Gen chem questions - acids/bases

[happyfellow]

All of the following properties are higher for a 1.0M HCl(aq) solution than for a 1.0M HF(aq) solution except:

A. boiling point
B. freezing point
C. electrical conductivity
D. osmotic pressure

My reasoning: HF can hydrogen bond and therefore has a higher BP than HCl. The rest have to do with colligative properties and since HCl fully dissociates they are true statements. However, the answer is B and I don't know why (and there's no explanation!). What's wrong with my reasoning?

Which of the following would yield the greatest hydroxide concentration when added to water?
A. Mg2+
B. H+
C. Cl-
D. NH2-

It says the answer is D but I don't know why. I had originally thought Mg2+ because it binds to OH- and therefore would promote the autoionization of water due to Le Chatliers principle (taking away a product in the reaction H2O --> H + OH). What's wrong with my reasoning?

I am grateful to any clarification on these topics. Thank you in advance!

-happy

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[Emerald84]

Hi Happy,

1. Remember for colligative properties, it is freezing point depression so if there are more particles we expect the freezing point to be lower. HCL will have more particles in solution than HF (weak acid)

2. I think this is because NH2- is a wicked strong base therefore yielding more OH- in solution ( measure of basicity) Mg2+ isn't really basic, Cl- is the conjugate base of a strong acid, so is weak and H+ is an acid so wouldn't increase the OH-

 

[Insulinshock]

Just to clarify what Emerald said on the 1st part:

HCl is a strong acid, and will therefore dissociate more than HF. This dissociation will raise the boiling point more than HF because the ions created have stronger interactions, and since there's more ions in an HCl solution than an HF solution this makes sense. This is why it's not A.

The same reason that boiling point is elevated for an HCl solution is why the freezing point is lower, not higher than HF. Stronger intermolecular forces make it harder to freeze because there's more ions which bring stronger forces.

You may have just read the question wrong, and that's why you answered like you did, but nonetheless this is the reasoning.

 

[PiBond]

.

 

[happyfellow]

thanks everyone for your responses. but isn't the boiling point of HF higher than the boiling point of HCl because of hydrogen bonding? so wouldn't answer choice A be correct as well? I understand the freezing point depression caused by the ions of HCl but the boiling point is also lower for HCl too. I looked up boiling points and noticed HF is 19.5 degrees and HCl is -85 degrees.

 

[Insulinshock]

I have no data to back this up, but I believe those boiling points are in reference to the actual compounds HCl and HF.

Remember, we're talking about ions in water. The boiling point the question is refering to is the boiling point of water, and how its affected by the solute. The ions are present, but aren't even close to the mole fraction of water; therefore, we are looking at the boiling of point of water, and the effects that HCl and HF will have on it, not on the boiling points of HCl and HF themselves.

Hydrogen bonds are strong, but not as strong as the bonds that are created with Cl- ions.

EDIT: Think of it this way: Proteins are held together largely by hydrogen bonds, right? Your body creates the blood pH buffer system because if the the blood gets, say, too acidic, the excess H+ will overpower the hydrogen bonds, unfolding the protein and destroying it.

H+ > hydrogen bonds

Same principle

 

[happyfellow]

I have no data to back this up, but I believe those boiling points are in reference to the actual compounds HCl and HF.

Remember, we're talking about ions in water. The boiling point the question is refering to is the boiling point of water, and how its affected by the solute. The ions are present, but aren't even close to the mole fraction of water; therefore, we are looking at the boiling of point of water, and the effects that HCl and HF will have on it, not on the boiling points of HCl and HF themselves.

Hydrogen bonds are strong, but not as strong as the bonds that are created with Cl- ions.

EDIT: Think of it this way: Proteins are held together largely by hydrogen bonds, right? Your body creates the blood pH buffer system because if the the blood gets, say, too acidic, the excess H+ will overpower the hydrogen bonds, unfolding the protein and destroying it.

H+ > hydrogen bonds

Same principle

Ah, ok! Thanks so much for your help! I really appreciate it

 

[godcyning]

I had originally thought Mg2+ because it binds to OH- and therefore would promote the autoionization of water due to Le Chatliers principle (taking away a product in the reaction H2O --> H + OH). What's wrong with my reasoning?
-happy

What's wrong with your reasoning is that first, if Mg++ is BINDING to OH-, then it's not increasing [OH-]. It's increasing [Mg(OH)2], or [MgOH+] if that's what it made.

Yes, LeChatelier's principle would occur if this happened, and it would increase the amount of water that is autoionized. But it won't raise [OH-] that way, since you had to take away [OH-] to induce it. It's like if you had some reaction: A + B --> C + D. You want to increase the concentration of D, so you boil it out of your flask. Yeah, that drives the rxn but you really aren't increasing [D]. You are increasing C, however, so this works if D is say, a water byproduct and C is the product of interest.

You can't cheat equilibrium thermodynamics. I know, it's a bitch.

NH2- is the answer because as Emerald84 said, it's a wicked strong base (i.e. it REALLY wants an H+), so it takes it away from water easily, leaving OH-. In a fight between OH- and NH2- for an H+, NH2- wins hands down.

 

[BennieBlanco]

All of the following properties are higher for a 1.0M HCl(aq) solution than for a 1.0M HF(aq) solution except:

A. boiling point
B. freezing point
C. electrical conductivity
D. osmotic pressure

My reasoning: HF can hydrogen bond and therefore has a higher BP than HCl. The rest have to do with colligative properties and since HCl fully dissociates they are true statements. However, the answer is B and I don't know why (and there's no explanation!). What's wrong with my reasoning?

Which of the following would yield the greatest hydroxide concentration when added to water?
A. Mg2+
B. H+
C. Cl-
D. NH2-

It says the answer is D but I don't know why. I had originally thought Mg2+ because it binds to OH- and therefore would promote the autoionization of water due to Le Chatliers principle (taking away a product in the reaction H2O --> H + OH). What's wrong with my reasoning?

I am grateful to any clarification on these topics. Thank you in advance!

-happy

lol with all the complex answers:

more impurities in a solution: Increases BP and lowers FP. done.

NH2- gets the most hydroxide concentration because it is the strongest base!!!!!

Simplicity.

 

[BennieBlanco]

What's wrong with your reasoning is that first, if Mg++ is BINDING to OH-, then it's not increasing [OH-]. It's increasing [Mg(OH)2], or [MgOH+] if that's what it made.

Yes, LeChatelier's principle would occur if this happened, and it would increase the amount of water that is autoionized. But it won't raise [OH-] that way, since you had to take away [OH-] to induce it. It's like if you had some reaction: A + B --> C + D. You want to increase the concentration of D, so you boil it out of your flask. Yeah, that drives the rxn but you really aren't increasing [D]. You are increasing C, however, so this works if D is say, a water byproduct and C is the product of interest.

You can't cheat equilibrium thermodynamics. I know, it's a bitch.

NH2- is the answer because as Emerald84 said, it's a wicked strong base (i.e. it REALLY wants an H+), so it takes it away from water easily, leaving OH-. In a fight between OH- and NH2- for an H+, NH2- wins hands down.

and Mg2+ is not an acid. Typically Q's like the one above are looking for strong acids/bases. Binding to the small % of ionized water doesn't do anything.

 

[boaz]

What's wrong with your reasoning is that first, if Mg++ is BINDING to OH-, then it's not increasing [OH-]. It's increasing [Mg(OH)2], or [MgOH+] if that's what it made.

Yes, LeChatelier's principle would occur if this happened, and it would increase the amount of water that is autoionized. But it won't raise [OH-] that way, since you had to take away [OH-] to induce it. It's like if you had some reaction: A + B --> C + D. You want to increase the concentration of D, so you boil it out of your flask. Yeah, that drives the rxn but you really aren't increasing [D]. You are increasing C, however, so this works if D is say, a water byproduct and C is the product of interest.

You can't cheat equilibrium thermodynamics. I know, it's a bitch.

NH2- is the answer because as Emerald84 said, it's a wicked strong base (i.e. it REALLY wants an H+), so it takes it away from water easily, leaving OH-. In a fight between OH- and NH2- for an H+, NH2- wins hands down.

and Mg2+ is not an acid. Typically Q's like the one above are looking for strong acids/bases. Binding to the small % of ionized water doesn't do anything.

Mg2+ actually IS an acid.

Mg2+ + H2O ----> MgOH+ + H+

[and Mg(OH)2]

H+ is a product in the reaction above. Putting Mg2+ into solution will result in lowering of pH.

 

[BennieBlanco]

Mg2+ actually IS an acid.

Mg2+ + H2O ----> MgOH+ + H+

[and Mg(OH)2]

H+ is a product in the reaction above. Putting Mg2+ into solution will result in lowering of pH.

hmmm. So is Na+ an acid? MgOH2 is a very strong base... So I guess Cl- is now a good base?

I'm thinking typical strong acids/weak acids, not those that are so negligible that they exhibit hardly any effect (although they are strictly acid/base)...

I guess then distilled water is an acid too, but questions like these aren't testing that. If distilled water was on the list would we use that as the answer? nope, but it is an acid in water.

 

[boaz]

.

hmmm.

So is Na+ an acid?

No. Na+ affinity for OH- is very very low. That's why NaOH is extremely soluble in water. So, putting Na+ in water has almost no effect on pH.

MgOH2 is a very strong base...

Yes, of course it is a strong base. Mg(OH)2 dissociates to form OH-...

So I guess Cl- is now a good base?

I don't follow the logic?

I'm thinking typical strong acids/weak acids, not those that are so negligible that they exhibit hardly any effect (although they are strictly acid/base)...

Mg(OH)2 <------> Mg2+ + 2OH-

Ksp=[Mg2+][OH-]^2/[Mg(OH)2]=1.5*10^-11

Just from looking at the Ksp, you can see that Mg2+ has a strong affinity for OH-.

I guess then distilled water is an acid too,

Go ahead and find the pH of a 1M solution of MgCl2 and then compare with the above.
but

questions like these aren't testing that.

They actually are tested. You should know that putting MgCl2 into water will result in a lower pH. I believe this is mentioned very prominently in the most widely used general chemistry textbook (Brown Lemay Burstein).

 

[BennieBlanco]

Mg(OH)2 <------> Mg2+ + 2OH-

Ksp=[Mg2+][OH-]^2/[Mg(OH)2]=1.5*10^-11

Just from looking at the Ksp, you can see that Mg2+ has a strong affinity for OH-.

hmmmmmm.

Wow. Ksp's don't include solid's in the expression in the denominator... this is very fundamental. That is something actually tested on the MCAT.

SUPER CONFUSED. This is the first thing you learn about equilibrium. hmmm.

By trying to correct me on a technicality, you tried to go further than your actually knowledge permits.

You are just blabbering nonsense at this point. Magnesium chloride in water is......... Neutral! not acidic.

Magnesium Chloride — PH: 7.0 (source: chemgroup)

MSDS says: it is neutral.

you are just talking and using basic knowledge to answer things that #1 aren't MCAT tested, #2 aren't even true.

Magnesium Chloride is a neutral or 7.0 ph

you should know that putting MgCl2 into water will result in a lower pH.

I should know that???? why should I know something wrong and not even important to the MCAT? This is nonsense you have put together from stretching basic concepts.

They actually are tested. You should know that putting MgCl2 into water will result in a lower pH. I believe this is mentioned very prominently in the most widely used general chemistry textbook (Brown Lemay Burstein).

I have it in front of me. Please list the page # and chapter. Oh wait? What you are saying is wrong. I am trying to wrap my head around something being PROMINENTLY mentioned yet 100% incorrect.

How do they test things that aren't true on the MCAT?

 

[boaz]

.

Ksp's don't include solid's in the expression in the denominator...

Thanks for correcting me. That was a mistake.

Magnesium chloride in water is......... Neutral! not acidic.

No, sir. Check back on your notes and/or textbook and you will remember that metal cations do react with water in the way I described above. If you don't find it there, then you missed an important part of the subject. Certain ions (e.g. Na+) react only very weakly, while others (e.g. Pd2+) react very strongly. Mg2+ reacts moderately, so it WILL lower the pH significantly.

Magnesium Chloride — PH: 7.0 (source: chemgroup)

This is wrong. Please provide a direct link to your source.

MSDS says: it is neutral.

"pH: 5% in water is neutral to litmus."
That's what it says. It does not say pH=7.0

In fact, pH is about 6.

I have it in front of me. Please list the page # and chapter.

In the 10th edition, it's page 701 and 708. If you have another edition, just look in the index under "Cations, reactions with water."

 

[boaz]

.

I'm looking back at your post, and you seem to have gotten terribly overheated.

It's OK to have a normal relaxed discussion; there's no need to get overexcited.

 

[BennieBlanco]

Thanks for correcting me. That was a mistake.



No, sir. Check back on your notes and/or textbook and you will remember that metal cations do react with water in the way I described above. If you don't find it there, then you missed an important part of the subject. Certain ions (e.g. Na+) react only very weakly, while others (e.g. Pd2+) react very strongly. Mg2+ reacts moderately, so it WILL lower the pH significantly.


This is wrong. Please provide a direct link to your source.



"pH: 5% in water is neutral to litmus."
That's what it says. It does not say pH=7.0

In fact, pH is about 6.






In the 10th edition, it's page 701 and 708. If you have another edition, just look in the index under "Cations, reactions with water."

Please quote your source that the pH of MgCl2 is 6. My point is whether or not it drops the pH, the point is that this is not even mentioned in any prep book's acid base section. Knowing that the CONJUGATE acid of a STRONG base is... acidic, isn't tested. Yes it is acidic, it is the conjugate of a strong base, but the point is, how much?


ok. I will believe you that this concept is tested on the MCAT if you can give me 1 question that tests that Mg2+ added to water decreases the pH.

Honestly, I should say 5 questions out of the thousands of possible Q's you have to chose from. If this was so important than finding 5 (much less than 1%) should be easy.

I have all the following, so chose from any of these:

Every AAMC FL
Kaplan 1-11 FLs
BR Chem (any passage or discrete)
EK 1001 chem
EK chem

I'm thinking this is not really tested but I will believe you if you give one example from this list of over 1000 MCAT questions on chemistry.

 

[BennieBlanco]

Please quote your source that the pH of MgCl2 is 6. My point is whether or not it drops the pH, the point is that this is not even mentioned in any prep book's acid base section. Knowing that the CONJUGATE acid of a STRONG base is... acidic, isn't tested. Yes it is acidic, it is the conjugate of a strong base, but the point is, how much?


ok. I will believe you that this concept is tested on the MCAT if you can give me 1 question that tests that Mg2+ added to water decreases the pH.

Honestly, I should say 5 questions out of the thousands of possible Q's you have to chose from. If this was so important than finding 5 (much less than 1%) should be easy.

I have all the following, so chose from any of these:

Every AAMC FL
Kaplan 1-11 FLs
BR Chem (any passage or discrete)
EK 1001 chem
EK chem

I'm thinking this is not really tested but I will believe you if you give one example from this list of over 1000 MCAT questions on chemistry.

lol, better yet. Find a question in the Brown book testing that adding Mg2+ to water decreases the pH. I don't even think the text cares to test this. Seriously, find one example in that Brown text book.

If it is so prominent there should be at least 1 example in an 1100 pg textbook devoted to chemistry.

I think you are just using interesting chem trivia that isn't relevant. But showing multiple examples proves me wrong. Please do.

 

[BennieBlanco]

I'm looking back at your post, and you seem to have gotten terribly overheated.

It's OK to have a normal relaxed discussion; there's no need to get overexcited.

I am an overexcited person.

No harm is meant.

 

[boaz]

The data is from: Daniel Harris: Quantitative Chemical Analysis, page 111.

For the reaction:

Mg2+ + H2O <----------> MgOH+ + H+

the equilibrium constant is [MgOH+][H+]/[Mg2+] = 10^-11.4

Setting up a table (for a 1M soln of MgCl2) results in a quadratic equation, with the result of [H]+ = 10^.5.7

pH=5.7

A more extreme case would be a 1M soln of Pd2+. The pH of such a solution would be about 1.0--a pretty 'strong' weak acid.

Sure, it's not as typical as the classic acetic acid type of acid. But I think it's fair game on the MCAT. It doesn't take a stretch of the imagination to realize that metal cations will react with water. It's not trivia.

I took the MCAT a half-year ago, so I'm not going to remember any specific practice questions. Nor will I go searching for one now just to prove my point . But just taking a look at EK gen chem, 7th edition, page 101 (section 6.6) we see that it is an important concept to know. It is written that "metal cations act as weak Lewis acids," though it would have been better if it would have shown the reaction so as to differentiate it from most Lewis acids. A Lewis acid does not necessarily affect [H+], but in this case it does.

I can very well imagine this concept showing up on the MCAT. So it's worth knowing.

Good luck.

 

[BennieBlanco]

The data is from: Daniel Harris: Quantitative Chemical Analysis, page 111.

For the reaction:

Mg2+ + H2O <----------> MgOH+ + H+

the equilibrium constant is [MgOH+][H+]/[Mg2+] = 10^-11.4

Setting up a table (for a 1M soln of MgCl2) results in a quadratic equation, with the result of [H]+ = 10^.5.7

pH=5.7

A more extreme case would be a 1M soln of Pd2+. The pH of such a solution would be about 1.0--a pretty 'strong' weak acid.

Sure, it's not as typical as the classic acetic acid type of acid. But I think it's fair game on the MCAT. It doesn't take a stretch of the imagination to realize that metal cations will react with water. It's not trivia.

I took the MCAT a half-year ago, so I'm not going to remember any specific practice questions. Nor will I go searching for one now just to prove my point . But just taking a look at EK gen chem, 7th edition, page 101 (section 6.6) we see that it is an important concept to know. It is written that "metal cations act as weak Lewis acids," though it would have been better if it would have shown the reaction so as to differentiate it from most Lewis acids. A Lewis acid does not necessarily affect [H+], but in this case it does.

I can very well imagine this concept showing up on the MCAT. So it's worth knowing.

Good luck.

I don't think so. The fact that:

#1: Not in 1 AAMC exam
#2: BR doesn't mention it once (and they tend to over do it with prep)
#3: The book that you quoted, Brown had over 50 problems, and not one dealing with Mg2+ as an acid
#4: Not in any of the other Acid/Base Q's (EK 1001, etc)

If all your saying is EK mentioning offhand, "metal cations are weak lewis acids" well sure, there are tons of metals. Mg2+ is negligible.

I wasn't asking you to slave over finding a question. There are thousands of questions, there are even sections in prep books dealing exclusively with Acid/Base and I've never seen this asked once. If you asked me to find a question concerning an MCAT topic I could find it in under 3 min. It isn't even asked once in the Brown textbook. hard to believe they will be testing for the acidity of Mg2+.

I guess in the end I find it funny that, although you said it was a prominent topic, there is no ability to even reference 1 question in any BR/EK/AAMC FL/Kaplan FL. I would think there would be at least one Q on it if it was an important topic. Offhand single sentences (in 1100 pg text and 200 pg EK book) referring to all metal cations doesn't lead me to believe the acidity of Mg2+ is important

 

[boaz]

.

I don't think so. The fact that:

#1: Not in 1 AAMC exam
#2: BR doesn't mention it once (and they tend to over do it with prep)
#3: The book that you quoted, Brown had over 50 problems, and not one dealing with Mg2+ as an acid
#4: Not in any of the other Acid/Base Q's (EK 1001, etc)


If all your saying is EK mentioning offhand, "metal cations are weak lewis acids" well sure, there are tons of metals. Mg2+ is negligible.

EK is the first book I looked at. As it says in the introduction, anything written in brown is there to "remind you what is and is not an absolute must for the MCAT."

I have not looked anywhere for practice questions on this subject, so I cannot address your "absence-of-proof" argument.

For you, I would recommend that you just skip this subject altogether.

For others who are reading this, I'd recommend following EK's sage advice and remember this small, but important, detail.

Best of luck on your MCATs.

 

[BennieBlanco]

I don't think so. The fact that:



EK is the first book I looked at. As it says in the introduction, anything written in brown is there to "remind you what is and is not an absolute must for the MCAT."

I have not looked anywhere for practice questions on this subject, so I cannot address your "absence-of-proof" argument.

For you, I would recommend that you just skip this subject altogether.

For others who are reading this, I'd recommend following EK's sage advice and remember this small, but important, detail.

Best of luck on your MCATs.

For others I recommend with paying attention to topics tested on:

Berkeley Review / AAMC FL / Kaplan FL / EK 1001 / EK's actual questions / (which likely means ANY prep books questions)


My argument about asking for one question is valid, because there are none. Anyone has the ability to look, I've looked, there are NONE. zero. zip.

The rest is chem trivia... Mg2+ being acidic is NOT tested on the MCAT. EK mentions that metal cations are weak acids, well yeah but we are talking specially about Mg2+ which NEVER comes up. I looked through all the Q's and was unable to find one, not even in the "prominent" textbook that was referred to.

It is funny you recommend BR but they don't even mention this nor one of their 140 chem passages have a single Q on it.