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Relationships between ka, pka, and ph

Posted: Sun Dec 08, 2019 12:15 am
by Frederick Keith_4C
What exactly is the relationship between Ka, pKa, and pH? I don't understand how to calculate them or what they mean either... Someone please explain before the final tomorrow morning ;-;

Re: Relationships between ka, pka, and ph

Posted: Sun Dec 08, 2019 2:19 am
by Julie_Reyes1B
For the scope of this class, KA is just a measure of the relative strength of an acid. The higher the KA, the stronger the acid. The pKA is the -log KA. The lower the pKA, the stronger the acid. There is also KB and pKB (the same definitions but for a base instead. Usually, you will be given KA and asked to solve for pKA, or vice versa. You would use the equation above.

pH is a measure of whether something is an acid or a base. For this class, anything pH<7 is acidic, pH>7 is basic, and pH=7 is neutral (values will fall between 0 and 14). In terms of calculation, pH is the -log of the concentration of H+ ions. You can also calculate pH if you are given pOH, with the equation pH+pOH=14. If you are given concentration of OH- ions, you can find pOH and solve for pH as well.

pH and pKA are not directly related through calculations. But, something with a low pH (indicates a strongish weak acid) will most likely have a low pKA too.

Re: Relationships between ka, pka, and ph

Posted: Sun Dec 08, 2019 2:52 am
by Ashley Kim 3F
Ka is the strength of an acid - a high Ka value means that the acid is strong, while a low Ka value indicates a weak acid. pKa is inverse - a high pKa value means that the acid is weak, while a low pKa value means that the acid is strong.
pH is -log[H3O+] and is a measure of the concentration of hydronium ions.

Re: Relationships between ka, pka, and ph

Posted: Sun Dec 08, 2019 2:59 am
by rachel liu 3k
Julie_Reyes1G wrote:For the scope of this class, KA is just a measure of the relative strength of an acid. The higher the KA, the stronger the acid. The pKA is the -log KA. The lower the pKA, the stronger the acid. There is also KB and pKB (the same definitions but for a base instead. Usually, you will be given KA and asked to solve for pKA, or vice versa. You would use the equation above.

pH is a measure of whether something is an acid or a base. For this class, anything pH<7 is acidic, pH>7 is basic, and pH=7 is neutral (values will fall between 0 and 14). In terms of calculation, pH is the -log of the concentration of H+ ions. You can also calculate pH if you are given pOH, with the equation pH+pOH=14. If you are given concentration of OH- ions, you can find pOH and solve for pH as well.

pH and pKA are not directly related through calculations. But, something with a low pH (indicates a strongish weak acid) will most likely have a low pKA too.

So there is no way to get from Ka to pH? Because there are no equations to relate the two? Hope you can help, thanks!

Re: Relationships between ka, pka, and ph

Posted: Sun Dec 08, 2019 3:00 am
by rachel liu 3k
Julie_Reyes1G wrote:For the scope of this class, KA is just a measure of the relative strength of an acid. The higher the KA, the stronger the acid. The pKA is the -log KA. The lower the pKA, the stronger the acid. There is also KB and pKB (the same definitions but for a base instead. Usually, you will be given KA and asked to solve for pKA, or vice versa. You would use the equation above.

pH is a measure of whether something is an acid or a base. For this class, anything pH<7 is acidic, pH>7 is basic, and pH=7 is neutral (values will fall between 0 and 14). In terms of calculation, pH is the -log of the concentration of H+ ions. You can also calculate pH if you are given pOH, with the equation pH+pOH=14. If you are given concentration of OH- ions, you can find pOH and solve for pH as well.

pH and pKA are not directly related through calculations. But, something with a low pH (indicates a strongish weak acid) will most likely have a low pKA too.

Also do you remember what professor was saying about how there is not pKa or Ka or a strong acid or pKb or Kb of a strong base because it will be infinity? why is that?

Re: Relationships between ka, pka, and ph

Posted: Sun Dec 08, 2019 3:17 am
by Julie_Reyes1B
rachel liu 3k wrote:
Julie_Reyes1G wrote:For the scope of this class, KA is just a measure of the relative strength of an acid. The higher the KA, the stronger the acid. The pKA is the -log KA. The lower the pKA, the stronger the acid. There is also KB and pKB (the same definitions but for a base instead. Usually, you will be given KA and asked to solve for pKA, or vice versa. You would use the equation above.

pH is a measure of whether something is an acid or a base. For this class, anything pH<7 is acidic, pH>7 is basic, and pH=7 is neutral (values will fall between 0 and 14). In terms of calculation, pH is the -log of the concentration of H+ ions. You can also calculate pH if you are given pOH, with the equation pH+pOH=14. If you are given concentration of OH- ions, you can find pOH and solve for pH as well.

pH and pKA are not directly related through calculations. But, something with a low pH (indicates a strongish weak acid) will most likely have a low pKA too.

So there is no way to get from Ka to pH? Because there are no equations to relate the two? Hope you can help, thanks!


There actually is a way to get from KA to pH, but take this explanation with a grain of salt. Here is an equation Dr. Lavelle hinted at very briefly:
KA = [H+][A-]/[HA]. If you were given KA, [A-], and [HA] explicitly, you could technically rearrange the equation and solve for [H+]. Then you could do pH=-log[H+] to find the pH. This is the only type of problem he could give you to find pH given the KA (and I highly doubt it, since it would just be a straight calculation). If you weren't given this information explicitly, you would need to create an ICE chart. However, ICE charts are the first unit of Chem 14B. Dr. Lavelle said explicitly you will NOT need to know them for the final. So yes, there is a way to find one given the other, but besides the unlikely situation I gave you, you do not yet have the tools to calculate it. So I would not worry too much :)

Re: Relationships between ka, pka, and ph

Posted: Sun Dec 08, 2019 3:31 am
by Julie_Reyes1B
rachel liu 3k wrote:
Julie_Reyes1G wrote:For the scope of this class, KA is just a measure of the relative strength of an acid. The higher the KA, the stronger the acid. The pKA is the -log KA. The lower the pKA, the stronger the acid. There is also KB and pKB (the same definitions but for a base instead. Usually, you will be given KA and asked to solve for pKA, or vice versa. You would use the equation above.

pH is a measure of whether something is an acid or a base. For this class, anything pH<7 is acidic, pH>7 is basic, and pH=7 is neutral (values will fall between 0 and 14). In terms of calculation, pH is the -log of the concentration of H+ ions. You can also calculate pH if you are given pOH, with the equation pH+pOH=14. If you are given concentration of OH- ions, you can find pOH and solve for pH as well.

pH and pKA are not directly related through calculations. But, something with a low pH (indicates a strongish weak acid) will most likely have a low pKA too.

Also do you remember what professor was saying about how there is not pKa or Ka or a strong acid or pKb or Kb of a strong base because it will be infinity? why is that?

KA measures the extent to which an acid dissociates, and since a strong acid dissociates completely/infinitely, the value is basically infinity. If you want to know why, I can explain in more depth below (but for all intents and purposes, explaining it that way is enough for the final).
I'm going to give you kind of a math-y explanation, so bear with me lol. This has to to with the definition of strong acids/bases. In theory, strong acids and bases dissociate completely in water. So the strong acid HCl, for example, will dissociate completely into H+ and Cl- in water. There will be no HCl molecules left.
Mathmatically, KA=[H+][A-]/[HA]. So for HCl it would be KA=[H+][Cl-]/[HCl]. But as I said, since the acid completely dissociates, there is no concentration of HCl left. So [HCl] would equal 0. If we substitute that in, we get KA=[H+][Cl-]/0. Anything with a denominator of 0 approaches infinity. That's the theory behind it, but probably something Dr. Lavelle will explain more in Chem 14B.

Re: Relationships between ka, pka, and ph

Posted: Sun Dec 08, 2019 10:40 am
by rachel liu 3k
Julie_Reyes1G wrote:
rachel liu 3k wrote:
Julie_Reyes1G wrote:For the scope of this class, KA is just a measure of the relative strength of an acid. The higher the KA, the stronger the acid. The pKA is the -log KA. The lower the pKA, the stronger the acid. There is also KB and pKB (the same definitions but for a base instead. Usually, you will be given KA and asked to solve for pKA, or vice versa. You would use the equation above.

pH is a measure of whether something is an acid or a base. For this class, anything pH<7 is acidic, pH>7 is basic, and pH=7 is neutral (values will fall between 0 and 14). In terms of calculation, pH is the -log of the concentration of H+ ions. You can also calculate pH if you are given pOH, with the equation pH+pOH=14. If you are given concentration of OH- ions, you can find pOH and solve for pH as well.

pH and pKA are not directly related through calculations. But, something with a low pH (indicates a strongish weak acid) will most likely have a low pKA too.

Also do you remember what professor was saying about how there is not pKa or Ka or a strong acid or pKb or Kb of a strong base because it will be infinity? why is that?

KA measures the extent to which an acid dissociates, and since a strong acid dissociates completely/infinitely, the value is basically infinity. If you want to know why, I can explain in more depth below (but for all intents and purposes, explaining it that way is enough for the final).
I'm going to give you kind of a math-y explanation, so bear with me lol. This has to to with the definition of strong acids/bases. In theory, strong acids and bases dissociate completely in water. So the strong acid HCl, for example, will dissociate completely into H+ and Cl- in water. There will be no HCl molecules left.
Mathmatically, KA=[H+][A-]/[HA]. So for HCl it would be KA=[H+][Cl-]/[HCl]. But as I said, since the acid completely dissociates, there is no concentration of HCl left. So [HCl] would equal 0. If we substitute that in, we get KA=[H+][Cl-]/0. Anything with a denominator of 0 approaches infinity. That's the theory behind it, but probably something Dr. Lavelle will explain more in Chem 14B.

This was super helpful!! Thanks so much!