Calculating pH Of Strong And Weak Acids Flashcards
What is pH a measure of?
The concentration of H+ ions in solution
What does pH tell us?
How acid/alkaline a solution is
Lower pH= higher conc of H+
Higher pH= lower conc of H+
What method do we use to find the pH of a strong acid?
The equation for pH if we find [H+] using a log scale
Why can we find the pH of strong acids so easily?
We know the conc of the acid so because they fully dissociate into H+ ions, we know the conc of H+ ions is either equal or x2/x3
So can use the equation
Equation of pH
pH = -log10[H+]
monoprotic acids
Only dissociates 1 H+ ion so only 1 Hydrogen is in the acid
Finding pH of strong monoprotic acid
Ratio of conc of acid: conc of H+ is 1:1
So pH = -log(conc of acid)
Finding pH of strong polyprotic acid
Multiply conc of acid by 2 or 3 depending on how many H+ ions released in dissociation (same value as number of hydrogen in acid)
-log(H+)
How can we find the concentration of any acid (strong or weak) if we are given the pH?
Conc of H+ = 10⁻ᵖᴴ
Finding conc of a strong polyprotic acid if given the pH…
Conc of H+ = 10⁻ᵖᴴ
The conc of H+ ions is larger than conc of acid
So divide this value by 2 if conc of acid: H+ is 1:2 etc
What do we have to use to find pH of weak acids?
The acid dissociation constant Ka
Why do we need to use the acid dissociation constant to find pH of a weak acid?
Because finding the conc of H+ ions is hard as there isn’t a full dissociation of H+ ions so it isn’t equal (or proportion) to the conc of the acid
Ka expression for a weak acid
[H+] [whats left of acid]
——————————— = Ka
[HA]
How do we use the expression of Ka to find the concentration of H+ ions?
Rearrange by plugging in value of Ka and the conc of HA
Simplify [H+] [HA] to [H+]^2
Divide Ka by [HA] and square thus value
Assumptions made to find value of H+ using Ka
That [H+] and [A-] are equal so we can simplify to [H+]^2 because dissociation of acid is greater than dissociation of water so all H+ ions are from the acid, cos acid dissociates into [H+] and [A-] in a 1:1 ratio
That [HA] at start is [HA] at equilibrium cos dissociation amount is so small
Units of Ka always are…
moldm^-3
Once we have used value of Ka to find [H+] for weak acids, what now?
pH = -log(H+)
Why does using the Ka not work for strong acids?
Because the dissociation of acids is so much that [Acid] at equilibrium and [Acid] initially are not equal
Difference is too significant
Where does using the Ka even come from?
Because the dissociation of a weak acid is at equilibrium so we can use a type of equilibrium constant called acid dissociation constant using same method as equilibrium constant
How can we find the concentration of a weak acid if given the Ka and pH?
Use [H+] = 10⁻ᵖᴴ to find the concentration of H+ and A-
Rearrange Ka constant so:
[HA] = [H+]^2 divided by Ka
What is pKa?
A value that measures the strength of an acid but using a negative logarithmic scale to Ka
How do we find pKa?
pKa = -log(Ka)
Higher the value of Ka…
The stronger the acid, indicates the POE of dissociation of acid is further to the right as there is a higher conc of H+ ions thus higher value of Ka
What does a smaller value of pKa show?
The stronger the acid
How to find Ka from pKa
Ka = 10 ⁻ᵖᴷᵃ
