Gchem Question

[Lonely Sol]

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1. How many ml of water must be added to 65 ml of a 5.5m soln of NaOH in order to prepare a 1.2M NaOH solution?

a. 230ml
b. 235ml
c. 229ml
d. 1L
e. None of the above


2. What is the best explanation for why Ca2+ has a larger atomic radius than Ar?

a. The ca2+ has less electrons
b. The ca2+ has more electron
c. The ca2+ has less protons
d. The ca2+ has more protons
e. The ca2+ is not a noble gas

 

[poc91nc]

1. How many ml of water must be added to 65 ml of a 5.5m soln of NaOH in order to prepare a 1.2M NaOH solution?

a. 230ml
b. 235ml
c. 229ml
d. 1L
e. None of the above


2. What is the best explanation for why Ca2+ has a larger atomic radius than Ar?

a. The ca2+ has less electrons
b. The ca2+ has more electron
c. The ca2+ has less protons
d. The ca2+ has more protons
e. The ca2+ is not a noble gas

[5.5(65/1000)]/[65/1000 + x] = 1.2 solve for x and convert to ml

 

[BodybldgDoc]

2. Answer is d. Because the ca has less electrons so its effective nuclear charge will be higher and therefore its protons will be more spread out leading to a bigger atomic radius

 

[poc91nc]

2. Answer is b. Because the ca has less electrons so its effective nuclear charge will be higher and therefore its protons will be more spread out leading to a bigger atomic radius

Ca2+ has the same electron config as Ar.

 

[Lonely Sol]

2. Answer is b. Because the ca has less electrons so its effective nuclear charge will be higher and therefore its protons will be more spread out leading to a bigger atomic radius

Yea, thats what I thought, if you lose elctrons, the atom get bigger.

Y3+>Y2+>Y>Y->Y2-...

but the answer says it is because it has less protons, but that doesnt make sense to me so i thought i would ask you guys for opinion!

 

[BodybldgDoc]

Yea, thats what I thought, if you lose elctrons, the atom get bigger.

Y3+>Y2+>Y>Y->Y2-...

but the answer says it is because it has less protons, but that doesnt make sense to me so i thought i would ask you guys for opinion!

less protons?😕 If it has less protons then it cant have a larger radius. are you sure its not a mistake in the answer key?

 

[scottyhoop]

1. How many ml of water must be added to 65 ml of a 5.5m soln of NaOH in order to prepare a 1.2M NaOH solution?

a. 230ml
b. 235ml
c. 229ml
d. 1L
e. None of the above


You don't have to worry about changing mL to L if you keep the units the same on both sides so just use (65)(5.5)=(1.2)(V2)-----V2=298mL

That is if you meant Molarity (M) for the 65 mL solution and not molality (m). Hope that helps!
Scott

 

[Lonely Sol]

less protons?😕 If it has less protons then it cant have a larger radius. are you sure its not a mistake in the answer key?

Well no answer key is error free, maybe it is wrong!

 

[BodybldgDoc]

Ca2+ has the same electron config as Ar.

there has to be something wrong with the answer key but your right they would have the same # of electrons, however it still would not make Ar have more protons than Ca.

 

[Lonely Sol]

1. How many ml of water must be added to 65 ml of a 5.5m soln of NaOH in order to prepare a 1.2M NaOH solution?

a. 230ml
b. 235ml
c. 229ml
d. 1L
e. None of the above


You don't have to worry about changing mL to L if you keep the units the same on both sides so just use (65)(5.5)=(1.2)(V2)-----V2=298mL

That is if you meant Molarity (M) for the 65 mL solution and not molality (m). Hope that helps!

Scott

The answer is B, and it is molality. If it was in M, it would be simple but I am getting confused with the units!

 

[poc91nc]

My equation gives 233ml.

5.5mol/kg x 1kg/L = 5.5M molality and molarity are roughly equal at room temp. In this particular case you aren't even given a percent mass. Not sure why it would be easier if it was M. If a percent mass was given....then it might be a different story.

Using the M1V1 = M2V2 method is fine if you can remember to subtract 65ml from V2.

 

[poc91nc]

You can only know the "true" molality if given a percent mass...then you could actually find the difference in molality and molarity...but the difference would still be negligable.

 

[scottyhoop]

My equation gives 233ml.

5.5mol/kg x 1kg/L = 5.5M molality and molarity are roughly equal at room temp. In this particular case you aren't even given a percent mass. Not sure why it would be easier if it was M. If a percent mass was given....then it might be a different story.

Using the M1V1 = M2V2 method is fine if you can remember to subtract 65ml from V2.

Good points...I would go with the MV=MV and subtract original volume. Seems the easiest route, but I'm lazy!
Scott

 

[doc toothache]

there has to be something wrong with the answer key but your right they would have the same # of electrons, however it still would not make Ar have more protons than Ca.

The answer is d.There is nothing wrong with the answer key.

See: http://forums.studentdoctor.net/showpost.php?p=4736228&postcount=13

 

[Lonely Sol]

The answer is d.There is nothing wrong with the answer key.

See: http://forums.studentdoctor.net/showpost.php?p=4736228&postcount=13

Thanks, it makes sense now!!

 

[DentalKitty]

The answer is d.There is nothing wrong with the answer key.

See: http://forums.studentdoctor.net/showpost.php?p=4736228&postcount=13

Ummm, but comparing Ca2+ (20p, 18e) and Ar (18p, 18e) means that the Ca2+ ion has more positive charge in its nucleus pulling on the same number of electrons. Your linked example clearly shows that. Therefore the radius of Ca2+ should be smaller than Ar, not larger like the original questions states. So the original question is written wrong. It should say:

"What is the best explanation for why Ca2+ has a smaller atomic radius than Ar?" if answer d were to be correct.

Also, this: "Y3+>Y2+>Y>Y->Y2-" is backwards I believe. The signs should be reversed.

 

[DentalKitty]

My equation gives 233ml.

5.5mol/kg x 1kg/L = 5.5M molality and molarity are roughly equal at room temp. In this particular case you aren't even given a percent mass. Not sure why it would be easier if it was M. If a percent mass was given....then it might be a different story.

Using the M1V1 = M2V2 method is fine if you can remember to subtract 65ml from V2.

That's what I get by approximating as molarity as well. However, none of the answers shown is 233ml. It says the answer is 235ml. With the closeness of the choices I don't think we can round to 235 and call it a day. Also, the molarity approximation only works if the solution is dilute, correct? I don't think 5.5M NaOH is dilute. That's 220g per 1000g of water which is 18% of the total solution weight. We're missing something....

 

[DailyDrivenTJ]

I agree with you dentalkitty. I hate this type of question, where do you draw the line? Answer can be E as well.

 

[doc toothache]

Ummm, but comparing Ca2+ (20p, 18e) and Ar (18p, 18e) means that the Ca2+ ion has more positive charge in its nucleus pulling on the same number of electrons. Your linked example clearly shows that. Therefore the radius of Ca2+ should be smaller than Ar, not larger like the original questions states. So the original question is written wrong. It should say:

"What is the best explanation for why Ca2+ has a smaller atomic radius than Ar?" if answer d were to be correct.

Also, this: "Y3+>Y2+>Y>Y->Y2-" is backwards I believe. The signs should be reversed.

There two other posts that have discussed the atomic radius of atoms, ions, and isoelectric ions and atoms.

http://forums.studentdoctor.net/showpost.php?p=464304&postcount=1 and
http://forums.studentdoctor.net/showpost.php?p=4716210&postcount=1.
(if you have problem loading from the link, look under posts by Awuah29 from 2-7-07) and lor from1-21-07).


In an isoelectric series of ions and atoms support for the size of the atom to be a function of the nuclear charge, Z, can be found in
http://antoine.frostburg.edu/chem/senese/101/periodic.

Other evidence show Ca+2 to have an atomic diameter of 114 pm (www.chemicool.com/elements/calcium/html and Ar at 98 pm (www.chemicool.com/elements/argon/html.

With this particular question you have two choices:

1.decide that the premise of the question is incorrect and argue your point after the exam or
2. accept the premise of the question to be correct. In this case we can eliminate a,b, and c. Answer e is correct but has little bearing on the size of the atomic radius. That leaves us with d. Here you can make an argument that the increase in size due to the additional 2 protons in calcium atom has a greater influence on the size of the atomic radius than the decrease in size from the loss of 2 electrons. (Keep in mind that the atomic size of calcium is 197 pm).

see also www.chemguide.co.uk/atoms/properties/atradius.html

 

[DentalKitty]

There two other posts that have discussed the atomic radius of atoms, ions, and isoelectric ions and atoms.

http://forums.studentdoctor.net/showpost.php?p=464304&postcount=1 and
http://forums.studentdoctor.net/showpost.php?p=4716210&postcount=1.
(if you have problem loading from the link, look under posts by Awuah29 from 2-7-07) and lor from1-21-07).


In an isoelectric series of ions and atoms support for the size of the atom to be a function of the nuclear charge, Z, can be found in
http://antoine.frostburg.edu/chem/senese/101/periodic.

Other evidence show Ca+2 to have an atomic diameter of 114 pm (www.chemicool.com/elements/calcium/html and Ar at 98 pm (www.chemicool.com/elements/argon/html.

With this particular question you have two choices:

1.decide that the premise of the question is incorrect and argue your point after the exam or
2. accept the premise of the question to be correct. In this case we can eliminate a,b, and c. Answer e is correct but has little bearing on the size of the atomic radius. That leaves us with d. Here you can make an argument that the increase in size due to the additional 2 protons in calcium atom has a greater influence on the size of the atomic radius than the decrease in size from the loss of 2 electrons. (Keep in mind that the atomic size of calcium is 197 pm).

see also www.chemguide.co.uk/atoms/properties/atradius.html

Yeah, I guess the reasoning would have to be that the two extra protons made the radius larger, but it's just frustrating because everything I've studied so far clearly says that for isoelectronic species, more protons means more nuclear pull and a smaller radius. Hopefully the question would be stated this way so you'd be forced to pick d instead of having a choice between larger and smaller or something....

 

[jdmsamurai]

as you move down a row in the periodic table the proton # increases and the radii gets smaller....the reason is for this is because there is more protons which then has more positive charge which then binds more tightly to the electrons...as you move down a group/column the radii gets bigger due to bigger electron clouds....

 

[Awuah29]

My equation gives 233ml.

5.5mol/kg x 1kg/L = 5.5M molality and molarity are roughly equal at room temp. In this particular case you aren't even given a percent mass. Not sure why it would be easier if it was M. If a percent mass was given....then it might be a different story.

Using the M1V1 = M2V2 method is fine if you can remember to subtract 65ml from V2.

Hey guys,
correct me if i am wrong. I get 298ml
using mivi=m2v2
solving for vi= 65ml*5.5/1.2 = 298🙁

 

[Lonely Sol]

Hey guys,
correct me if i am wrong. I get 298ml
using mivi=m2v2
solving for vi= 65ml*5.5/1.2 = 298🙁

After you get 298, you have substract 65 ml in order to get the amount of h20 added, since 298 equals the amt of h20 + NaOH

 

[joonkimdds]

My equation gives 233ml.

5.5mol/kg x 1kg/L = 5.5M molality and molarity are roughly equal at room temp. In this particular case you aren't even given a percent mass. Not sure why it would be easier if it was M. If a percent mass was given....then it might be a different story.

Using the M1V1 = M2V2 method is fine if you can remember to subtract 65ml from V2.

I get 233mL, too and that's not the answer choice.
and he said it's molality instead of molarity... and I believe
it's either typo or something that i've never seen before.
I think it has to be molarity.

I never heard anything about molarity = molality at the same temperature.

 

[jdmsamurai]

Um Ca2+ has a smaller atomic radius then Ar

We all know as you move left to right across the periodic table the atomic radius gets smaller

We also know as one moves down a group the atomic radius gets bigger

We also know having the same electron configuration, the one with the more # of protons has a smaller atomic radius

Therefore Since the electron configuration of both Ca2+ and Ar are equal, and Ca2+ having more protons it is the smaller atom.