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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: jsmith613 on December 19, 2009, 04:14:02 PM

Title: Conjugates
Post by: jsmith613 on December 19, 2009, 04:14:02 PM
to find conjugate base pair of an acid - remove a H+
to find conjugate acid pair of a base - add an H+
BUT
how do you find conjugate base pair of a cation (e.g: Na+ / K+)

Furthermore

how can one use hydrolyzed salts to find the pH of something (e.g: CaCO3 / Na2CO3) - if pH is too difficult how can you work out if the salt would be an acid or base in aqueous form
Title: Re: Conjugates
Post by: Grundalizer on December 19, 2009, 04:27:18 PM
Na+ is the conjugate acid of the base NaOH  K+ is the conjugate acid of the base KOH.  I feel strange saying, the conjugate base of Na+ is NaOH.

The carbonate Ion CO32-  is a base since it can accept protons

CO32− +2 H2O ⇋ HCO3− + H2O + OH− ⇋ H2CO3 +2 OH−

You can look up the pKa values of those salts, but you'd need to do experiments to figure out their pH in solution, I don't know anyway to tell you the exact number just from its makeup.
Title: Re: Conjugates
Post by: jsmith613 on December 19, 2009, 04:51:57 PM
Quote from: Grundalizer on December 19, 2009, 04:27:18 PM
Na+ is the conjugate acid of the base NaOH  K+ is the conjugate acid of the base KOH. 
so then how is the conjugate base of Ca2+ CaOH- should it not be Ca(OH)2
Title: Re: Conjugates
Post by: Grundalizer on December 19, 2009, 05:20:19 PM
The conjugate base of Ca2+ is Ca(OH)+ (it can donate an OH- group)

Read this thread.

http://www.chemicalforums.com/index.php?topic=38413.0

Remember, there are three definitions of an acid/base.  http://en.wikipedia.org/wiki/Acid
Title: Re: Conjugates
Post by: jsmith613 on December 19, 2009, 05:32:28 PM
Quote from: Grundalizer on December 19, 2009, 05:20:19 PM
The conjugate base of Ca2+ is Ca(OH)+ (it can donate an OH- group)

what confuses me is why can there note be 2 OH groups here - it is not stable whereas NaOH is

please explain
Title: Re: Conjugates
Post by: Grundalizer on December 19, 2009, 06:26:52 PM
Did you read that other thread?

NaOH and Ca(OH)2 are both metallic bases.  Na forms the Na+ ion, Ca forms the 2+ ion (look at the periodic table to see why).  Na is in group 1 (needs to lose 1 electron to form a stable octet), Ca is in group 2 (needs to lose 2 electrons to form a stable octet).  NaOH is a "strong" base, it completely dissociates into OH- and Na+ when placed in water. 

Most hydroxides are not very soluble, the exceptions being NaOH, KOH, and to some extent, Ca(OH)2.  Calcium hydroxide is not as strong a base as KOH and NaOH.

IN WATER:
Ca(OH)2  ::equil:: Ca(OH)+ + OH-  ::equil:: Ca2+ + 2OH-

So as in the previous thread I linked on there, the middle equilibrium is not as stable as the first or last state, but nevertheless, as also mentioned in the last thread, it does play a role in helping to say why calcium hydroxide isn't as strong a base as NaOH and KOH