Salts

Question 1

What is the same and what is different in equimolar solutions of weak and strong acids or bases?

Answer 1

same-stoichiometry of reactions

different-pH values, conductivity, reaction rates

Question 2

what is the acid dissociation constant represented by?

Answer 2

Ka or by pKa

Ka = [H3O+][A-]/[HA]
where A- = conjugate base of acid
H3O+ = conc. of hydronium ions
HA= conc. of the acid

pKa = -logKa

Question 3

what formula is used for the approximate pH of a weak acid?

Answer 3

pH = 1/2pKa -1/2longc

Question 4

Difference between Ka, pKa and pH?

Answer 4

Ka=strength of an acid in solution (high value=strong acid) (a measure of the degree of dissociation)

pKa=indicates whether an acid is strong or weak (high pKa=weaker acid)

pH= concentration of hydronium ions

Question 5

What does a soluble salt of strong acid and strong base dissolve in water to produce?

Answer 5

neutral solution

Question 6

What does a soluble salt of weak acid and strong base dissolve in water to produce?

Answer 6

alkaline solution

Question 7

What does a soluble salt of strong acid and weak base dissolve in water to produce?

Answer 7

acidic solution

Question 8

how can the changes in concentration of H3O+ and OH- ion of salt solutions be explained?

Answer 8

using the appropriate equilibria

Question 9

What is a buffer solution?

Answer 9

one in which the pH remains approximately constant when small amounts of acid, base or water (diluted) are added

Question 10

What does an acid buffer consist of?

Answer 10

a solution of weak acid and one of its salts made from a strong base

(salt must relate to acid)

Question 11

how does an acid buffer solution work?

Answer 11

An acidic buffer contains a weak acid and its conjugate base.

When Acid (H⁺) is Added:
The conjugate base reacts with the extra H⁺ which neutralizes the added H⁺ and prevents a big drop in pH.

When Base (OH⁻) is Added:
The weak acid donates H⁺ to neutralize OH⁻ which minimizes the increase in pH.

Question 12

What does a basic buffer consist of?

Answer 12

a solution of a weak base and one of its (related) salts

Question 13

How does a basic buffer solution work?

Answer 13

A basic buffer contains a weak base and its conjugate acid (salt).

When Acid (H⁺) is Added:
The weak base reacts with the H⁺ which neutralizes the added H⁺, keeping pH from dropping much.

When Base (OH⁻) is Added:
The conjugate acid donates H⁺ to neutralize the OH⁻ which prevents the pH from rising sharply.

Question 14

What are indicators?

Answer 14

weak acids for which dissociation can be represented as:

HIn(aq) + H2O(l) <—> H3O+(aq) + In-(aq)

Question 15

In aqueous solution, what is the colour of an acid indicator distinctly different from?

Answer 15

That of its conjugate base

Question 16

How is the colour of an indicator determined?

Answer 16

ratio of [HIn] to [In-]

HIn = weak acid indicator

In- = conjugate base

Question 17

What is the theoretical point at which colour change occurs with an indicator?

Answer 17

when [H3O+] = KIn

Question 18

When is the indicator colour change assumed to be distinguishable?

Answer 18

when [HIn] and [In-] differ by a factor of 10

Question 19

What does the expression, pH = pKIn +- 1, estimate?

Answer 19

The pH range over which a colour change occurs

Question 20

salt from strong acid v strong alkali

Answer 20

pH=7 neutral
There are no weak ions from strong acids and strong alkalis

eg sodium chloride

Question 21

salt from weak acid v strong alkali

Answer 21

pH>7 alkaline

eg sodium ethanoate, potassium carbonate

Question 22

salt from strong acid v weak alkali

Answer 22

pH<7 acidic

eg ammonium chloride, ammonium nitrate

Question 23

How do you work out the pH of a salt

Answer 23

-Show the reaction
-take the products and split them into ions + their OH- and H+ from first reactants
-work out what side equilibria lies towards for each equation (ions or molecule)

Question 24

What is an example of a salt made from a weak acid and strong base

Answer 24

soaps (alkaline salt)

Question 25

What is the equation when dealing with ammonia/amines and why is it different?

Answer 25

NH3 + H2P <— -> NH4 + OH- Ammonia is a weak base. It doesn’t contain hydroxide ions but reacts with water to produce them

Question 26

What is the calculation to calculate the pH of a weak acid from its pam’s value and its concentration?

Answer 26

(Ka=[H3O+]x[A-]/[HA]) but [H3O+]=[A-] so Ka=[H3O+]^2/[HA]

Question 27

How do indicators change colour at different pHs?

Answer 27

Indicator is a weak acid If added to acid, H+ concentration increases, so equilibrium will shift to the left to use up H+ If added to alkali, OH- will react with H+, so equilibrium will shift to the right to produce more H+

Question 28

indicator for strong acid/strong base

Answer 28

indicator must change colour about pH 7

Question 29

indicator for strong acid/weak base

Answer 29

indicator must change colour about pH 5 methyl orange

Question 30

indicator for weak acid/strong base

Answer 30

indicator must change colour about pH 9 phenolphthalein

Question 31

indicator for a weak acid/weak base

Answer 31

titration hardly ever use the equivalence point changes from case to case

Question 32

How can buffers be used to calibrate pH meters?

Answer 32

To calibrate a pH meter, normally we use 3 types of buffer solutions: pH4, pH7 and pH10. The electrode will be placed inside each buffer (ascending pH order) and then the calibration button is pressed each time. When you use a pH meter you want to be sure that the pH meter is working ok before hand and displays the right pH values before you measure the pH of your required.

Question 33

Why does adding water to a buffer not change the pH?

Answer 33

The concentration of the acid and salt will change by the same amount