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This question is about acids and alkalis - AQA - GCSE Chemistry - Question 8 - 2022 - Paper 1
Question 8
This question is about acids and alkalis. 1. Explain why the pH of an acid depends on: • the strength of the acid: • the concentration of the acid. 2. A stud... show full transcript
Worked Solution & Example Answer:This question is about acids and alkalis - AQA - GCSE Chemistry - Question 8 - 2022 - Paper 1
Step 1
Explain why the pH of an acid depends on the strength of the acid:
Answer
The pH of an acid is influenced by the strength of the acid, which refers to its ability to dissociate in aqueous solution. A stronger acid ionizes more completely, producing a greater concentration of hydrogen ions (H⁺) in solution. Therefore, the more H⁺ ions present, the lower the pH value.
Step 2
Explain why the pH of an acid depends on the concentration of the acid:
Answer
Additionally, the concentration of an acid affects its pH. A higher concentration of acid means a greater number of H⁺ ions are present per unit volume. As a result, an increase in concentration leads to a lower pH, because the pH scale is logarithmic.
Step 3
Explain why the student used a volume of 23.50 cm³ of barium hydroxide solution in the titration calculation:
Answer
The student used a volume of 23.50 cm³ of barium hydroxide because it represents the mean volume calculated from the five titrations, excluding an anomalous result. This ensures the most accurate representation of the average volume needed for the reaction.
Step 4
Calculate the concentration of the hydrochloric acid in mol/dm³:
Answer
First, calculate the moles of barium hydroxide reacted:
ext{moles Ba(OH)}_2 = rac{23.50}{1000} imes 0.100 = 0.00235
According to the reaction, two moles of HCl react with one mole of Ba(OH)₂:
Next, calculate the concentration of HCl:
ext{concentration} = rac{0.00470}{0.025} = 0.188 ext{ mol/dm}³
Step 5
Explain why the electrical conductivity of the mixture was zero when the sulfuric acid had just been neutralised:
Answer
At the point of neutralisation, the sulfuric acid has fully reacted with the barium hydroxide to form barium sulfate, which is a solid that precipitates, along with water. No ions are left in solution to carry electric charge, resulting in zero electrical conductivity.
Step 6
Give one reason why the electrical conductivity of the mixture increased after adding a further 10 cm³ of barium hydroxide solution:
Answer
The increase in electrical conductivity is due to the additional barium hydroxide providing more hydroxide ions (OH⁻) into the solution, thereby increasing the overall concentration of ions available to carry electric current.