Acids, bases and the environmentIB MYP Chemistry: Subtopic test

1. 1
   Researchers in Sweden find that the pH of a lake has fallen from 6.8 to 4.9 over thirty years. Many of the fish have died and nearby trees are losing their leaves. A coal-burning power station, which has no gas filters on its chimney, operates 200 km upwind of the lake.

   (a)

   Which gas released when coal is burned is mainly responsible for acid rain?

   [1 mark]

   • ACarbon monoxide
   • BSulfur dioxide
   • COxygen
   • DNitrogen

   (b)

   Which substance could be added to the lake to raise its pH?

   [1 mark]

   • ASulfuric acid
   • BSodium chloride
   • CCalcium carbonate
   • DCarbon dioxide

   (c)

   Explain how burning coal can make the rain acidic.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student in Cairo compares three antacid tablets, X, Y and Z, sold for indigestion. For each test she crushes one tablet, adds it to 25 cm³ of 0.10 mol dm⁻³ hydrochloric acid, which represents stomach acid, stirs for two minutes and measures the pH with a pH meter. The acid on its own has a pH of 1.0. After tablet X the pH is 3.2, after tablet Y it is 5.1 and after tablet Z it is 6.9. She concludes that tablet Z is the best antacid.

   (a)

   Which of these must be kept the same for every tablet so that the test is fair?

   [1 mark]

   • AThe type of tablet
   • BThe final pH
   • CThe identity of the active ingredient
   • DThe volume and concentration of the hydrochloric acid

   (b)

   Which pair of compounds are commonly the active ingredients in antacid tablets?

   [1 mark]

   • AMagnesium hydroxide and calcium carbonate
   • BSodium hydroxide and potassium hydroxide
   • CHydrochloric acid and citric acid
   • DSulfuric acid and sodium chloride

   (c)

   Suggest one improvement to the method that would make the conclusion more reliable, and explain how it helps.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Marine scientists monitor the seawater around a coral reef in Australia. Over 40 years the average pH of the surface seawater has fallen from 8.11 to 8.04, while the amount of carbon dioxide in the air has risen. Corals and shellfish build their skeletons and shells from calcium carbonate.

   (a)

   Explain how an increase in carbon dioxide in the air lowers the pH of seawater.

   [3 marks]

   (b)

   Explain why this change in pH harms corals and shellfish, and suggest one way to slow the change.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A farming cooperative in Kenya grows maize on soil with a pH of 4.5. Maize grows best in soil with a pH of 6 to 7. An adviser recommends spreading powdered lime (calcium oxide or calcium carbonate) on the fields. Lime must be bought, transported to the farms and spread again every few years. The cooperative could instead do nothing or change to a crop that tolerates acidic soil.

   (a)

   Discuss and evaluate whether the cooperative should treat its soil with lime. Reach a justified conclusion.

   [6 marks]

   (b)

   Design an investigation to find how the mass of powdered calcium carbonate added to a soil sample affects its pH. Include the variables and a safety precaution.

   [6 marks]

   Total for question 4: 12 marks

End of questions