Gen Chem question

[Albuterol]

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Hey guys this question is from the Kaplan BB paper test.

When 100g of an unknown compound is dissolved in water to produce 1.00 liters of solution, the pH of the resulting solution is measured as 4.56. Which of the following statements is most likely true of the unknown compound?

the first 2 answer choices says strong base and weak base. I know its not those 2 since the final pH is below 7 but I don't know how to decide on the correct answer from the next 3 choices:

C. it is a strong acid with a formula weight of less than 50g/mol
D. it is a weak acid with a pKa of more than 1
E. it is a weak acid with a formula weight of less than 10g/mol

the answer is D...but i find kaplans explanation kind of confusing. thanks for the help!

 

[alanan84]

I think this is a good question but am a little stumped as well. I was able to eliminate choice C as well. I just solved for the pH of 100g in 1 liter of a strong acid with a formula weight of 50g/mole. I got something really small but as the formula weight would increase, the molarity would decrease, and consequently the pH would increase.

Anyway, I'm not sure how you would be able to eliminate E

 

[ZenoVT]

I think it is because for a weak acid the pKa is about the same as the pH and so if it were a weak acid then the pKa would be around 4.56, which is greater than 1. That's what I am thinking, but not sure if thats right.

 

[alanan84]

I think it is because for a weak acid the pKa is about the same as the pH and so if it were a weak acid then the pKa would be around 4.56, which is greater than 1. That's what I am thinking, but not sure if thats right.

Yeah I thought the same thing, it certainly makes sense. Anyone know how to definitively eliminate answer choic E

 

[Albuterol]

Kaplans answer is...Choice E is incorrect because there is no acid (weak or strong) that has a formula weight of 10g/mol.

 

[ZenoVT]

Hahaha I guess that's true... but I would've never thought that.

 

[Albuterol]

me either, thats why when i read the choices i was like wtf? stupid kaplan haha...thanks for the help guys!

 

[alanan84]

Kaplans answer is...Choice E is incorrect because there is no acid (weak or strong) that has a formula weight of 10g/mol.

Wow... that's all I can say, wow.