Which is more basic?

[151AND8TH]

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how do you find out which is more basic or which is more acidic just by knowing the Molarity?


more acidic?

0.10M H2CO3 or 0.10M NH4CL



more basic?
0.10M NH3 or 0.10M NaF

thanks

 

[bigstix808]

how do you find out which is more basic or which is more acidic just by knowing the Molarity?


more acidic?

0.10M H2CO3 or 0.10M NH4CL



more basic?
0.10M NH3 or 0.10M NaF

thanks

When all the M values are the same, you need to know the properties of the actual acids/bases. with these types of problems, i think the easiest thing to do is just memorize some of the common pKa and pKb values, or at least know how strong or weak some of them are are in relation to each other.


#1) Carbonic acid is a weak acid (you should know that), and NH4Cl is just an ammonium salt which is soluble in H2O. Now the problem is quite simple with a little background knowledge: h2co3 has a lower pKa1 (~6.5) than NH4+ (~9.5) and thus is more acidic. h2co3 is also diprotic (can dissociate more than one H) which would cause a drop in pH as well. But if you didn't know the pKa's of either of these then you can just remember that, acids prefer to dissociate to form the weakest conjugate bases over those forming stronger conjugate bases [NH3 (conjugate base of NH4+) is a much stronger base than hco3- (conjugate base of h2co3)]

#2) kind of same principles applied here just in reverse. NaF will dissolve in h2o. F- is the conjugate base of HF (a weak acid). Stronger the acid, the weaker the conjugate base (for example: HCl = very strong acid / Cl- = very weak base). so based on that alone...HF is stronger (more acidic) than NH4+ (conjugate acid of NH3) so thus F- must be a weaker base than NH3. so the answer is NH3 would form the more basic solution.