Dilution/Electron Config Problem

[doc1986]

This is a dilution problem on a practice test that I am having trouble with:

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

A. 25 mL
B. 50 mL
C. 75 mL
D. 125 mL
E. 150 mL

I used MiVi = MfVf
So: (18 M)(25 mL) = (3 M)(x)
x = 150 mL
*but the answer key says 125 mL is the right answer. Is this a mistake or am I missing something? Maybe it has something to do with the molar equivalents (2H's that dissociate from sulfuric acid)...I don't know.

Also:

Which of the following is the electron configuration of Mn2+?
A. [Ar]4s2 3d5
B. [Ar]3d5
C. [Ar]4s2 3d5
D. [Ar]3d3
E. [Ar]4s2 3d7

I got C. but the key says B.

Help please!
Thanks

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[nze82]

This is a dilution problem on a practice test that I am having trouble with:

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

A. 25 mL
B. 50 mL
C. 75 mL
D. 125 mL
E. 150 mL

I used MiVi = MfVf
So: (18 M)(25 mL) = (3 M)(x)
x = 150 mL
*but the answer key says 125 mL is the right answer. Is this a mistake or am I missing something? Maybe it has something to do with the molar equivalents (2H's that dissociate from sulfuric acid)...I don't know.

Also:

Which of the following is the electron configuration of Mn2+?
A. [Ar]4s2 3d5
B. [Ar]3d5
C. [Ar]4s2 3d5
D. [Ar]3d3
E. [Ar]4s2 3d7

I got C. but the key says B.

Help please!
Thanks

Hi Doc1986:
You almost got this question right. Here's where you're making a mistake:
MiVi = MfVf
Vf = (18)(25)/3 = 150ml
But remember that:
Vf = V(H2SO4) + V(H2O)
So:
V(H2O) = Vf - V(H2SO4) = 150 - 25 = 125ml

Hope this helped!

 

[bebebebebebe]

And for the second one, it says Mn2+ not Mn

Mn2= [Ar] 3d^5 4s^2

Mn2+ = [Ar] 3d^5

 

[nze82]

Regarding your electron configuration problem...
The electron configuration of Mn is as follows:

Mn: [Ar], 4s2, 3d5

Notice that the 4s2 energy level contains 2 paired electrons, while the 3d5 energy level contains 5 unpaired electrons. Therefore, 3d5 is less stable, and is therefore, the energy level that loses electrons to make the Mn2+ cation. So:

Mn2+: [Ar], 4s2, 3d3

But, it doesn't end here. At this point we have a 3d energy level that has 3 unpaired electrons and 2 empty orbitals. Under such circumstances, the two electrons from the 4s2 energy level jump to the 3d energy level to fill the empty orbitals. But why?

Notice that, when this jump occurs, we will have:

Mn2+: [Ar], 3d5

A 3d energy level with 5 unpaired electrons is always more stable than a 3d energy level with 3 unpaired electrons and 2 empty orbirals. This is why you have the transfer of electrons from the 4s energy level.

 

[bebebebebebe]

nze82:

I don't agree completely; I understand what you mean but what you saying applies when you filling out the orbitals. For instance, Cr has 3d5 4s1 instead of 3d6 4s2, Because 5 es in d orbital is more stable than 6 in d.
However, once the electrons are actually in their orbitals, the energy order changes - and in all the chemistry of the transition elements, the 4s orbital behaves as the outermost, highest energy orbital.
The reversed order of the 3d and 4s orbitals only applies to building the atom up in the first place. In all other respects, you treat the 4s electrons as being the outer electrons. So for ions, you should take electrons from the outermost which is 4s here.

 

[nze82]

nze82:

I don't agree completely; I understand what you mean but what you saying applies when you filling out the orbitals. For instance, Cr has 3d5 4s1 instead of 3d6 4s2, Because 5 es in d orbital is more stable than 6 in d.
However, once the electrons are actually in their orbitals, the energy order changes - and in all the chemistry of the transition elements, the 4s orbital behaves as the outermost, highest energy orbital.
The reversed order of the 3d and 4s orbitals only applies to building the atom up in the first place. In all other respects, you treat the 4s electrons as being the outer electrons. So for ions, you should take electrons from the outermost which is 4s here.

As far as I'm concerned, orbitals containing paired electrons are always more stable than half-filled orbitals. The reason is that the opposite spin of two paired electrons creates more attraction between them, which will in turn make it harder to remove electrons from such orbitals. At least this is how I've always solved electron configuration questions and it has worked for me so far. I hope doc1986 can pick the explanation that makes most sense to him/her.
Thanks for your post though

 

[nze82]

Here's why I think 3d is less stable than 4s. Notice that in the following figure 3d has a higher energy state (less stable) than 4s, which has a lower energy state (more stable):

 

[Sublimation]

This is a dilution problem on a practice test that I am having trouble with:

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

A. 25 mL
B. 50 mL
C. 75 mL
D. 125 mL
E. 150 mL

I used MiVi = MfVf
So: (18 M)(25 mL) = (3 M)(x)
x = 150 mL
*but the answer key says 125 mL is the right answer. Is this a mistake or am I missing something? Maybe it has something to do with the molar equivalents (2H's that dissociate from sulfuric acid)...I don't know.

Also:

Which of the following is the electron configuration of Mn2+?
A. [Ar]4s2 3d5
B. [Ar]3d5
C. [Ar]4s2 3d5
D. [Ar]3d3
E. [Ar]4s2 3d7

I got C. but the key says B.

Help please!
Thanks

Let me give u some advice friend. Ive been teaching an SAT and ACT class for kaplan for the past 3 years. Let me just say this, dont ever pick the answer that looks right. Always go with ur gut. Dont second guess urself and dont fall for their trap. they put some attractive answers so that if u narrow the answer down ull pick the wrong one just cuz it looks right. All standardized tests are the same.

For the second Q. seriously i dont see wats so complicated that we are talking about orbital energy levels. They are never gonna ask u anything like that. Look Ar has 18 e-. Mn has 25 in nature. When its an ion its 23. Wats the diff? 5. there is only one answer with 5 electrons lol. im sitting here reading this **** and ppl are overcomplicating it, dont get me wrong im not saying dont learn it. im just saying Q's like this shouldnt take more then 15 sec. and its not made to trick u. lol Thanks for the explanation guys i always learn something new when i get on here lol Gluck Guys.

 

[nze82]

Let me give u some advice friend. Ive been teaching an SAT and ACT class for kaplan for the past 3 years. Let me just say this, dont ever pick the answer that looks right. Always go with ur gut. Dont second guess urself and dont fall for their trap. they put some attractive answers so that if u narrow the answer down ull pick the wrong one just cuz it looks right. All standardized tests are the same.

For the second Q. seriously i dont see wats so complicated that we are talking about orbital energy levels. They are never gonna ask u anything like that. Look Ar has 18 e-. Mn has 25 in nature. When its an ion its 23. Wats the diff? 5. there is only one answer with 5 electrons lol. im sitting here reading this **** and ppl are overcomplicating it, dont get me wrong im not saying dont learn it. im just saying Q's like this shouldnt take more then 15 sec. and its not made to trick u. lol Thanks for the explanation guys i always learn something new when i get on here lol Gluck Guys.

No wonder KAPLAN courses aren't worth anyone's time! Now I know why! They advise their students to go with their gut and not their logic!!
True! time is an important factor on the real DAT. But, you missed one important fact! This is not the real DAT. We're here to help someone understand the concept, so that he/she would be able to use the same concepts and principles to answer similar type of questions. We're discussing different ideas so that we can learn something in depth. If after 3 yrs of teaching at KAPLAN you have not learned to respect people's views and ideas you neither belong there as a teacher nor in dental school as a student.

 

[bebebebebebe]

100% agree with u nze82 on this one!lol Finally
(To nze82 )

 

[Sublimation]

No wonder KAPLAN courses aren't worth anyone's time! Now I know why! They advice their students to go with their gut and not their logic!!
True! time is an important factor on the real DAT. But, you missed one important fact! This is not the real DAT. We're here to help someone understand the concept, so that he/she would be able to use the same concepts and principles to answer similar type of questions. We're discussing different ideas so that we can learn something in depth. If after 3 yrs of teaching at KAPLAN you have not learned to respect people's views and ideas you neither belong there as a teacher nor in dental school as a student.

Let me give u some advice friend. Ive been teaching an SAT and ACT class for kaplan for the past 3 years. Let me just say this, dont ever pick the answer that looks right. Always go with ur gut. Dont second guess urself and dont fall for their trap. they put some attractive answers so that if u narrow the answer down ull pick the wrong one just cuz it looks right. All standardized tests are the same.

For the second Q. seriously i dont see wats so complicated that we are talking about orbital energy levels. They are never gonna ask u anything like that. Look Ar has 18 e-. Mn has 25 in nature. When its an ion its 23. Wats the diff? 5. there is only one answer with 5 electrons lol. im sitting here reading this **** and ppl are overcomplicating it, dont get me wrong im not saying dont learn it. im just saying Q's like this shouldnt take more then 15 sec. and its not made to trick u. lol Thanks for the explanation guys i always learn something new when i get on here lol Gluck Guys.

My post was in no way meant to belittle any1s efforts. I highly appreciate it when ppl take their time to explain things in detail on SDN. I guess im just so used to teaching ppl to take the SAT(aptitude test btw) which doesnt really test wat u already know, but a test of reasoning skills. Ur smart enough to understand taht when i said go with ur gut i meant it to apply to questions that you have hit a deadend with. Im not advising you to go with ur gut on everyquestion. Obviously u use logic to get to the right answer, BUT there are those concepts that you will forget, and when u forget them, and u dont have the time to think them threw logically (cuz if u can think it thru logically u would have gotten the answer by then) you go with ur gut. Time is precious on the DAT. Last time i checked we are all entitled to our opinions on SDN im sorry if u disagree with my methods but its up to the OP to decide wat course of action he wishes to take i just wish him/her and urself the best. I hope u dont take this the wrong way but after i voiced my feelings towards the explanations i plauded the efforts and the goodheart, i dont see how that is disrespectful to ur views and ideas.

 

[nze82]

It's all good dude! No hard feelings. We're all trying to help each other here!

My post was in no way meant to belittle any1s efforts. I highly appreciate it when ppl take their time to explain things in detail on SDN. I guess im just so used to teaching ppl to take the SAT(aptitude test btw) which doesnt really test wat u already know, but a test of reasoning skills. Ur smart enough to understand taht when i said go with ur gut i meant it to apply to questions that you have hit a deadend with. Im not advising you to go with ur gut on everyquestion. Obviously u use logic to get to the right answer, BUT there are those concepts that you will forget, and when u forget them, and u dont have the time to think them threw logically (cuz if u can think it thru logically u would have gotten the answer by then) you go with ur gut. Time is precious on the DAT. Last time i checked we are all entitled to our opinions on SDN im sorry if u disagree with my methods but its up to the OP to decide wat course of action he wishes to take i just wish him/her and urself the best. I hope u dont take this the wrong way but after i voiced my feelings towards the explanations i plauded the efforts and the goodheart, i dont see how that is disrespectful to ur views and ideas.

 

[doc1986]

Thanks for the help guys!