Predicting conjuates effect on pH

[Doctor D]

I understand that all strong acids have weak bases and strong bases have weak acid conjugates therefore the conjugate base of HCl with the conjugate acid of NaOH for example (NaCl) does not effect the pH. So when you pair a strong acid conjugate to a strong base conjugate it is neutral. When you pair a strong acid conjugate to a weak base conjugate it is acidic? When you pair a weak acid conjugate to a strong base conjugate it is basic? Anyone wish to clarify please....

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[koopa_troopa]

I'm not sure if you are using the terminology correctly. What do you mean by conjugate base of NaOH?

A conjugate is the resulting molecule from the interaction of an acid or base with water.

For example

HA + H2O <--> A- + H3O+

HA is the acid, A- is the conjugate base. The pH is determined by the ratio of the of A- : HA and the pKA (H-H equation). Adding a strong base or acid will shift the ratio and change the pH. I'm not sure if this is what you are asking???

 

[Doctor D]

sorry, I meant to say conjugate acid of NaOH which is Na+. What I am asking about is when you combine two conjugates how do you predict the resulting pH. For example, CO3 in water is basic because it will steal a proton from water leaving behind an OH. So if we combined The conjugate of bicarbonate (CO3) and say NH4 (the acid conjugate of NH3) what would be the result.

 

[161927]

sorry, I meant to say conjugate acid of NaOH which is Na+. What I am asking about is when you combine two conjugates how do you predict the resulting pH. For example, CO3 in water is basic because it will steal a proton from water leaving behind an OH. So if we combined The conjugate of bicarbonate (CO3) and say NH4 (the acid conjugate of NH3) what would be the result.

Na+ is a spectator ion and H2O is the conjugate acid of NaOH. When bicarbonate and NH4 are mixed, there's a rapid proton transfer. Bicarbonate takes a proton from NH4. CO3 is a dianion; it has a 2- charge. It easily abstracts a proton from positively charged nitrogen (NH4). It gets iffy on the second deprotonation, because oxygen can better handle a negative charge, so taking a hydrogen from NH3 is probably unlikely.

 

[Kaustikos]

Looking and memorizing/understanding titration curves helps alot for these questions. If we know we're reacting weak acid/bases' conjugates together, we can say that it will have the same affect as mixing strong acids with strong bases. So when one has NH4 with HCO3-, we can, from knowing that NH4 is a strong conjugate acid and HCO3 is a strong conjugate base, conclude that the result will be liken to adding HCL with NaOH. They readily react with each other.

Hope that helps.

 

[xanthomondo]

I understand that all strong acids have weak bases and strong bases have weak acid conjugates therefore the conjugate base of HCl with the conjugate acid of NaOH for example (NaCl) does not effect the pH. So when you pair a strong acid conjugate to a strong base conjugate it is neutral. When you pair a strong acid conjugate to a weak base conjugate it is acidic? When you pair a weak acid conjugate to a strong base conjugate it is basic? Anyone wish to clarify please....

First, the difference between a strong acid/base and a weak acid/base is that the strong completely dissociates while the weak exists in an equilibrium. The conjugate of a weak base is a weak acid, however there does not exist an acid/base conjugate for strong acid and bases.

For the HCl + NaOH situation, as long as moles = moles, yes they will "cancel out". When you start titration and mixing weak acids/bases things get a little more shaky. Especially when you throw in polyprotic acid combinations.

Know that the reverse reaction the K is 1/K for the constant given. Also Kb (of the conjugate) is equal to Kw/Kb (because KaKb = Kw) and when you add chemical equations you multiply the constants

But to calculate the pH in your example you would have to take into consideration the following:

CO3 2- (reacting with NH4+, H20, H2CO3, H3O+)
HCO3- (reacting with H20, NH3, NH4+, OH-, H3O+)
NH4+ (reacting with CO3 2-, HCO3-, H20, OH-)

etc etc for NH3, H3CO3...

There's just too many components and reactions to follow, and while its possible (thanks analytical chem!), you're probably not going to have something so complicated on the MCAT

It you're mixing monoprotic/basic components it's a lot easier to calculate pH