Acids, Bases and SaltsIB MYP Chemistry: Topic test

1. 1
   A teacher in Madrid dissolves dry hydrogen chloride gas in two liquids. Solution A is hydrogen chloride in water. Solution B is hydrogen chloride in methylbenzene, a liquid in which it stays as separate molecules and forms no ions. The teacher dips a strip of dry blue litmus paper into each solution. The paper in solution A turns red, but the paper in solution B stays blue.

   (a)

   Which ion in solution A turns the blue litmus paper red?

   [1 mark]

   • AOH⁻ ions
   • BH⁺ ions
   • CCl⁻ ions
   • DNa⁺ ions

   (b)

   Which statement explains why the litmus paper stays blue in solution B?

   [1 mark]

   • AThe hydrogen chloride stays as molecules and does not produce hydrogen ions
   • BMethylbenzene is a strong alkali that neutralises the hydrogen chloride
   • CLitmus paper only turns red when hydroxide ions are present
   • DHydrogen ions are present but their concentration is too low to affect dry litmus paper

   (c)

   Name the ion present in all acidic solutions and the ion present in all alkaline solutions.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A hospital laboratory in Doha measures the pH of four body fluids with a pH probe. Gastric juice has a pH of 1.5, urine has a pH of 6.0, saliva has a pH of 6.8 and pancreatic juice has a pH of 8.5.

   (a)

   Which of these fluids is alkaline?

   [1 mark]

   • AGastric juice
   • BUrine
   • CPancreatic juice
   • DSaliva

   (b)

   Which statement about gastric juice and urine is correct?

   [1 mark]

   • AUrine is more acidic than gastric juice, because its pH number is higher
   • BGastric juice and urine have the same hydrogen ion concentration, because both are below pH 7
   • CGastric juice is less acidic than urine, because its pH number is smaller
   • DGastric juice has a much higher hydrogen ion concentration than urine, because its pH is lower

   (c)

   State two advantages of measuring the pH of these fluids with a pH probe rather than with universal indicator paper.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Students in Istanbul want to find out which of three brands of vinegar, X, Y and Z, is the most acidic. They have 0.50 mol dm⁻³ sodium hydroxide solution, universal indicator, a burette and flasks.

   (a)

   Identify the independent variable, the dependent variable and one control variable in this investigation.

   [3 marks]

   (b)

   Describe how the students should carry out the investigation so that the results are reliable and the work is safe.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A gold mine in Ghana releases wastewater that contains dilute sulfuric acid and has a pH of 3. The wastewater flows into a series of settling ponds where the company adds powdered limestone, which is calcium carbonate, before the water is released into a river used by local farmers and fishers.

   (a)

   Explain how adding calcium carbonate changes the wastewater, including the type of reaction, the products and what happens to the pH.

   [6 marks]

   (b)

   Discuss the benefits and drawbacks of neutralising the wastewater with limestone before it is released, and reach a justified conclusion.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   A student in Oslo makes crystals of magnesium sulfate. She warms 40 cm³ of dilute sulfuric acid and adds magnesium oxide powder a little at a time, stirring, until some powder no longer dissolves. She filters the mixture, heats the filtrate until about half of the water has evaporated, and leaves it to cool.

   (a)

   Why does she keep adding magnesium oxide until some powder no longer dissolves?

   [1 mark]

   • ATo make sure all of the acid has reacted
   • BTo make the solution change colour
   • CTo make the reaction release hydrogen gas
   • DTo make the crystals larger

   (b)

   Which pair of products forms in this reaction?

   [1 mark]

   • AMagnesium sulfate and hydrogen
   • BMagnesium sulfide and water
   • CMagnesium sulfate and water
   • DMagnesium sulfate, water and carbon dioxide

   (c)

   Explain why the student heats the filtrate only until about half of the water has evaporated, instead of until it is completely dry.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   In Agra, India, the surface of old buildings made of marble, which is mostly calcium carbonate, is slowly being worn away. Nearby factories and power stations burn coal that contains sulfur. Scientists find that the local rainwater has a pH of 4.2.

   (a)

   What happens when this acid rain falls on marble?

   [1 mark]

   • AThere is no reaction, because calcium carbonate is insoluble
   • BThe marble reacts to give hydrogen gas and a salt
   • CThe marble is oxidised to calcium oxide
   • DThe marble reacts to give a soluble salt, water and carbon dioxide, so the surface wears away

   (b)

   Which action would reduce the damage caused by acid rain?

   [1 mark]

   • ABurning coal with more sulfur in it
   • BRemoving sulfur dioxide from power station waste gases before they are released
   • CAdding more acid to the soil near the buildings
   • DWashing the buildings with water once a year

   (c)

   Explain how burning coal that contains sulfur makes the rainwater acidic.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   Biologists in Ontario, Canada survey seven lakes of similar size and record the pH of the water and the number of fish species in each. The lakes have pH values of 4.0, 4.5, 5.0, 5.5, 6.0, 6.5 and 7.0 and contain 0, 2, 5, 8, 11, 12 and 12 fish species respectively. Only one lake was surveyed at each pH.

   (a)

   Describe the pattern in the results, and state the pH at which the number of species stops increasing.

   [3 marks]

   (b)

   Evaluate whether these results prove that acidic water kills fish, and suggest two improvements to the survey.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   Dentists in Mexico report rising tooth decay in children. Bacteria in the mouth turn sugar into acid, and sugary soft drinks themselves have a pH of about 3. Many toothpastes are mildly alkaline, with a pH of about 9. The government is considering a tax on sugary soft drinks and free toothpaste in schools.

   (a)

   Explain how the pH in the mouth changes after eating sugar, and how an alkaline toothpaste helps to reverse the change, using ideas about ions and neutralisation.

   [6 marks]

   (b)

   Discuss the benefits and drawbacks of taxing sugary soft drinks to reduce tooth decay, and reach a justified conclusion.

   [6 marks]

   Total for question 8: 12 marks

End of questions