Acids, bases and pH

Question 1

Define a Bronsted-Lowry acid

Answer 1

Proton donor

Question 2

Define a Bronsted-Lowry base

Answer 2

Proton acceptor

Question 3

What 2 ions causes a solution to become acidic?

Answer 3

H+ and H3O+ (if H2O is present)

Question 4

What ion causes a solution to become alkaline?

Answer 4

OH-

Question 5

Give an example of a monobasic acid?

Answer 5

HCL

Question 6

Give an example of a dibasic acid?

Answer 6

H2SO4

Question 7

Give an example of a tribasic acid?

Answer 7

H3PO4

Question 8

What is the difference between concentrated and strong?

Answer 8

Concentration means mol per dm-3 and strong means the dissociation

Question 9

What is the acid dissociation constant called?

Answer 9

Ka

Question 10

What does a larger Ka value mean?

Answer 10

The greater the dissociation

Question 11

Write the equation used to convert Ka into pKa

Answer 11

pKa = -log(Ka)

Question 12

Write the equation used to convert pKa into Ka

Answer 12

10^-pKa

Question 13

What is the relationship between pKa and strength of the acid?

Answer 13

Smaller the pKa the stronger the acid

Question 14

Write the equation used to convert concentration of H+ into pH

Answer 14

pH = -log(H+)

Question 15

Write the equation used to convert pH into concentration of H+

Answer 15

(H+) = 10^-pH

Question 16

If 2 solutions have a pH difference of 1, what is the difference in (H+)?

Answer 16

A factor of 10

Question 17

(H+) of a strong acid is equal to what?

Answer 17

(H+) = (HA)

Question 18

Write the equation used to calculate the (H+) of weak acids?

Answer 18

(H+)2 = Ka*(HA)

Question 19

What 2 assumptions are made when calculating pH of weak acids?

Answer 19

1. HA dissociate to produce equilibrium concentrations of H+ and A- that are equal
2. (HA)eqm=(HA)start

Question 20

Write the expression for the ionic product of water, Kw

Answer 20

Kw= ( H+) (OH-)

Question 21

What is the value of Kw at 298k

Answer 21

1.0*10^-14

Question 22

What physical factors affect the value of Kw?

Answer 22

Temp only - if temp is increased, the equilibrium moves to the right so Kw increases and the pH of water decreases

Question 23

Write the equation used to calculate (H+) of strong bases

Answer 23

(H+)=Kw/OH-

Question 24

A solution of calcium dihydrogenphosphate, Ca(H2PO4)2, in water acts as a buffer solution. Suggest, with the aid of equations, how this buffering action takes place.

Answer 24

• H2(PO4)2 produced from Ca(H2PO4)2
• H2PO4+ H+ ⇌ H3PO4 acts as a base
• H2PO4+ OH– ⇌ H2O + HPO42–

Question 25

State what is meant by a buffer solution.

Answer 25

A solution that minimises/resists pH changes

Question 26

What is the equation for buffers?

Answer 26

[H+] = Ka × [HA] / [A-]

Question 27

What factors determine the pH of a buffer solution?

Answer 27

• Temperature
• Concentration of weak acid and conjugate base

Question 28

Nitric acid, HN03, is sold by a chemical supplier as a 65% solution, by mass. As
supplied, each cubic decimetre of this nitric acid has a mass of 1400g.
Calculate the pH of this solution.

Answer 28

1. Calculate 65% of 1400g
2. Divide it by the Mr of HNO3
3. pH = –log[H+]

Question 29

Give one reason why the pH scale is a
more convenient measurement for measuring acid concentrations than [H+]

Answer 29

[H+] deals with negative indices over a very wide range

Question 30

A student adds an excess of aqueous ethanoic acid to solid calcium carbonate.The resulting solution is able to act as a buffer solution, explain why.

Answer 30

Solution contains both CH3COOH and CH3COO-

Question 31

The student adds 50.0 cm3 of 0.250 mol dm–3 butanoic acid to 50.0 cm3 of 0.0500 mol dm–3
sodium hydroxide. A buffer solution forms.
(i) Explain why a buffer solution forms.

Answer 31

• CH3(CH2)2COO– forms
• CH3(CH2)2COOH is in excess