AAMC #8 question gen-chem #38 #40

[Shipley231]

Okay so the question asks -- If red litmus paper is dipped into a solution of Na2CO3, it will:
A)remain red, because carbonate is an acidic salt
B)remain red, because carbonate is neutral
C)turn blue, because carbonate reacts with water to produce OH-
D)turn blue, because sodium ions form sodium hydroxide in water

The answer was C, but how are we supposed to know that? The explanation just provides the equation like we just have it memorized.

Also -- According to the passage, the gas that evolves is:
A)sulfur dioxide
B)sulfur trioxide
C)Carbon dioxide
D)carbon monoxide

The answer is C, but I'm also unsure why. The passage states that HCl is added to a solution containing CO3^2- but I'm unsure why I'm supposed to know that it evolves CO2.

Thanks!

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[MajorBubblez]

For the first questions, I had no idea what Red Litmus paper did, so I answered it correctly by knowing how to identify acidic and basic salts. Acidic salts have cations that are stronger acids than water, which generally include cations not in group 1 or the large group 2 cations (they're every other cation, ex. NH4+, Cu2+ etc) . Basic salts have anions that are non the conjugate of a strong acid (F-, ClO-, etc.). So for Na2CO3, it's not acidic (Na is Group 1) but it is basic (CO3 2- isn't a conj of a strong acid). Therefore the overall salt is basic. This rules out A, B, and D. Therefore, it must be C. Which makes sense, a basic salt is more reactive than water, so it would react with water to form OH-. Conversely an acidic salt would react with water to form H3O+.

For the second question, when you add an acid, the acid is generally oxidized, meaning the compound it's added to is reduced. Reduction is gain of electrons, gain of hydrogen, loss of oxygen. Losing one oxygen produces CO2. I don't know if this is the correct way to do it, but it was my approach and I was right so maybe I was just lucky.

Just took the exam, very interesting physical sciences section lol.

 

[gettheleadout]

AAMC material questions belong in the appropriate designated threads. Thread closed.