Calculating pH Of Strong And Weak Acids

Question 1

What is pH a measure of?

Answer 1

The concentration of H+ ions in solution

Question 2

What does pH tell us?

Answer 2

How acid/alkaline a solution is
Lower pH= higher conc of H+
Higher pH= lower conc of H+

Question 3

What method do we use to find the pH of a strong acid?

Answer 3

The equation for pH if we find [H+] using a log scale

Question 4

Why can we find the pH of strong acids so easily?

Answer 4

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

Question 5

Equation of pH

Answer 5

pH = -log10[H+]

Question 6

monoprotic acids

Answer 6

Only dissociates 1 H+ ion so only 1 Hydrogen is in the acid

Question 7

Finding pH of strong monoprotic acid

Answer 7

Ratio of conc of acid: conc of H+ is 1:1
So pH = -log(conc of acid)

Question 8

Finding pH of strong polyprotic acid

Answer 8

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+)

Question 9

How can we find the concentration of any acid (strong or weak) if we are given the pH?

Answer 9

Conc of H+ = 10⁻ᵖᴴ

Question 10

Finding conc of a strong polyprotic acid if given the pH…

Answer 10

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

Question 11

What do we have to use to find pH of weak acids?

Answer 11

The acid dissociation constant Ka

Question 12

Why do we need to use the acid dissociation constant to find pH of a weak acid?

Answer 12

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

Question 13

Ka expression for a weak acid

Answer 13

[H+] [whats left of acid]
——————————— = Ka
[HA]

Question 14

How do we use the expression of Ka to find the concentration of H+ ions?

Answer 14

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

Question 15

Assumptions made to find value of H+ using Ka

Answer 15

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

Question 16

Units of Ka always are…

Answer 16

moldm^-3

Question 17

Once we have used value of Ka to find [H+] for weak acids, what now?

Answer 17

pH = -log(H+)

Question 18

Why does using the Ka not work for strong acids?

Answer 18

Because the dissociation of acids is so much that [Acid] at equilibrium and [Acid] initially are not equal
Difference is too significant

Question 19

Where does using the Ka even come from?

Answer 19

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

Question 20

How can we find the concentration of a weak acid if given the Ka and pH?

Answer 20

Use [H+] = 10⁻ᵖᴴ to find the concentration of H+ and A-
Rearrange Ka constant so:
[HA] = [H+]^2 divided by Ka

Question 21

What is pKa?

Answer 21

A value that measures the strength of an acid but using a negative logarithmic scale to Ka

Question 22

How do we find pKa?

Answer 22

pKa = -log(Ka)

Question 23

Higher the value of Ka…

Answer 23

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

Question 24

What does a smaller value of pKa show?

Answer 24

The stronger the acid

Question 25

How to find Ka from pKa

Answer 25

Ka = 10 ⁻ᵖᴷᵃ