Salts Flashcards

1
Q

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

A

same-stoichiometry of reactions

different-pH values, conductivity, reaction rates

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2
Q

what is the acid dissociation constant represented by?

A

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

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3
Q

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

A

pH = 1/2pKa -1/2longc

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4
Q

Difference between Ka, pKa and pH?

A

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

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5
Q

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

A

neutral solution

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6
Q

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

A

alkaline solution

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7
Q

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

A

acidic solution

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8
Q

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

A

using the appropriate equilibria

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9
Q

What is a buffer solution?

A

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

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10
Q

What does an acid buffer consist of?

A

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

(salt must relate to acid)

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11
Q

how does an acid buffer solution work?

A

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.

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12
Q

What does a basic buffer consist of?

A

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

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13
Q

How does a basic buffer solution work?

A

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.

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14
Q

What are indicators?

A

weak acids for which dissociation can be represented as:

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

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15
Q

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

A

That of its conjugate base

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16
Q

How is the colour of an indicator determined?

A

ratio of [HIn] to [In-]

HIn = weak acid indicator

In- = conjugate base

17
Q

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

A

when [H3O+] = KIn

18
Q

When is the indicator colour change assumed to be distinguishable?

A

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

19
Q

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

A

The pH range over which a colour change occurs

20
Q

salt from strong acid v strong alkali

A

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

eg sodium chloride

21
Q

salt from weak acid v strong alkali

A

pH>7 alkaline

eg sodium ethanoate, potassium carbonate

22
Q

salt from strong acid v weak alkali

A

pH<7 acidic

eg ammonium chloride, ammonium nitrate

23
Q

How do you work out the pH of a salt

A

-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)

24
Q

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

A

soaps (alkaline salt)

25
Q

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

A

NH3 + H2P <— -> NH4 + OH-

Ammonia is a weak base. It doesn’t contain hydroxide ions but reacts with water to produce them

26
Q

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

A

(Ka=[H3O+]x[A-]/[HA])

but [H3O+]=[A-]
so

Ka=[H3O+]^2/[HA]

27
Q

How do indicators change colour at different pHs?

A

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+

28
Q

indicator for strong acid/strong base

A

indicator must change colour about pH 7

29
Q

indicator for strong acid/weak base

A

indicator must change colour about pH 5

methyl orange

30
Q

indicator for weak acid/strong base

A

indicator must change colour about pH 9

phenolphthalein

31
Q

indicator for a weak acid/weak base

A

titration hardly ever use

the equivalence point changes from case to case

32
Q

How can buffers be used to calibrate pH meters?

A

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.

33
Q

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

A

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