The pH scaleIB MYP Chemistry: Subtopic test

1. 1
   A swimming pool technician in Dubai measures the water with a digital pH probe and records a pH of 6.4. The recommended range for safe swimming is about 7.2 to 7.8.

   (a)

   What does a pH of 6.4 show about the pool water?

   [1 mark]

   • AIt is neutral
   • BIt is weakly acidic
   • CIt is strongly alkaline
   • DIt is strongly acidic

   (b)

   The technician adds an alkaline chemical and the pH rises to 7.5. What has happened to the concentration of hydrogen ions?

   [1 mark]

   • AIt has decreased
   • BIt has increased
   • CIt has stayed the same
   • DIt has become zero

   (c)

   Explain why a pH probe is a better choice than universal indicator for the technician.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student starts with hydrochloric acid of pH 1.0. She mixes 10 cm³ of this acid with 90 cm³ of distilled water and measures the pH of the new solution with a probe: it is 2.0. She then mixes 10 cm³ of this new solution with another 90 cm³ of distilled water, and the pH is 3.0.

   (a)

   How does the concentration of hydrogen ions in the pH 3.0 solution compare with the original pH 1.0 acid?

   [1 mark]

   • A2 times lower
   • B3 times lower
   • C100 times lower
   • D100 times higher

   (b)

   What would the pH be after a third identical dilution?

   [1 mark]

   • A3.0
   • B3.5
   • C5.0
   • D4.0

   (c)

   Describe the trend in pH as the acid is diluted and explain it in terms of hydrogen ions.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student in Cape Town tests five solutions, P to T, with a pH probe. The readings are: P, pH 1.0; Q, pH 4.0; R, pH 7.0; S, pH 10.0; T, pH 13.0.

   (a)

   State which solution is the most acidic, which is neutral and which is the most alkaline.

   [3 marks]

   (b)

   Solutions P and Q are both acids. Compare the concentration of hydrogen ions in P and Q, giving the factor of the difference.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A tea farm in Sri Lanka grows its best crop in slightly acidic soil of pH 4.5 to 5.5. Soil in a new field has a pH of 7.8, measured with a probe. The farm manager could add powdered sulfur, which soil bacteria slowly turn into an acid, or could plant a different crop that grows well at pH 7 to 8.

   (a)

   Explain what the pH reading shows about the soil, what must change for tea to grow well, and how adding powdered sulfur would bring about this change.

   [6 marks]

   (b)

   Evaluate the two options available to the farm manager: treating the soil with sulfur or planting a different crop.

   [6 marks]

   Total for question 4: 12 marks

End of questions