Polyprotic acids

[deleted388502]

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Just wanted to clear up a couple scenarios involving a polyprotic acid/base because I feel like I'm confusing various concepts.

So say I have H2Po4. If I am given its pH and I have to calculate it's concentration, I do not account for the fact that it is polyprotic. If I am give its concentration and have to calculate its pH, I still do not account for the fact that it is polyprotic?

I'm confused as to when you actually have to account for the fact that you have 2H's in the molecule when you're doing out calculations. This question specifically:

The solution says that only the first hydrogen dissociates completely. Cool, makes sense. But then, when you look at bases:

Solution says that both OHs dissociate completely so you have to account for the double concentration of OH.

I'm conceptually thrown off by why you do not have to account for the presence of double the hydrogen concentration at all times in a polyprotic acid, but the OH situation seems to be different.

Thanks.

 

[Chrisz]

The difference lies in the fact hso4- is a weak acid, so it only dissociate to an extent in accordance with its ka value. Since the contribution of h+ is only slight, so it would lower the ph little bit lower than 3

 

[nugang]

Exactly what Chrisz said and to add onto that, the reason you double the H+ concentration for the Barium Hydroxide is because it is a STRONG base, therefore the [OH] will dissociate into two [OH] ions per 1 mole of Ba[OH]2

 

[deleted388502]

Thank you!