Hydrolysis of salts

[stuw]

So when NaHSO4 is placed in water, what pH will the salt solution be in? (basic, acidic, neutral)




Right off the bat, group 1 ions don't react with water (this is due to NaOH being a strong base, dissociating completely to sodium and hydroxide, leaving no conjugate acid properties (right?)), so it's neutral. But for HSO4, I approached it saying that HSO4 was the conjugate base for H2SO4, a strong acid, declaring it to be neutral. However, the answer key states that HSO4 is actually a conjugate acid of SO4 2- (a weak acid). Could someone explain to me why this is? It's been tripping me up on some problems. Thanks a bunch 🙂

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[siffster]

I think your rationale that the conjugate base of a strong acid is neutral only works for binary acids like HI, HCl etc etc. For polyprotic acids like H2SO4, after it dissociates into HSO4- and H+, it still has another hydrogen that it can donate, so I'd say the answer is acidic although a weak acid.

 

[brood910]

You should remember that HSO4- is amphoteric. It can be either base/acid depending on the situation.

HSO4- is more likely to give H+ away than accepting it because H2SO4 is a strong acid.
So, even if it accepts H+, it will dissociate back.

Thus, HSO4- + H2O => SO4- + H3O+

ACIDIC.

If you used NaHCO3 instead, it is different.

HCO3- is from a weak acid H2CO3, so it will not dissociate back much when it gains H+.

HCO3- + H2O => H2CO3 + OH- <-> CO2 + H2O

BASIC

 

[stuw]

I think your rationale that the conjugate base of a strong acid is neutral only works for binary acids like HI, HCl etc etc. For polyprotic acids like H2SO4, after it dissociates into HSO4- and H+, it still has another hydrogen that it can donate, so I'd say the answer is acidic although a weak acid.

Yeah definitely noting that trend. Thanks man.

You should remember that HSO4- is amphoteric. It can be either base/acid depending on the situation.

HSO4- is more likely to give H+ away than accepting it because H2SO4 is a strong acid.
So, even if it accepts H+, it will dissociate back.

Thus, HSO4- + H2O => SO4- + H3O+

ACIDIC.

If you used NaHCO3 instead, it is different.

HCO3- is from a weak acid H2CO3, so it will not dissociate back much when it gains H+.

HCO3- + H2O => H2CO3 + OH- <-> CO2 + H2O

BASIC

oi you're absolutely right. I guess my issue was thinking about each successive deprotonation making the molecule less acidic...but considering that h2so4 completely dissociates, your reasoning makes complete sense 🙂

Great responses, guys. Thanks again!

 

[Cmdr_Shepard]

Since they're not here to accept, I will accept your thanks on their behalf.

You're welcome, it was my pleasure!