GS-1 Physical Sciences Question 23

[sunscreenjoanna]

23. The CO32- anion is approximately the same size as:

The answer talks about the solubility of SrCO3 being less than Ba. I don't see how it explains the SIZE in comparison to CO3 anion.

What is the question really asking?

[admin]

The question is only asking: which of the following cations must be about the same size as the carbonate anion?

Please note that before the Explanation begins, in bold it says "P4, T" which means that the answer can be found by looking at Paragraph 4 and the Table.

So here is P4:

However, there is one exception to these general rules. If the cation of the salt is approximately the same size as the anion, the arrangement of ions in the crystal lattice is more uniform and hence the lattice is more stable and ?Hlatt is more negative.

In other words: same size, very stable as a crystal therefore poor solubility. Then by looking at the table provided, the carbonate with the lowest solubility involves the cation Sr++ (it is the cation of the salt).

[samnpdd8521]

I understand the comments above a little, but I dont understand how the anion CO3 is the same size as Sr?? are they going by molecular weight?? When I look at the periodic table Sr is approx. a little bigger than the CO3 anion

[admin]

I will answer your question solely because of your personal interest. It's important for you to know that the question can be answered based on the passage (as described in the Explanation and above) and the background information below is useful general knowledge but not necessary to solve this problem.

OK, normally, if you say that someone is "big" you might mean that they are tall or it might mean that they are wide. In the end, without saying it, most people are thinking of volume. Often volume is related to mass but a big box is big because of its volume not necessarily because if its mass.

Now onto the periodic table: Sr has a far larger atomic radius (therefore greater volume as an atom) than either C or O since Sr is in Group 5 and to the left while C and O are only in Group 2 and to the right. If you had time to waste (which you never will on the MCAT!), you would calculate the volume of Sr based on data (atomic radius) and then the volumes of the 4 atoms in carbonate and then calculate the total volume of carbonate and only then will you be able to determine that the information in the passage is correct: the size of Sr approximates the size of the carbonate anion.

Trends in the Periodic Table:

CHM 2.2 in The GS
http://chemistry.about.com/od/periodictableelements/a/periodictrends.htm

[adbassett7072]

I didn't understand this question at first but I believe I understand it now, please correct me if im wrong. So basically, the question is asking which cation is relatively the same size as the carbonate anion thus making the molecule very stable acording to the information in the passage about it having a stable lattice. Thus the most stable molecule has this lattice configuration due to the cation adn the anion beign relatively the saem size. From the table we can infer that the molecule that is the least stable is the most soluble and thus this is the molecule that was formed from a cation and anion that are relatively the same size.

[admin]

Your logic is correct! However, I would not have used the word "molecules" in my explanation only because these are crystalline arrangements (ie the stable lattice) and so the traditional understanding of a molecule is less important when imagining how size allows these atoms to fit into a nice stable crystal.

[asfi]

So for the salt to be more soluble we want the ΔHlatt to increase and ΔHsolv to be more negative?

[admin]

The passage does not address your question directly and deeper understanding (beyond the passage) is not required in order to accurately answer the question. Nonetheless, according to the passage, we know that "ΔHlatt and ΔHsolv undergo similar changes" (second to last paragraph) and that they are both exothermic reactions (P2).

[gautamg7053]

I believe the equation was looking at the solubility of something...if something is more soluble, then you would assume it would be an exothermic reaction if you take Gibbs free energy into account (G = H - TS). Thus, because the stated equation states that Hsol = Hsolv - Hlat, then you can assume that a more negative Hlat (which is what you get with a cation and anion of similar size), would result in a more positive Hsol, resulting in a decreased solubility.

I hope I'm kind of right...

[sleepyzboy2302]

I thought that the greater molar solubility would be more soluble?

[geoffn045751]

"If the cation of the salt is approximately the same size as the anion, the arrangement of ions in the crystal lattice is more uniform and hence the lattice is more stable"

So more stable means less soluble, since SrCO3 has the lowest solubility it can be inferred that Sr is similar in size to CO3.

[hua8986059]

didn't really need to know anything to answer this, more stable = less soluble = smaller solubility number from graph = similar cation/anion size as stated in passage.

EZ question but required inference.

Oh and I feel like a dummy missing it now.

[abbiearmst4898]

how do i know that an increase in stability means decrease solubility?

[mohsinalam5969]

@ abbiearmst 4898...
There are many reason
1.the molecular size (it increases as you move downward) makes it more difficult to dissolve, they just physically take up more space so less will fit in a given amount of solution (its like the difference the number of tennis balls you can fit in a container versus the number of bowling balls).

2. elements at the top of a group are more ionic in behaviour (less electron transfer needed to reach desired energy level )
on the metal side they just need to transfer 1 0r 2 electrons while on the non-metal side they just need to gain 1or 2

as you move down and across they start to share and wont even disassociate at all when place in a solution/solutions.