, it could be a decent review, better than crap Princeton anyway. I'll start:

For a capacitor, C=EA/d, where E(epsilon) is the dielectric coefficient (i think air/vacuum? is 1 and everything else is greater), A is area of plates, and d is distance separating plates. A key point is that smaller distance->greater C.

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[cfdavid]

Did you (or anyone else) finish every single question on those 1001 books? Thanks. I ask because the image of fish question appears to be at the very end of the 1001 physics book...

Actually, I was just referencing the picture in the EK physics book, not from the 1001 problem set.

I did not do every 1001 physics or chem question.

 

[LabMonster]

Energy of a photon = hf = hc/lambda(wavelength) <--- Seems like there is always one of these buggers.

Radioactivity: Alphas and Gammas aren't too goofy, but Betas... Beta particles: B- emmitted when a neutron decays into a proton and an electron, the proton number (Z) increases by one and the electron escapes at a high speed, while the nucleon number does not change (A).

a/z(Parent nucleus)--> a/z+1(Daughter Nucleus) + 0/-1e (Beta particle/ electron)

B+ decay: When a nucleus "captures" an inner orbital electron, a proton will convert into a neutron so the atomic number will Decrease by one and the nucleon number "A" will remain the same. The particle emitted is a positron, or a particle having the same mass as an electron, but equal and opposite charge e+.

a/z(Parent nucleus)--> a/z-1(Daughter nucleus) + 0/1e (Positron/B+ particle)

 

[BaylorGuy]

So basically, with B- decay the parent nucleus is going to go up one space in the periodic table and with B+ decay the parent nucleus is going to go down one space??
(I.E. 59/27Co---> 59/28Ni + 0/-1e for B-decay
59/27Co---> 59/26Fe + 0/+1e for B+decay)

I always get these confused for some reason

 

[cfdavid]

So basically, with B- decay the parent nucleus is going to go up one space in the periodic table and with B+ decay the parent nucleus is going to go down one space??
(I.E. 59/27Co---> 59/28Ni + 0/-1e for B-decay
59/27Co---> 59/26Fe + 0/+1e for B+decay)

I always get these confused for some reason

That sounds correct to me. The number of protons determines the element, not the number of neutrons + protons, since the same element can have different numbers of neutrons (i.e. isotopes).

 

[LabMonster]

Anyone have any clever methods to quickly hash out Keq's pKa's etc? Any ideas would be interesting...

 

[sahil22]

for EMF of a cell:

EMF = E(reduction at cathode) + E(oxidation at anode)
ie EMF = E(reduction at cathode) - E(reduction at anode)

Basically EMF is always the net reduction potential.

You should learn the series for decreasing E (reduction) potentials. It basically says :
K Na Ba Ca Mg Al Zn Fe Sn Pb H Cu Au Ag Pt
This series says that an ion of the metal coming before in the series will be formed spontaneously from the respective metal.
ie K = K+ion + electron (spontaneous) (this reaction has a specific E reduction)
In all electrochemical reactions, electrons should be balanced. Therefore in a cell, this electron is gained by the 'ion' of another metal lower in the series which forms the corresponding metal.
.5 Cu2+ + electron = .5 moles of Cu (this reaction has a specific E reduction which is equal to negative of the E oxidation of the reaction).

Thus EMF = E (K/K+) - 0.5 E(Cu/Cu 2+)

If this yields negative value reaction is not spontaneous. If it yields a +ve value, it is.

If you work out the numbers for a galvanic cell, they come out to be +ve.

Also you should know that E(any metal / metal ion) is measured by making a cell with it being the cathode and having a Standard Hydrogen Electrode (which has a E reduction = 0).

Also delta G = -nFE , so if E is +ve, delta_G is negative (spontaneous).

"Redox: Galvanic cell--OxidAtion occurs at Anode. anode is also negative pole. ReduCtion occurs at cathode. Ca+hode is positive pole. This reaction occurs spontaneously, thus emf is negative.
Electrolytic...same thing occurs, only anode is positive pole, cathode is negative pole. Rxn is non-spontaneous, emf is positive."

Just to clear up some possible confusion that might have come from a previous post...

A reaction in a Galvanic (Voltaic) Cell is spontaneous so the delta G is negative, However from the equation delta G = -nFEcell if the reaction is spontaneous Ecell (EMF of cell) must be positive. The EMF by the same relation for an electrolytic cell would be reversed on the same premesis (it would be negative). Not exactly sure if I am correct as I have not verified 100% that I am right but I'm pretty sure it's correct as I stated.

 

[sahil22]

Guys, remember that Alpha or Beta emission is always supplemented by Gamma decay. Gamma decay in a nuclear reaction is like the energy release in a chemical reaction. it is always there in some amount when alpha or beta are emitted.

Energy of a photon = hf = hc/lambda(wavelength) <--- Seems like there is always one of these buggers.

Radioactivity: Alphas and Gammas aren't too goofy, but Betas... Beta particles: B- emmitted when a neutron decays into a proton and an electron, the proton number (Z) increases by one and the electron escapes at a high speed, while the nucleon number does not change (A).

a/z(Parent nucleus)--> a/z+1(Daughter Nucleus) + 0/-1e (Beta particle/ electron)

B+ decay: When a nucleus "captures" an inner orbital electron, a proton will convert into a neutron so the atomic number will Decrease by one and the nucleon number "A" will remain the same. The particle emitted is a positron, or a particle having the same mass as an electron, but equal and opposite charge e+.

a/z(Parent nucleus)--> a/z-1(Daughter nucleus) + 0/1e (Positron/B+ particle)

 

[sahil22]

Guys,.. i need help
I gave my mcats in my freshman year (actually in the august before starting freshman year).. it was more for fun.. i got a 31Q with a 15/15 in physical sciences.
But this year I am giving it again. I am in sophomore year (though I am graduating an year early).
Any hints where I should do practice tests or study. I havent really dont any tests till now. I think they just tend to make you overconfident. Only thing I am reading is Kaplan.. again and again.

 

[sahil22]

Anyone have any clever methods to quickly hash out Keq's pKa's etc? Any ideas would be interesting...

suppose you have a reaction:
aA + bB <-> cC + dD
ie a moles of A , b moles of B give c modes of C and d moles of D

Then the reaction if let to stay will reach a point where the concentrations of A, B , C, D wont change.

AT THIS POINT, the ratio of the concentrations of products raised to the power their stiochiometric coefficients to the reactants is the Keq for the 'forward' reaction.
Keq = [C]^c [D]^d / [A]^a ^b
Remember that equilibrium is reached how so ever you start the reaction, any amount of A or B or C or D.
Also Keq only changes with temperature. If you change pressure or concentration or volume, the equilibium concentrations change in such a way that Keq remains constant.

Note that concentrations of pure solid and liquids is 1 (its actually not but we assume solution is dilute and there will be no significant change. it also simplifies things).

So if you have a acid or base, they will dissociate to give H or OH till some extent. If its a strong acid (like H2SO4 HNO3 HCL ) Keq (or Ka) would be close to infinity as concentrations of products would be >>>> conc of reactants. Thus for strong acids and bases Keq (which is called Ka) is not that important.
For weak bases, Ka is impt. because there is tells you how much the acid or base will dissociate. ie at equilibrium point how much H+ it will give..
HA <-> H+ and A- (some books call H+ H3O+ due to solvation..)
Ka = [H+][A-]/[HA]
same for weak bases Kb = [OH][A-]/[AOH]

to note:
we are considering only monoprotic acids and monohydroxy bases. If there are 2 H+ or 3OH- to be given, they will not be given together but in steps. Each step will have a different K

pK's:::
remember when you first learnt algebra, addition was so much easier than multiplication same way all equations of equilibrium are better written in log form so that instead of multiplying numbers and taking their powers, we just add them. The convention followed for this is p of something = -log of that something. You might ask why -, thats coz mostly numbers we have are so small that they are of the form aE-x, so taking -ve log would make it xloga.
If you are confused with pK's, work backwards to concentrations and then use your stiochiometric concepts and then get back to pH or pOH values.

 

[cfdavid]

Remember that the photoelectric effect occurs when electrons FALL from higher to lower energy levels.

But, if the energy of the photon is sufficient and exact enough to push it directly to the next energy level, then there is no energy to 'emit' since all was absorbed and required to push it to the next shell, where it stays.

 

[sahil22]

Remember that the photoelectric effect occurs when electrons FALL from higher to lower energy levels.

But, if the energy of the photon is sufficient and exact enough to push it directly to the next energy level, then there is no energy to 'emit' since all was absorbed and required to push it to the next shell, where it stays.

I dont think the last sentence is right. Basically an electron is like a ball. When a ball drops from a height, PE is released. Same way photons (photon is basically an electromagnetic flux entity used as unit of energy for EM waves) are released when electrons fall from high to low energy level.

Is it right that if a photon has energy sufficient (doesnt need to be exact, i has to be greater than some quantized energy value) to push an electron to next level, it will happen. But this electron will fall again when the photon is not there and light will be emitted. This phenomenon is called phosphorescence. Its analogous to holding a ball up from the ground. If you have enough energy to lift it, you can lift it but it will stay there as long as u hold it up. Not a second after that.

ps: photons have an intensity and a frequency. also one of the hidden fundas behing photoelectric effect is that one electron is hit by one photon only at a time. so you cannot say that because two photons hit an electron, it will go higher in the hierarchy of energy states. the intensity of the photon will define the work function and kinetic energy of the ejected electon.

 

[UCdannyLA]

I recall coming across questions of this nature:

"There is a ship filled with cannonballs floating on a lake...if the cannonballs are dropped into the lake, will the water level of the lake rise, fall, or remain the same? and, will the ship sink more into the lake, rise out of the lake, or stay the same?"


or...

"a brick is floating on a piece of styrofoam in water...if the brick is dropped into the water...will the level of water rise, fall, or remain the same?"

I remember that the answers are that the water levels will fall.

can anyone explain whY?

 

[Shredder]

Humm out of most of the things mentioned here, I'm a little concerned about that floating brick one. Will someone assess its likelihood of showing up, and provided that this likelihood is significantly high then please go on to explain, otherwise why worry, this is the nature of studying efficiently, you have to know what's useful knowledge and what's not since you can know some of it perfectly, all of it well, but not all of it perfectly well.

 

[LabMonster]

Guys, remember that Alpha or Beta emission is always supplemented by Gamma decay. Gamma decay in a nuclear reaction is like the energy release in a chemical reaction. it is always there in some amount when alpha or beta are emitted.

Yep - but most of the problems I've seen tell you to ignore it... Usually on the basis that if you're going to use conservation of energy to find a momentum - working the problem WITH gamma energy adds more calculations...

 

[LabMonster]

"a brick is floating on a piece of styrofoam in water...if the brick is dropped into the water...will the level of water rise, fall, or remain the same?"

I remember that the answers are that the water levels will fall.

can anyone explain whY?

Ok, I just did this in the lab here (slow night)...
Here are the, er, results

----->floating object, brick on top

----->floating object, "brick" submerged

----->normal level

I think the tricky part is that when you initially think about this problem, your mind assumes the level will go up, but that is only relative to the "normal" level.

The styrofoam and brick must be displacing enough water to float. The bricks added weight requires additional bouyant force, so more water must be displaced than if just the styrofoam were floating. This total displacement is greater than the displacement observed when the brick is simply submerged (and the foam is floating on top.)

 

[Shredder]

Okay I found this, makes sense

When the boulder is in the boat it displaces its weight in water. When the boulder is thrown into the lake it only displaces an amount of water equal to its volume. As the boulder is denser than water, (it sinks), the volume of water equal to its weight is larger than the volume of the boulder.

 

[jon0013]

shredder
so how does that account for the water level becoming lower?

 

[sahil22]

shredder
so how does that account for the water level becoming lower?

It states that 'Loss in weight of an object in a liquid is equal to the weight of the liquid displaced when the object is immersed in the liquid'.
This statement explains it all.

Assumption::: Density of Canon >>>> Density of ship

A boat is like a broad cup. You carry cannons in it, you increase the weight of the ship without increasing the volume (density increases), so to float, the ship sinks a little as this will increase the water displaces and increase the loss in weight which is necessary for loweing the effective density (effective weight / original volume) and floating (floating condition is that 'overall' density of ship assuming its a closed body is = that of water)

Since more water is now being displaced, water cannot flow out from the sea like it would in the case of a container filled to brim with water. So water level rises.

Now suppose the cannon was thrown out, it will sink. Cannon has a high density small volume it cannot float in water => so it will sink and displace water (which will rise) equal to its volume.
But throwing the cannon will cause the ship to loose weight while retaining its original volume which will need to be less submerged to cause the loss in weight.
So now if what I wrote makes sense, think of a cannon thrown from a ship as first the cannon removed from the ship (which will cause an decrease in height of water since weight of ship is less) and then being thrown into water which causes a small increase in water level. Therefore net water level falls down.
Basically (increase in height on droppign cannon)/decrease in height = density of ship+canon/density of canon.

AS A POINT: WHENEVER A DENSE OBJECT IS INSIDE AN OPEN OBJECT, it causes the overall density to go up without increase in volume. When this dense object is thrown out, water level will drop. !!

 

[jon0013]

thanks sahil for the extended explanation

so we should just know if a canon is tossed out of a boat the boat will rise a bit due to less weight in it while the water level of the lake falls?

 

[UCdannyLA]

Can someone explain how to use this formula for Dopper effect?

&#916;f/f = -v/c

 

[sahil22]

thanks sahil for the extended explanation

so we should just know if a canon is tossed out of a boat the boat will rise a bit due to less weight in it while the water level of the lake falls?

Yes. Its a good thing to remember right off hand. Ever thought if you jumped into a river from a boat, the water level would rise or fall? It definitely would fall coz the boat got so much lighter.

 

[BaylorGuy]

Can someone explain how to use this formula for Dopper effect?

&#916;f/f = -v/c

The formula i know is F'=F(Vd+/-V)/(Vs-/+V)

It basically breaks down to a few things you need to remember...first off, the equation...it isnt that hard. Just remember that D comes before S...therefore D is above S, hence Vd/Vs

now, Vd=velocity of detector (I.E. a person or something), Vs=velocity of source (I.E. car, police, ambulance). Now, plus in the numbers...if you don't want to remember the sign config, which I personally confuse and hate to, just remember that if something is moving towards the source, F' is greater than F...is moving away F' is less than F...and vice versa for the detector moving.

 

[tank you]

Can someone explain how to use this formula for Dopper effect?

&#916;f/f = -v/c

delta f is the change in frequency from source to observer.
f is the frequency of the source.
c is usually the speed of sound in that medium (usually given)
v is the only hard part, its the relative velocity. so if, for example, an ambulance was coming towards u at 30 m/s and u were running away from it at 5 m/s, the relative velocity would be 25 m/s.

in all doppler frequency problems, three of the variables are given, so u can solve for the unknown.

 

[UCdannyLA]

delta f is the change in frequency from source to observer.
f is the frequency of the source.
c is usually the speed of sound in that medium (usually given)
v is the only hard part, its the relative velocity. so if, for example, an ambulance was coming towards u at 30 m/s and u were running away from it at 5 m/s, the relative velocity would be 25 m/s.

in all doppler frequency problems, three of the variables are given, so u can solve for the unknown.

in regards to the relative velocity...when do you use a positive and when do you use a negative value?

If the ambulance is coming to you do you leave it as - 25/ c? or would it be 25/c? What about if it is going away from you?

haha sorry for the questions..

 

[Mediculous]

in regards to the relative velocity...when do you use a positive and when do you use a negative value?

If the ambulance is coming to you do you leave it as - 25/ c? or would it be 25/c? What about if it is going away from you?

haha sorry for the questions..

You don't have to memorize whether the relative velocity is negative or positive---just think of the results of the Doppler Effect: a wave approaching you has an increased frequency(positive change) while a wave going away from you has a lower frequency(negative change).

 

[jon0013]

just remember that frequency will increase if the object is gettin closer to you (the observer)....so if you are moving away then use a negative speed for Vd....if the object is coming towards u then use a negative Vs...just whatever makes Frequency bigger #...

 

[LabMonster]

Can someone explain how to use this formula for Dopper effect?

?f/f = -v/c

This equation is the doppler effect for electromagnetic waves... fo=fs(1-(v/c)

and v must be << than c

 

[sahil22]

This equation is the doppler effect for electromagnetic waves... fo=fs(1-(v/c)

and v must be << than c

for all those who thought this was for sound.. its not! This equation is for light or any EM transverse wave.

But its based on the same principal::

When the source is moving towards you, you encounter the next wave faster than the last wave. (Wavelength of the wave remains same).

therefore cfo = v'fa (ie speed of light times origianl freq = apparent frequency times relative speed)

 

[jon0013]

remember Molecular geometry vs Electron Geometry especially for you non kaplan students...Princeton review never made this distinction..

Molecular Geometry does not take into consideration electrons so XeF2 is considered linear because 3 lone pairs dont count

electron geometry XeF2 would be trigonal bipyramidal...

this question got me on a full length...

 

[KiTmAn]

remember Molecular geometry vs Electron Geometry especially for you non kaplan students...Princeton review never made this distinction..

Molecular Geometry does not take into consideration electrons so XeF2 is considered linear because 3 lone pairs dont count

electron geometry XeF2 would be trigonal bipyramidal...

this question got me on a full length...

oh ma lord ...that explains soooo much..!!! appreciate it man!!