Question

I don't want a straight out answer for this, can someone explain this to me?

The table below provides percent ionization values to indicate the extent that 0.1M solutions of certain weak acids ionize in water according to the generic equation, HA + H2O 2-way arrow H3O+ + A−. How are percent ionization values similar to equilibrium constant values? Explain your reasoning and identify which weak acid would have the smallest equilibrium constant.


Weak Acid

Percent Ionization


HNO2

6.2%


HClO2

28.2%


HCN

0.007%

Answer 1

\(K_a=\dfrac{[H_3O^+] [A^-]}{[HA]}\); \([H_3O^+]=[A^-]=\%ionization[HA]\)

so for \(HNO_2\)

\(K_a=\dfrac{[H_3O^+] [NO_2^-]}{[HNO_2]}=\dfrac{0.062*[HNO_2]}{[HNO_2]}\)

they're related in a way that they can give you the concentration of ions in solution. You can figure out one given the other, as i posted above.

Answer 2

sorry this should've been squared:

\((0.062*[HNO_2])^2\)

Answer 3

I don't really get this. I now I'm supposed to find the smallest equilibrium constant which is Ka. But how I get that is a bit lost to me.

Answer 4

Ka is simply just a measure of how much of the original species dissociated/ionization.

the higher the Ka the higher dissociation/ionization.

Answer 5

dissociated/ionized*

Answer 6

Ok, so this is acidic dissociation. Then at the final part with 0.062∗[HNO2][HNO2]^2 would it be...0.00384?

Answer 7

we'll without the concentration of HNO3 you can't really give the Ka value.

though if you were comparing the species above,

HNO2 6.2% HClO2 28.2%

HClO2 has a larger Ka because it has a greater dissociation/ionization percent

Answer 8

we'll actually, you can if you assume [HNO3]=1.

Answer 9

So HCN has the smallest equilibriun constant or does HNO2? With the assumption that HNO3=1. I mean HCN has the smallest dissociation/ionization percentage I was going to choose that as my first choice.

Answer 10

yep HCN has the smallest Ka, therefore the smallest ionization %.

They're proportional

Answer 11

Which is what I figured! To keep equilibrium the proportions shift from each side, smaller Ka equals smallest ionization percentage and vice versa. SO I could say that Ionization percentages are similar to equilibrium constants because they're proportionally linked to one another based on their characteristics? Like HCN has a trait of ionizing in water quickly.

Answer 12

it's good, up until "based on their characteristics? Like HCN has a trait of ionizing in water quickly." The last part is not necessarily true. rates of dissociation aren't specified by equilibrium constants, those are in the realm of kinetics.

Answer 13

Yeah I caught myself there later, I have to keep molecular motion in the equation when talking about dissociation. But I guess I could just keep it at they're proportional and are linked with their values. Small Ka is a small ionization percentage and leave it at that.

Answer 14

yeah thats good. Equilibrium constants tell you "how much", rate constants tell you "how fast". It's unfortunate that their symbols are K and k, though.

Answer 15

Yeah I probably got mixed up with that somewhere along the lines. Anyways, thank you for your time, I was lost there for awhile!

Answer 16

no problem, glad you got things figured out !