The Acidic environment

Topic Test

FEBRUARY 28, 2015

The first two questions (1 and 2) refer to the following diagram:

1. Which choice shows the acid arranged according to increasing pH?(a) HA, HB, HC(b) HC, HB, HA(c) HA, HC, HB(d) HC, HA, HB

2. Which solution will have the highest pH?(a) NaA(b) NaB(c) HA(d) HB

C

3. The acidities of aqueous solutions of some substances are as follows:(i) soda water, slightly acidic(ii) detergent for an automatic dishwasher, highly alkaline(iii) detergent for dishwashing by hand, slightly alkaline(iv) detergent for washing hands and face, neutral(v) vinegar, highly acidic

What colours would you expect the solutions to show if drops of the following substances were added in?

(a) Litmus

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(b) Methyl orange

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(c) Phenolphthalein

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4. Describe what you would observe when each of the following experiments was performed. Write equations for any reactions which occur.(a) Black copper oxide powder was slowly added to dilute nitric acid solution with continuous

stirring.

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(b) Colourless barium hydroxide solution was added slowly to a dilute solution of hydrochloric acid containing a few drops of litmus indicator.

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(c) Dilute sulfuric acid was added to a suspension of brown iron(III) hydroxide in water with continuous stirring.

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(d) Brown silver oxide powder was added to dilute nitric acid solution with stirring.

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(e) White solid calcium oxide (quick lime) was added to water in a beaker with stirring; a few drops of litmus indicator solution were added to the mixture, and then dilute hydrochloric acid solution was slowly added.

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5. What is the hydrogen ion concentration in each of the following solutions at 25oC, and calculate its pH:(a) 0.010 mol/L sodium hydroxide

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(b) 5.0 x 10–5 mol/l calcium hydroxide

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(c) 6.7 × 10–3 mol/L barium hydroxide

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(d) 0.0024 mol/L lithium hydroxide

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6. Calculate the pH of(a) 0.255 mol/L nitric acid

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(b) 0.168 mol/L sodium hydroxide.

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50 mL of each of these two solutions are mixed together. Calculate the pH of the resulting solution.

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7. Calculate the pH of:(a) 0.0155 mol/L hydrochloric acid

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(b) 0.0078 mol/L barium hydroxide.

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100 mL of each of these solutions are mixed together. Calculate the pH of the resulting solution.

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8. The pH of the solution in the human stomach is normally about 0.9. Indigestion is generally caused by an increase in stomach acidity. Some people take the ‘bicarbonate of soda’ (sodium hydrogen carbonate) to relieve indigestion.(a) What hydrogen ion concentration corresponds to a pH of 0.9?

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(b) Does indigestion cause stomach pH to rise or to fall?

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(c) Write an equation for the reaction of sodium hydrogen carbonate which relieves indigestion.

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(d) Why is there a tendency to ‘belch’ (bring up wind) after taking bicarbonate of soda?

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9. A 0.074 mol/L solution of nitrous acid has a pH of 2.15. Is this acid strong or weak? Explain. (If it is weak calculate the degree of ionisation in this solution.)

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10. To determine the concentration of acetic acid (ethanoic acid) in a particular brand of vinegar, a student first diluted 25 mL of the vinegar accurately to 100 mL, and then titrated 10 mL of the diluted solution with 0.097 mol/L sodium hydroxide. 17.2 mL was required. Calculate the molarity of the dilute solution and of the original vinegar. Calculate the mass of acetic acid per 100 mL of the vinegar.

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11. The electrical conductivity of solutions (of the same concentration) of nitric acid, potassium nitrate, potassium hydroxide and six other substances labelled P to U was measured and recorded in the table below. Pieces of litmus paper were dipped into the solutions and the final colour of the paper recorded in the table; NCC means no colour change: red litmus stayed red and blue litmus stayed blue.

Substance HNO3 KNO3 KOH P Q R S T URelative conductivity

125 45 90 0 3 110 50 4 85

Litmus Colour

Red NCC blue NCC blue red NCC red blue

From the compounds P to U, identify the following, giving reasons for each choice.

(a) a strong acid

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(b) a strong base

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(c) a weak acid

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(d) a weak base

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(e) a salt

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12. Draw all products and reactants involved in the reaction to form ethyl methanoate

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13. Explain why methyl ethanoate has a much lower boiling point than ethanoic acid (57oC compared with 118oC), despite having a higher molecular weight.

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14. Explain why ethanoic acid has a much greater solubility in water than methyl methanoate.

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15. A sample of octyl ethanoate (which is insoluble in water) was contaminated with a small amount of ethanoic acid which is soluble in water. Explain how you would use water and a separating funnel to purify the octyl ethanoate. Sketch the separating funnel.

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16. The salt, sodium hydrogen phosphate (Na2HPO4) can be prepared by reacting sodium hydroxide solution with the required amount of phosphoric acid (H3PO4) solution.

(a) Write a balanced equation for the reaction.

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(b) When some crystals of Na2HPO4 were dissolved in water, the pH of the resulting solution was found to be 9.5. Calculate the hydrogen ion concentration of this solution.

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(c) Write an equation for the reaction of the HPO42- ion with water to account for the measured pH.

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