Question 10

[mueller86606]

I do not understand this question at all. All parts of it. Is there way it could be further clarified? The explanation is not great.

[mcat_premed3832]

Please excuse the length of the answer (I presume that some of it you know already but I just want to be sure).

Though his question does not require previous knowledge on this topic, let's first clarify what you are expected to know: the natural of a salt.

In chemistry, salts are ionic compounds which can result from the neutralization reaction of acids:

The classic of course is table salt (NaCl):

HCl + NaOH ---> Na+ + Cl- + H2O (neutral compound)

So salts are ionic compounds composed of cations (Na+ above) and anions (Cl-). In general, the ions can be inorganic such as chloride (Cl−), as well as organic such as acetate (CH3COO−) and monoatomic ions such as iodide (I−; famous for being added to table salt to reduce the risk of thyroid disease), as well as polyatomic ions such as sulfate [(SO4)2−] or nitrate in our problem.

The question establishes the following: a Group I nitrite is stable (this is understood because it is the product of a decomposition; if it were not stable then we would expect the decomposition to continue until some other more stable product would be formed).

So the question asks: why is this nitrate salt more stable than the salt with the smaller oxide?

Now we must ask, after reviewing the 3 main characteristics in stability given in the passage: what feature could the nitrite have that the smaller oxide does not have?

The first paragraph establishes 3 main principles for stable salts of s-block elements: greater charge, smaller ions and comparable size. Nitrite does NOT have a greater charge (it is -1 and oxide -2); nitrite is NOT smaller (the question tells is oxide is smaller plus we could be expected to figure that out based on the molecular formula!); and so, we are left with the only conclusion, nitrite must be a similar size to the cation portion of the salt.


Side note: There are some features of the new MCAT that can really help you with efficiency during the exam. One is the highlighter. Just so you know, when I quickly took a peak at the passage, I just naturally highlighted the following words in the first paragraph:

thermal stability
s block
greater the charge
smaller the ions
comparable size

Then when I looked at the question, I instantly had access to the key ideas before even looking at the answer choices.

[GraemeMatt8033]

I don't understand. It says that if they are not of comparable size they will decompose to the smaller anions. Isn't NO2 a smaller anion than NO3?

[jellywing_2058]

Yes it is smaller; however, there is no mention of NO3- as the decomposition product, it is NO2- versus the oxide O2-. The reason it decomposes to NO2- is that the N and O are covalently bonded and you obtain a polyatomic ion as a product of decomposition in place of the oxide regardless of the fact that the oxide is smaller. Also, D is correct because the charge distribution around the NO2- makes it smaller than the charge distribution around O2-. Thus, D is fine.

[btdumford5156]

"Probably, About" Tough way to sway an opinion.