Common acids/alkalis; H⁺(aq) concept; pH determination: แบบฝึกหัด

A solution has a pH of 2. Which of the following statements about this solution is correct?

Calculate the pH value of a solution that contains a hydrogen ion concentration of \(1.0 \times 10^{-9} \text{ mol dm}^{-3}\).

An aqueous solution of sodium hydroxide has an initial pH of \(14\). If a chemist takes \(1.0\text{ cm}^3\) of this solution and dilutes it to exactly \(1000\text{ cm}^3\) with deionized water, what will be the pH of the diluted solution?

Which of the following substances would turn blue litmus paper red?

A clear solution turns methyl orange yellow, litmus paper red, and phenolphthalein colourless. Which of the following represents the most likely pH range for the solution?

State the basicity of ethanoic acid (\(CH_3COOH\)) and explain your answer briefly.

A solution of a strong acid has a pH of \(2.0\). If \(50.0 \text{ cm}^3\) of this acid is diluted with distilled water to a total volume of \(5.0 \text{ dm}^3\), what will be the pH of the diluted solution?

Solution:
Given:
Initial pH of strong acid = \(2.0\)
Initial volume of acid (\(V_1\)) = \(50.0 \text{ cm}^3 = 0.050 \text{ dm}^3\)
Final volume after dilution (\(V_2\)) = \(5.0 \text{ dm}^3\)

First, calculate the initial concentration of \(H^+\) ions:
\([H^+]_1 = 10^{-pH_1} = 10^{-2.0} = 0.01 \text{ mol dm}^{-3}\)

Next, calculate the initial moles of \(H^+\) ions:
\(Moles_{H^+} = [H^+]_1 \times V_1 = 0.01 \text{ mol dm}^{-3} \times 0.050 \text{ dm}^3 = 0.0005 \text{ mol}\)

Now, calculate the new concentration of \(H^+\) ions in the diluted solution:
\([H^+]_2 = \frac{Moles_{H^+}}{V_2} = \frac{0.0005 \text{ mol}}{5.0 \text{ dm}^3} = 0.0001 \text{ mol dm}^{-3}\)

Finally, calculate the pH of the diluted solution:
\(pH_2 = -\log_{10}[H^+]_2 = -\log_{10}(0.0001) = 4.0\)

The pH of the diluted solution will be 4.0.

Sulphuric acid (\(H_2SO_4\)) is a common mineral acid used in the laboratory.

(a) State the basicity of sulphuric acid.
(b) Write a chemical equation to show the complete dissociation of sulphuric acid in water.
(c) Explain, in terms of ions, why an aqueous solution of sulphuric acid can conduct electricity whereas pure liquid sulphuric acid cannot.

A student prepared a sample of dilute nitric acid (\(HNO_3(aq)\)) and measured its pH value to be 2.30.

(a) Write an ionic equation to show the dissociation of nitric acid in water.
(b) Calculate the concentration of hydrogen ions, \([H^+(aq)]\), in this sample. (Give your answer to three significant figures).
(c) If \(20.0 \text{ cm}^3\) of this acid is diluted with distilled water to a total volume of \(200.0 \text{ cm}^3\), calculate the concentration of hydrogen ions in the diluted solution.
(d) Determine the pH of the diluted solution and state the general relationship between the concentration of hydrogen ions and the pH value of a solution.