What is pKa?
pKa is the quantitative measure of the strength of an acid. It tells us about how strong an acid holds a proton.
pKa is equivalent to the negative logarithm of Ka.
pKa= -logKa
- It tells us about how strong or weak an acid is.
- It is measured in mol/l.
What is Ka?
Ka or acid dissociation constant is the quantitative measure of the strength of an acid. It tells us about how much an acid dissociates in an aqueous solution.
- More the Ka more would be its dissociation in water.
- It is used to distinguish strong acids from weak acids.
Table of Content
- Understanding Ka and pKa in detail
- pKa to Ka
- pKa value for acid
- What is pH?
- Frequently Asked Questions – FAQs
Understanding Ka and pKa in detail
Ka pKa are essential tools for predicting whether a species will donate or accept protons at a specific pH value.
Ka or acid dissociation constant is the quantitative measure of the strength of an acid. It tells us about how much an acid dissociates in an aqueous solution.
- More the Ka more would be its dissociation in water.
- It is used to distinguish strong acids from weak acids.
pKa is the quantitative measure of the strength of an acid. It tells us about how strong an acid holds a proton.
pKa is equivalent to the negative logarithm of Ka.
pKa= -logKa
- It tells us about how strong or weak an acid is.
- It is measured in mol/l
Consider dissociation of acid HA:
HA ⥦ H+ + A–
For the above reaction, the dissociation constant Ka would be equivalent to
Whereas
pKa= -logKa
Here, quantities in the square brackets symbolise the concentration at equilibrium.
A strong acid can dissociate into its constituent ions rapidly. Thus we can conclude that a strong acid has higher Ka. If the acid-base reaction is favoured in the right direction, i.e. the concentration of both H+ and A- would be more, then the value of Ka would increase.
pKa to Ka
We can convert pKa to Ka as
pKa = –log Ka
– pKa = log Ka
10-(pKa) = Ka
Ka = 10-(pKa).
The dissociation constant value for strong acid is as high as 107, while for weak acid, it is as low as 10-12, which is quite tricky to remember. So to ease that, pKa came into existence.
We know that
pKa = -log Ka.
On putting the value of the dissociation constant in the equation, we get -7 pKa for a strong acid and 12 pKa for a weak acid.
Thus we can conclude that the lesser the pKa, the stronger the acid.
pKa value for acid
For a weak acid, the value of pKa varies from -2 to 12.
In contrast, for a strong acid, the value of pKa is less than -2
Value of pKa of a few acids
| S No | Name of the Acid | pKa value |
|---|---|---|
| 1 | Hydroiodic Acid | −9.30 |
| 2 | Sulfuric Acid | −2.00 |
| 3 | Nitric Acid | -1.37 |
| 4 | Hydronium Ion | 0.00 |
| 5 | Hydrofluoric Acid | 3.20 |
| 6 | Ammonium Ion | 9.25 |
| 7 | Water | 14.00 |
| 8 | Acetylene | 26.00 |
| 9 | Ammonia | 35.00 |
What is pH?
pH is the power of Hydrogen ions in an aqueous.
- It tells us about the concentration of hydrogen ions present in an aqueous solution.
- pH is equivalent to the negative logarithm of hydrogen ions.
pH= -logH+
- A pH of less than 7 indicates that the solution is acidic, while more than 7 indicates that the solution is basic.
- A neutral solution has a pH equivalent to 7.
Frequently Asked Questions on pKa to Ka
What is pKa?
pKa is the quantitative measure of the strength of an acid. It tells us about how strong an acid holds a proton.
pKa is equivalent to the negative logarithm of Ka.
pKa= – logKa
What is Ka?
Ka or acid dissociation constant is the quantitative measure of the strength of an acid. It tells us about how much an acid dissociates in an aqueous solution.
Consider dissociation of acid HA:
HA ⥦ H+ + A-
For the above reaction, the dissociation constant would be equivalent to
How can you convert pKa to Ka?
We can convert pKa to Ka as
pKa = –log Ka
– pKa = log Ka
10-(pKa) = Ka
Ka = 10-(pKa)
What is the significance of pKa?
The dissociation constant value for strong acid is as high as 107, while for weak acid, it is as low as 10-12, which is quite challenging to remember. So to ease that, pKa came into existence.
We know that
pKa = -log Ka.
On putting the value of the dissociation constant in the equation, we get -7 pKa for a strong acid and 12 pKa for a weak acid.
What is pH?
pH is the power of hydrogen ions in an aqueous.
It tells us about the concentration of hydrogen ions present in an aqueous solution.
pH is equivalent to the negative logarithm of hydrogen ions.
pH= -logH+
- A pH of less than 7 indicates that the solution is acidic, while more than 7 indicates that the solution is basic.
- A neutral solution has a pH equivalent to 7.
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