Question

1.) Determine the dissociation constants for the acids. Express the answers in proper scientific notation where appropriate.

Acid A: p?a=5.0 ?a=

Acid B: p?a=9.10 ?a=

Acid C: p?a=−2.0 ?a=

Which is the strongest acid?

2.) The ?bKb for an amine is 7.416×10−5.7.416×10−5. What percentage of the amine is protonated if the pH of a solution of the amine is 9.1309.130? Assume that all OH−OH− came from the reaction of B with H2O.H2O.

percentage protonated

Answer 1

Hi,

1) 
The dissociation constants are determined under the following formula: 



Applying antilogarithm properties is able to clear the Ka:



For acid A, B and C there are pKa values ​​that vary from -2 to 9.1.

ACID A
pKa = 5

ACID B 
pKa = 9,1

ACID C
pKa = -2,0



An acid is stronger when it has a small value of pKa and therefore a high value of ka(since they are inversely proportional), this means that ACID C, since it has a higher value of ka, completely dissociates into hydrogen ions and anions.

2) 
The protonation rate is determined with the following mathematical expression:





Relate the amount of OH released by the base during the reaction.

For this, we must know the OH concentration and the base concentration,

For the OH concentration:

The pH of the amine is known (9.13), therefore by difference the pOH is achieved and subsequently the OH concentration:


 
It's known that



Clearance, applying antilogarithm properties



Therefore the [OH] is equal to



On the other hand, for the concentration of the base is determined with the following equation:



Where X represents the OH concentration, such formula has:



Since we know the dissociation constant of the base, Kb (7.416x10-5) and the OH concentration (1.349x10-5), our incognita is the concentration of the base, clearing the previous formula:




Once both incognites are achieved, the protonation percentage formula is substituted to obtain the following result:


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