20 - Acids, bases and pH

Question 1

A Bronsted lowry acid

Answer 1

proton donor

Question 2

A Bronsted lowry base

Answer 2

proton acceptor

Question 3

Conjugate acid-base pairs

Answer 3

• two species that can be interconverted by transfer of a proton

Question 4

how can water act as a base

Answer 4

• accepts proton and becomes positive ion

Question 5

monobasic, dibasic, tribasic

Answer 5

• Refers to the total number of hydrogen ions in the acid that can be replaced per molecule in an acid- base reaction

Question 6

monobasic, dibasic, tribasic -HCL / CH3COOH / H2C02 / H3BO3

Answer 6

Question 7

low value of [H+]…

Answer 7

high value of pH

Question 8

high value of [H+]

Answer 8

low value of pH

Question 9

how is pH measured

Answer 9

logarithmic scales of hydrogen ions

Question 9

equation to convert H+ into pH

Answer 9

Question 10

equation to convert pH into H+

Answer 10

Question 11

dilution needed to convert a solution from pH 1 to pH 4

Answer 11

10 x 10 x 10 = 1000

dilution by factor of 1000

Question 12

general equation for strong (monobasic) acid dissociation

Answer 12

• completely dissociates

Question 13

for a strong acid dissociation what is special and needed for calculations

Answer 13

concentration of HA = concentration of H+

therefore - instead of using conc of H+ to find pH, can use conc of HA in -log equation

Question 14

Answer 14

Question 15

Answer 15

Question 16

dilution -

Answer 16

Question 17

strong and weak acids

Answer 17

fully vs partially dissociates

Question 18

Ka

Answer 18

acid dissociation constant

Question 19

Larger the value of Ka..

Answer 19

the further the equilibrium is to the right

greater dissociation and greater acid strength

Question 20

Ka formula

Answer 20

Question 21

why use pKa instead of Ka

Answer 21

Hard to compare Ka values because they have negative indices

Question 22

convert ka into pKa

Answer 22

Question 23

convert pKa into Ka

Answer 23

Question 24

if Ka increases... (effect on pKa)

Answer 24

pKa decreases

Question 25

how to calculate Ka from equilibrium concentrations

Answer 25

Question 26

approx 1 - Ka

Answer 26

* HA dissociates to produce equilibrium concentration of H+ and A- that are equal * Small concentration of H+ released from dissociation of water but this will be so small, it can be neglected compared to H+ of acid The contribution of H+ from the water is negligible.

Question 27

approx - 2 Ka

Answer 27

The dissociation of the HCOOH is small enough for the equilibrium concentration of HCOOH to be taken as the same as the initial concentration.

Question 28

problems with approx 1

Answer 28

* If pH > 6 = then conc of H+ from dissociation of water will be significant compared to dissociation of weak acid

Question 29

problems with approx 2

Answer 29

* Assumes that conc of acid is greater than the H+ conc at equilibrium * Fine for weak acids with low Ka values * When conc of H+ INCREASES = ‘stronger’ weak acids = it breaks down * Difference between conc of acid at equilibrium and conc of acid at start – conc of H+ at equilibrium doesnt work for stronger weak acids or very dilute solutions where the difference is significant

Question 30

using approx - how can Ka be simplified for equilibrium

Answer 30

Question 31

Calculate pH from Ka

Answer 31

* To calculate pH, need to find [H+] first. * Rearrange formula for H+ conc

Question 32

determination of Ka experimentally

Answer 32

* preparing a standard solution of weak acid of known concentration * measuring pH of standard solution using a pH meter * pH= acid concentration calculation or pH measurement

Question 33

how can water be an acid and a base

Answer 33

Question 34

Kw

Answer 34

ionic product of water - conc of H+ and OH- multiplied together

Question 35

Kw - 1 x 10^-14 calculate the pH of water

Answer 35

Question 36

For pH with whole numbers how to work out conc of H+ and OH-

Answer 36

indices add up to 14

Question 37

if pH is not a whole number how to work out conc of H+ and OH-

Answer 37

Question 38

how to find pH of a strong base

Answer 38

- as strong base - conc of base = conc of OH- - use OH- and Kw to find conc of H+ - minus log it

Question 39

state the two assumptions for ka / weak acid calculations

Answer 39

The contribution of H+ from the water is negligible. The dissociation of the HCOOH is small enough for the equilibrium concentration of HCOOH to be taken as the same as the initial concentration.