At 298K, water molecules dissociate into equal quantities of ions, and the pH is 7.
i)
Write an equation to show the dissociation of water.
ii)
At 313 K, the pH of water is 6.77. Explain why water is still neutral with a pH of 6.77.
Question 1b
Marks: 5
b)
The ionic product of water, Kw, can be used to find the pH of a strong base. Changing the temperature will affect the value for Kw.
i)
Give the expression and units for the ionic product of water, Kw
ii)
As temperature is increased, the value for Kw also increases. Explain why.
Question 1c
Marks: 3
c)
Determine the pH of pure water at 40 ℃.
Kw of pure water at 40 ℃ is 2.92 x 10-14 mol2 dm-3
Question 1d
Marks: 3
d)
Strong bases fully ionise in water, as shown by the equation of dissociation of sodium hydroxide:
NaOH (aq) → Na+ (aq) + OH- (aq)
At 298K, Kw is 1 x 10-14 mol2 dm-6.
Calculate the pH of a 0.05 mol dm-3 solution of NaOH at 298 K.
Question 2a
Marks: 1
a)
Weak acids do not fully ionise in solution. The acid dissociation constant, Ka is used to determine the hydrogen ion concentration.
Write an expression for the acid dissociation constant, Ka for the acid HA.
Question 2b
Marks: 3
b)
The pH of a 0.15 mol dm-3 solution of HCN is 5.08 at 298 K. Calculate the value of Ka for HCN at 298 K.
Give your answer to two decimal places.
Question 2c
Marks: 4
c)
A sample of 0.01 mol dm-3 butanoic acid has a Ka value of 1.51 x 10-5 mol dm-3.
i)
Write an expression for the acid dissociation constant, Ka, for butanoic acid.
ii)
Calculate the pH of the 0.01 mol dm-3 butanoic acid. Give your answer to two decimal places.
Question 2d
Marks: 4
d)
0.50 moles of ammonia was dissolved in water to make a 1.00 dm3 solution. This solution has a hydroxide ion concentration of 6.40 x 10-3 mol dm-3.
i)
Write an expression for the base dissociation constant, Kb, of ammonia.
ii)
Calculate a value for pKb for ammonia.
Question 3a
Marks: 3
a)
The pH curve shown below was obtained when a 0.150 mol dm–3 solution of sodium hydroxide was added to 25.0 cm3 of an aqueous solution of ethanoic acid.The half equivalence point is where halfof the volume of sodium hydroxide required for neutralisation has been added to the ethanoic acid.
i)
Label the graph with an X to show the position of the half equivalence point.
ii)
When half of the ethanoic acid solution has been neutralised, the remaining ethanoic acid concentration is equal to that of the sodium ethanoate that had formed. Calculate the pH at this point.
Ka of ethanoic acid = 1.75 x 10-5 mol dm-3.
Question 3b
Marks: 6
b)
A different titration was performed using 0.100 mol dm-3 ammonia solution, NH3 (aq) and 25.00 cm3 of 0.100 nitric acid, HNO3 (aq).
Using Section 21 of the Data Booklet, calculate the pH of the ammonia solution before it was added to the nitric acid.
Question 3c
Marks: 2
c)
The titration is repeated using 0.200 mol dm-3 sodium hydroxide, NaOH (aq), instead of ammonia.
Determine whether the salt formed in this titration will be acidic, basic or neutral.
Question 3d
Marks: 5
d)
Determine the pH of the solution if 150 cm3 of 0.30 mol dm-3 sodium hydroxide, NaOH (aq), is mixed with 200 cm3 0.10 mol dm-3 of nitric acid, HNO3 (aq).
Question 4a
Marks: 1
a)
Monochloroacetic acid, ClH2COOH, is a skin irritant that is used in “chemical peels” intended to remove the top layer of dead skin from the face and ultimately improve the complexion.
Write an expression for the acid dissociation constant, Ka, of monochloroacetic acid.
Question 4b
Marks: 4
b)
Calculate the pH of a 0.05 M solution of monochloric acid.
The value of Ka for monochloroacetic acid is 1.35 x 10-3 mol dm-3
Question 4c
Marks: 2
c)
Using Section 2 of the Data Booklet, calculate the value of [OH-] for the solution of monochloric acid.
Question 4d
Marks: 2
d)
Calculate the percentage dissociation for the solution of monochloric acid.
Question 5a
Marks: 2
a)
State the relationship between the following expressions for conjugate acid-base pair
i)
Ka and Kb
ii)
pKa and pKb
Question 5b
Marks: 4
b)
Using Section 21 of the Data Booklet, calculate the following for the conjugate bases at 298 K.
i)
pKb of CH3CH2COO-
ii)
Kb of CH(Cl2)COO-
iii)
Ka of CH(CH2)2COO-
Question 5c
Marks: 4
c)
A student performs a titration using a 0.10 mol dm-3 ammonia, NH3 (aq), and a hydrochloric acid and 0.10 hydrochloric acid, HCl (aq).
Volume of HCl (cm3)
i)
State the equation for the overall reaction that is occurring.
ii)
Mark on the curve the point at which the pOH is equal to pKb of the weak base and deduce the pKb of the acid.
Question 5d
Marks: 1
d)
The student repeats the titration with 0.10 mol dm-3 ethanoic acid, CH3COOH (aq) which has a pKa value of 4.76. A sketch of the pH curve obtained is shown below.
Explain why it is difficult to determine the equivalence point for this reaction accurately.
