18.2 Calculations Involving Acids & Bases

Nitric acid, HNO₃, and hydrocyanic acid, HCN, can be made from ammonia. Hydrocyanic acid has a pK_a of 9.21.

Formulate equations for the dissociation of each acid and distinguish between the terms strong and weak in this context.

Write an expression for the acid dissociation constant, K_a, of hydrocyanic acid and calculate the K_a at 298 K.

Determine the hydrogen ion concentration and pH of 0.15 mol dm^-3 hydrocyanic acid.

Write an expression to show the ionisation of the conjugate base of hydrocyanic acid and calculate it K_b value.

Calculate the pH of a solution made by mixing 50.0 cm³ of 0.200 mol dm^-3 HCl (aq) with 50.0 cm³ of 0.100 mol dm^-3 NH₃ (aq)

A 0.100 mol dm^-3 solution of NH₃ (aq) contains 1.28 x 10^-3 mol dm^-3 in hydroxide ion.

[3]

[2]

Determine the base dissociation constant, K_b for ammonia using the information in part b).

The pH of pure water is 6.92 at 328 K and K_b for NH₃ (aq) at this temperature is 1.80 x10^-5.

Determine the pK_a of [NH₄⁺] at this temperature.

Weak acids do not fully ionise in solution. The acid dissociation constant, K_a is used to determine the hydrogen ion concentration.

Write an expression for the acid dissociation constant, K_a for the acid HA.

The pH of a 0.15 mol dm^-3 solution of HCN is 5.08 at 298 K. Calculate the value of K_a for HCN at 298 K.

Give your answer to two decimal places.

A sample of 0.01 mol dm^-3 butanoic acid has a K_a value of 1.51 x 10^-5 mol dm^-3.

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. 

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 half of the volume of sodium hydroxide required for neutralisation has been added to the ethanoic acid.

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.

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.

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).

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, K_a, of monochloroacetic acid.

Calculate the pH of a 0.05 M solution of monochloric acid. 

The value of K_a for monochloroacetic acid is 1.35 x 10^-3 mol dm^-3

Using Section 2 of the Data Booklet, calculate the value of [OH^-] for the solution of monochloric acid.

Calculate the percentage dissociation for the solution of monochloric acid.

State the relationship between the following expressions for conjugate acid-base pair

Using Section 21 of the Data Booklet, calculate the following for the conjugate bases at 298 K.

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)

The student repeats the titration with 0.10 mol dm^-3 ethanoic acid, CH₃COOH (aq) which has a pK_a 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.