Test on The pH scale and indicators

Question 1: The pH scale was introduced by a German chemist named Søren Peder Lauritz Sørensen.

A. Yes

B. No

Explanation: The pH scale was introduced by Søren Peder Lauritz Sørensen, who was a Danish chemist, not a German chemist.

Question 2: According to the study materials, which of the following statements about the Ion Product of Water (Kw) at 25 °C is correct?

A. The value of Kw is 1.0 × 10⁻¹⁴.

B. It is defined by the product of the molar concentrations of hydrogen ions and hydroxide ions ([H⁺] · [OH⁻]).

C. If the concentration of H⁺ ions increases, the concentration of OH⁻ ions must decrease proportionally to maintain a constant Kw.

D. It indicates that in any aqueous solution, only H⁺ ions are present, not OH⁻ ions.

Explanation: The study materials explicitly state that 'At 25 °C, the value of Kw is 1.0 × 10⁻¹⁴,' making the first option correct. It also defines Kw as 'the product of the molar concentrations of hydrogen ions and hydroxide ions: Kw = [H⁺] · [OH⁻],' confirming the second option. Furthermore, the text explains that 'if the concentration of H⁺ increases, the concentration of OH⁻ must decrease proportionally to maintain the constant Kw,' validating the third option. The fourth option is incorrect because the study materials clearly state, 'in any aqueous solution, both H⁺ and OH⁻ ions are always present.'

Question 3: When a neutral solution becomes increasingly acidic, a universal indicator will transition through colors from green to yellow, then to orange, and finally to red.

A. Yes

B. No

Explanation: The study materials state that a universal indicator shows 'Green at neutral pH (around 7)'. It also notes that 'As the pH drops, the indicator will transition from green to yellow, then to orange, and finally to red, depending on the amount of acid added', directly confirming this sequence of color changes as a solution becomes more acidic.

Question 4: The specific pH range over which an indicator changes color is called its transition range.

A. Yes

B. No

Explanation: The study materials explicitly state that 'The specific pH range over which an indicator changes color is called its transition range.'

Question 5: Based on the provided study materials, what is the primary composition of a universal indicator?

A. A single indicator that has an exceptionally wide pH transition range.

B. A mixture of several different indicators, each with its own specific transition range.

C. A precise numerical pH meter combined with a visual display.

D. A single weak organic acid or base that changes color based on H⁺ concentration.

Explanation: The study materials state: 'A universal indicator is a mixture of several different indicators, each with its own specific transition range.' This combination allows it to produce a continuous spectrum of color changes across a wide pH range. A single indicator cannot cover the entire pH spectrum, and a pH meter is a separate tool for precise measurement, not the composition of an indicator solution.