Acid/ pH

[Dencology]

100g of a compound of unknown identity is dissolved in 10L of water to produce a solution with a pH of 2. which of the following statements with regard to the compound is most likely true?

a. the compound is a strong acid with a formula weight less than 100g/mol
b. the compound is a weak base with a formula weight greater than 100g/mol
c. " "" is a strong base with fromula weight less than 50g/mol
d. " " is a strong acid with a formula weight of greater than 100g/mol
e. the compound is a weak acid with a formula weight of less than 100g/mol

the answer is a but why guys?

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[Zerconia2921]

file:///C:/DOCUME%7E1/BEHNAM%7E1/LOCALS%7E1/Temp/moz-screenshot-7.jpg

100g of a compound of unknown identity is dissolved in 10L of water to produce a solution with a pH of 2. which of the following statements with regard to the compound is most likely true?

a. the compound is a strong acid with a formula weight less than 100g/mol
b. the compound is a weak base with a formula weight greater than 100g/mol
c. " "" is a strong base with fromula weight less than 50g/mol
d. " " is a strong acid with a formula weight of greater than 100g/mol
e. the compound is a weak acid with a formula weight of less than 100g/mol

the answer is a but why guys?

Common logic tells us the a pH of 2 is very acidic from that we can eliminate e, and c. Also a strong acid will dissociate and among the strong acids formula weight dosent exceed 100g/mol (from what I can think of) so this eliminaes answer b, and d. thats how i would reason this problem.

 

[doc3232]

It is definitely a strong acid because weak acids could never get to a pH of 2.
So the question is: Is it less than or more than 100 g/mol?
Try each one. If it is 100 grams/mol then it is will have a pH of 1 and if it is something like 20 grams/mol, then we get pH of 1.7. If you went the other way, saying it is greater than 100 grams/mol, then you would get a pH of less than 1. I don't know if this is the right way or if I am right but it sort of works.

 

[Zerconia2921]

It is definitely a strong acid because weak acids could never get to a pH of 2.
So the question is: Is it less than or more than 100 g/mol?
Try each one. If it is 100 grams/mol then it is will have a pH of 1 and if it is something like 20 grams/mol, then we get pH of 1.7. If you went the other way, saying it is greater than 100 grams/mol, then you would get a pH of less than 1. I don't know if this is the right way or if I am right but it sort of works.

doc3232
i was thinking of all of the strong acids like HCl for example its mw is 36g/mol right and whats another strong acid HI 56g/mol, HNO3 63g/mol are all below 100g/mol thats what i was thinking about from my post is this corret?

 

[Hugh Mannity]

Well I may be doing something wrong since my answer is D but here is how I tried to solve for the MW. pH=2 means [H+]=0.01. Now I set up a stoichiometry type thing like this: (100g)(1/(x g/mol))(1/(1 L)) = mol of acid. Since we treat it as a strong acid then [HA]i = [H+]eq. so 100/(10x) = 0.01. x=1000g/mol. But then why answer A? All common strong acids are under 100g/mol but according to wikipedia there are extremely strong acids that are above 100g/mol. If anyone can tell me why my reasoning is wrong it would be greatly appreciated because if the answer is A then it is obviously wrong.

 

[doc3232]

doc3232
i was thinking of all of the strong acids like HCl for example its mw is 36g/mol right and whats another strong acid HI 56g/mol, HNO3 63g/mol are all below 100g/mol thats what i was thinking about from my post is this corret?

I agree with you but if you do the math, none of those numbers will give a pH of 2.

 

[Dencology]

I like your approach.

 

[TempleDMDKrazd]

a stronger acid then all those are HClO4.....= molecular weight of 100g/mole