Find which nitrate has the lowest pH

[leroyjenkens]

Homework Statement

Which solution will have the lowest pH?
a) Mg(NO3)2
b) Ca(NO3)2
c) Al(NO3)3
d) KNO3
e) Zn(NO3)2

Homework Equations

None I know of.

The Attempt at a Solution

My reasoning was that when the metal dissociates from the nitrate, it will leave a negative charge on the nitrate. I need to find which one will have the lowest pH, so I figured that the KNO3 would dissociate and leave the NO3 with 1 negative charge, which would only attract 1 hydrogen ion, leaving the least amount of OH ions, meaning the solution would have a lower pH than the other ones, since they would dissociate and attract more hydrogens, leaving more OH ions, which would cause the pH to be higher.
But apparently that's not the case.

So I'm stumped. Please help.

 

[Borek]

Meⁿ⁺ + mH₂O <=> Me(OH)_m^(n-m)+ + mH⁺

 

[leroyjenkens]

Meⁿ⁺ + mH₂O <=> Me(OH)_m^(n-m)+ + mH⁺

I don't understand.

 

[Saitama]

I don't understand.

Borek wanted to convey that the metal ions would form hydroxides through reaction with water.

The metal ion which would use more OH^- will result in increased concentration of H⁺. (H₂O <=> OH^-+H⁺)

 

[Bandarigoda]

See which one gives,more
H3O+
H+

Lowest pH means acids

 

[Bandarigoda]

I learned weeks ago, that you can find it by oxidation number. If I remember clearly below number the metal has, it's acid

 

[Bandarigoda]

Is the the answer KNO3?

 

[Borek]

I learned weeks ago, that you can find it by oxidation number. If I remember clearly below number the metal has, it's acid

No.

Is the the answer KNO3?

No.

 

[Bandarigoda]

No.
No.

As usually I suck at chemistry. Could u explain more easy way to determine it? Sorry for my English

 

[Borek]

I already signaled how it should be done - you have to start with the Brønsted-Lowry's theory of acids and bases.

 

[leroyjenkens]

Borek wanted to convey that the metal ions would form hydroxides through reaction with water.

The metal ion which would use more OH^- will result in increased concentration of H⁺. (H₂O <=> OH^-+H⁺)

Oh, I was confused what Me and m were for. I guess m was just used as a variable and Me stands for metal.

So the opposite of what I thought was true. When the Al(NO3)3 dissociates, that leaves an Al with a +3 charge, which will bond to 3 OH, which will leave 3 H cations. For the one I chose, K will only bond to 1 OH, leaving just 1 H cation, which will have a higher pH. So the lowest pH will be the Al(NO3)3.

Thanks guys.

 

[Borek]

Oh, I was confused what Me and m were for. I guess m was just used as a variable and Me stands for metal.

Yes, Me is a metal, and m, n are just some integeres.

So the opposite of what I thought was true. When the Al(NO3)3 dissociates, that leaves an Al with a +3 charge, which will bond to 3 OH, which will leave 3 H cations. For the one I chose, K will only bond to 1 OH, leaving just 1 H cation, which will have a higher pH. So the lowest pH will be the Al(NO3)3.

Beware - these are all equilibrium reactions. It doesn't mean these reactions proceed to an end, so your conclusion is not necessarily correct. TBH without consulting solubility/stability constant tables I am not sure what is the correct answer here.