Acid base equilibria

Question 1

what is PH defined as in formula

Answer 1

-log[H+]
where [H+] is the concentration of hydrogen ions in mol dm–3

Question 2

How can you calculate concentration of hydrogen ions

Answer 2

[H+] = 10-pH

Question 3

What is a strong acid

Answer 3

an acid that dissociates almost completely in aqueous solutions

Question 4

what is a weak acid

Answer 4

an acid that partially dissociates in aqueous solutions so fewer hydrogen ions are formed

Question 5

What is the Ka expression

Answer 5

acid dissociation constant and has the units mol dm-3

Question 6

why is the concentration of hydrogen ions assumed

Answer 6

due to the ionisation of water is negligible

Question 7

what does a higher Ka value indicate

Answer 7

the more dissociated the acid and the stronger it is

Question 8

Why is total [H+] the same as the [HA]

Answer 8

The number of hydrogen ions formed from the ionisation of water is very small relative to the [H+] due to ionisation of the strong acid and can therefore be neglected

Question 9

what does the ionisation of sulphuric acid suggest about the concentration of hydrogen in dibasic acids

Answer 9

H2SO4 → HSO4- + H+

HSO4- ⇌ SO42- + H+

the first step is thought to be fully ionised, the second step is suppressed by the abundance of hydrogen ions from the first step creating an equilibrium

The result is that the hydrogen ion concentration is less than double the acid concentration

Question 10

When can the pH of weak acids be calculated

Answer 10

when the following are known:
- The concentration of the acid
- The Ka value of the acid

Question 11

what can we assume about Ka expression of equilibrium concentration of [H+] and [A-]

Answer 11

the same since one molecule of HA dissociates into one of each ion

Question 12

What is Kw

Answer 12

[H+ (aq) ][OH- (aq) ]

for all aqueous solutions is 1x10-14 mol2dm-6

Question 13

what is the relationship between Kw and pKw

Answer 13

pKw = -logKw

Question 14

what can you tell from a pH curve

Answer 14

pH of the acid
pH at equivalence point
volume of base at equivalence point

Question 15

Draw the pH curve for:
strong acid + strong base
weak acid + strong base
strong acid + weak base
weak acid + weak base

Question 16

what is an alkali

Answer 16

a soluble base

Question 17

what does end point refer to in comparison to equivalence point

Answer 17

change in colour of indicator whereas equivalence point is volume of base required to neutralise the acid

Question 18

what is a buffer solution

Answer 18

a solution which resists changes in pH when small amounts of acid or alkalis are added

Question 19

what happens when hydroxide ions are added to buffer solution

Answer 19

hydrogen ions react with them to form water and decrease in hydrogen ions would mean the pH will increase but equilibrium move to right to replace removed hydrogen ions and keep pH constant

Question 20

how is a acidic buffer solution created

Answer 20

from a weak acid and one of its salts

Question 21

what does a buffer consist of

Answer 21

weak acid and its conjugate base or a weak base and its conjugate acid

Question 22

what exactly is half equivalence point

Answer 22

stage of the titration at which exactly half the amount of weak acid has been neutralised

Question 23

The equivalence point in a weak acid - strong base titration is….

Answer 23

above 7

Question 24

whats a key thing to talk about in titration curve questions

Answer 24

ph range lies completely in vertical region and indicator will change colour in vertical region

Question 25

devise an experiment to find the Ka of a solution of ethanoic acid with access to a pH meter and a solution of NaOH

Answer 25

titrate ethanoic acid with sodium hydroxide
measure pH at regular intervals and plot a pH graph against volume
use the graph to find the pH half equivalence point

use phenolphthalein to find end point and add the same volume of acid to the mixture at end point
measure pH of resultant mixture with a pH meter

Ka = 10-pH

Question 26

how to obtain Ka from titration curve

Answer 26

read pH at half equivalence point as pH is pKa

Question 27

what happens if you use an unsuitable indicator for a titration

Answer 27

colour change will be complete before equivalence point is reached

Question 28

when can you use equilibrium amount in moles rather than equilibrium concentrations for calculating Kc

Answer 28

Same number of moles/molecules on both sides of the equation
(1)
(so) volume / V cancels in K expression

Question 29

why does pink colour of phenolphthalein fade after end point of titration is reached

Answer 29

the equilibrium shifts to the left
ОГ
the mixture absorbs carbon dioxide from the atmosphere
(1)
so the mixture is (becoming more)
acidic / the acid reforms

Question 30

describe a buffer region for potassium hydroxide and propanoic acid

Answer 30

buffering region

as a result of both acid and its conjugate base

as excess Ch3Ch2COOh has reacted with KOH

Question 31

Why is a pH greater than 7 formed using potassium hydroxide and propanoic acid

Answer 31

propanoate ions are present (conjugate base) at equivalence point

proponoate ions react with water to form KOH in excess

OH > H+

weak acid strong base titration

Question 32

How to find PH in such circumstances

HA in excess
A- in excess
HA = A-

Answer 32

HA in excess = Use [HA] and [A] along

A- in excess = with Ka to find [H*], then pH.

HA = A-
Use Kw to find [H*], then pH.
In this case, pKa is equal to pH, therefore find pa.