Percent Ionization problem

[ezsanche]

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[ezsanche]

This problem seems weird to me. I dont think there is enough info to solve it. Here is my thought process

HCN --> H+ + -CN

if were to take the Ka expression of this we should get

Ka=[H+][-CN]/[HCN]

solving for

Ka/[H+] x 100% =[-CN]/[HCN] x 100%

dont we need a Ka value to solve for the percent ionization?

 

[Kanye West]

is the answer B?

what I did is:

.339 M HCN means in solution you SHOULD have .339 M [H+]

but you have pH 4.89, which working backwards is like a little more than 1e-5 M [H+]

then i just divided 1e-5/.339 which is ~ 3.3e-5

but then it is % so you multiply by 100 and get ~ 3.3e-3 which is close to B

 

[ezsanche]

is the answer B?

what I did is:

.339 M HCN means in solution you SHOULD have .339 M [H+]

but you have pH 4.89, which working backwards is like a little more than 1e-5 M [H+]

then i just divided 1e-5/.339 which is ~ 3.3e-5

but then it is % so you multiply by 100 and get ~ 3.3e-3 which is close to B

yeah the answer is B but I just dont get why we are using the 1e-5 for the numerator.
I mean what if the pH was 11

then that would mean

10^-11/.331 x 100 = a really small number.

BUT!!!!! if the pH is 11 then the solution is really basic and you would expect the concentration of the -CN to be great. and the percent ionization to be near 100 percent not a small number.

does anyone see something wrong with my logic?

 

[ezsanche]

anyone?

 

[RogueUnicorn]

yeah the answer is B but I just dont get why we are using the 1e-5 for the numerator.
I mean what if the pH was 11

then that would mean

10^-11/.331 x 100 = a really small number.

BUT!!!!! if the pH is 11 then the solution is really basic and you would expect the concentration of the -CN to be great. and the percent ionization to be near 100 percent not a small number.

does anyone see something wrong with my logic?

if pH is 11 and the only thin in solution is HCN, this means precisely that there is a tiny amount of HCN dissolved.. it's an acid. the only way you can have a pH greater than its pKa would be to add its conjugate base.

 

[ezsanche]

if pH is 11 and the only thin in solution is HCN, this means precisely that there is a tiny amount of HCN dissolved.. it's an acid. the only way you can have a pH greater than its pKa would be to add its conjugate base.

okay so you agreeing with me that the percent of ionization will be greater.

 

[zmatrix]

BUT!!!!! if the pH is 11 then the solution is really basic and you would expect the concentration of the -CN to be great. and the percent ionization to be near 100 percent not a small number.

does anyone see something wrong with my logic?

What's screwing you up is that it's impossible to ask this question with a pH < 7. Pure water already has 1e-7M [H+]. HCN can only ADD to the number of [H+]. Therefore the question only makes sense if the pH is < 7. (At pH=7, there would be 0% ionization since pH=7 implies that no additional hydrogen ions have dissociated other than those from water alone.)


Solving for % ionization at pH = 11 will require a Ka for HCN, as well as a description of the base as either a strong base or provide a Kb. As you can already guess, you cannot solve this problem in the same manner as the original.

 

[zmatrix]

BUT!!!!! if the pH is 11 then the solution is really basic and you would expect the concentration of the -CN to be great. and the percent ionization to be near 100 percent not a small number.

does anyone see something wrong with my logic?

What's screwing you up is that it's impossible to ask this question with a pH > 7. Pure water already has 1e-7M [H+]. HCN can only ADD to the number of [H+]. Therefore the question only makes sense if the pH is < 7. (At pH=7, there would be 0% ionization since pH=7 implies that no additional hydrogen ions have dissociated other than those from water alone.)


Solving for % ionization at pH = 11 will require a Ka for HCN, as well as a description of the base as either a strong base or provide a Kb. As you can already guess, you cannot solve this problem in the same manner as the original.

 

[ezsanche]

What's screwing you up is that it's impossible to ask this question with a pH < 7. Pure water already has 1e-7M [H+]. HCN can only ADD to the number of [H+]. Therefore the question only makes sense if the pH is < 7. (At pH=7, there would be 0% ionization since pH=7 implies that no additional hydrogen ions have dissociated other than those from water alone.)


Solving for % ionization at pH = 11 will require a Ka for HCN, as well as a description of the base as either a strong base or provide a Kb. As you can already guess, you cannot solve this problem in the same manner as the original.

Thanks Zmatrix!