quick kaplan question...

[prydA]

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Hi, this Kaplan question has been annoying me.. anyone can explain it to me?

Concentrated H2SO4 is 18M. What volume of water would be needed to dilute 25 mL of concentrated H2SO4 solution to 3M?

I use MVi=MVi --> (18)(25)(2)=(3)(V) ... and get V= 300mL. The answer says 125mL though whyyyy??

Thanks!

 

[Orgodox]

You need to use Normality here N1 V1 = N2V2
You used normality on one side and molarity on other. Since they are both double from normality to molarity you can just ignor it

(18*25)=3V
v=150 but already hve 25 ml so only need to add 125

 

[prydA]

ah okk thanks... how about this next one..

Which of the following carbocation ions will be most stable?
A. Ph3C+
B. CH3CH2+
C. (CH3)2CH+
D. CH2=CHCH2+
E. +CH2Cl

I thought Ph3C+ and CH2=CHCH2+ had the same stability since both of them have conjugated double bonds to support the charge... so I was stumped. The answer says A.
Thanks againn

 

[tsav23]

Benzyl carbocations are more stable than allylic carbocations due to full conjugation of the benzene ring.

 

[Orgodox]

Yup benzylic carbonation can spread the charge over 4 C atoms, while in the allylic it is only on two

 

[sweetpea2]

I have a question pertaining to this topic. On achievere there is a question where it says CH2=CHCH2+ is more stable than (CH3)3C+. I thought it was the other way around but is the reasoning simply due to the conjugation of the double bond. Just wanna make sure, thanks!

 

[tsav23]

Achiever is correct because (CH3)3C+ is a tertiary alkyl carbocation, and allylic carbocations are more stable

Order of stability
Benzylic carbocation>allylic carbocations>alkyl carbocations

 

[Orgodox]

Yea I think the resonance would give the edge to the secondary allylic carbocation over the tertiary without resonance.

 

[prydA]

Thanks guys! How about this QR one too..

What is the value of x if SQRT(2(5/(x+3))) = 10?

and...

A committee of 2 people must be selected from a group of 16 people. In how many different ways may the committee be formed?

I would think this was just a simple permutation (16x15)... but it is apparently a combination (16x15/2). whyyy?

 

[tsav23]

From what use gave me I got x= -29/10, but that seems like a weird answer.
For the second problem you have to do 30 *29 / 2 *1 because you are selecting for 2 individuals vs. 1.

 

[250rsavage]

for the first QR question: I got the answer to be -2.9
first off square both side to remove the SQRT so you get 2(5/(x+3)=100
now divide by 2 so you get 5/(x+3)=50
divide 5 by 50 to get 1/10 = x+3
now (1/10)- (30/10)= X which equals -29/10 or -2.9

 

[prydA]

i got -2.9 too.. but kaplan says its 5/47 O___O. ima just trust you guys on this one lol

 

[prydA]

ok heres another prob.. but from destroyer because i don't want to start a brand new topic =)

What is the major product of the reaction below?

benzene-CH2-CHO + NH2CONHNH2 --> benzene-CH2-CH=NNHCONH2

my question is... why can't it be the other way around and be benzene-CH2CH=NCONHNH2? what makes it so that the NH2 next to the NH2 more reactive than the NH2 near the C=O? this is question #4 btw. thanks again!

 

[tsav23]

It is due to electron density. Since the Nitrogen is acting as the nucleophile when it attacks the carbonyl of (benzene-CH2-CHO) to from an imine, it needs to be the strongest nucleophile possible. The Nitrogen next to the carbonyl in (NH2CONHNH2) is more electron deficient due to the inductive effect of the oxygen on the carbonyl. This is why the Nitrogen furthest away from the carbonyl attacks the benzene compound. Hopefully that makes sense.

 

[MAK1186]

For your QR question that was "What is the value of x if SQRT(2(5/(x+3))) = 10?" If you were copying that from the test on Kaplan's site, you made a tiny error. The question actually states "What is the value of x if SQRT [2(5/x + 3)] = 10?" In this case, the answer would be 5/47. Let me know if you want me to explain how you get the answer.

 

[pistolpete007]

for the very first question how come u dont subtract 25ml from the final answer?.....when do you subtract what they give u with the volume u calculate?

 

[DentalDeac]

You need to use Normality here N1 V1 = N2V2
You used normality on one side and molarity on other. Since they are both double from normality to molarity you can just ignor it

(18*25)=3V
v=150 but already hve 25 ml so only need to add 125

You actually shouldn't use normality here, because all you are doing is a simple dilution and not a reaction involving the neutralization of an acid or a base. Regardless, you managed to get the right answer by doing M1V1=M2V2

You solve and get V2=150 mL which is the total volume, so subtract 25 mL from that and you end up with 125mL of water.

 

[Orgodox]

You actually shouldn't use normality here,

It doesnt matter because normality and molarity are = here both diprotic.

 

[Orgodox]

for the very first question how come u dont subtract 25ml from the final answer?.....when do you subtract what they give u with the volume u calculate?

I did check out the post from 6-8: I said "v=150 but already hve 25 ml so only need to add 125"

 

[pistolpete007]

well in that case how would the question have to be written so that u dont subtract the final from 25 i alwasy get confused when your suppose to and when u dont