Queensland Chemistry

QUEENSLAND CHEMISTRY

CHAPTER 8 WHAT’S IN OUR WATER?
Name:

QUESTIONS

1. How do you think this method of estimating concentration might be influenced by the colour of the original cordial?

8.1 Concentration

2. Explain whether a solution can be simultaneously both:

(a) dilute and saturated

(b) concentrated and saturated

(d) unsaturated and concentrated

(e) unsaturated and dilute

(f) saturated and unsaturated.

Use the solubility curves in figure8.4 to answer the following questions.

3. What is the solubility of:

(a) NaCl at 50°C

(b) KCl at 10°C?

4. At which temperature:

(a) are the solubilities of Na2SO4 and K2SO4 equal

(b) is the solubility of CuSO4·5H2O twice the solubility of Na2SO4 at 10°C

(d) is the solubility of CuSO4·5H2O equal to 25g per 60g H2O?

5. A student wants to prepare a range of solutions that will be saturated at 70°C. What is the minimum mass of each solid needed to prepare the following solutions?

(a) KClO3 in 100g H2O

(b) Na2SO4 in 50g H2O

6. Identify the following solutions as saturated, unsaturated or supersaturated.

(a) 20g NaCl in 80g H2O at 30°C

(b) 23g CuSO4·5H2O in 100g H2O at 40°C

(d) 5g K2SO4 in 25g H2O at 70°C

7. Consider the cordial solutions investigated at the beginning of the chapter.

(a) Calculate their concentrations.

(b) What is the best way to report their concentrations? pph? ppt? ppm? (Here, t represents thousand.)

8. When the water from a 375mL can of soft drink was evaporated and other materials were removed, 47.3g of sugar remained. Calculate the concentration of sugar in the original drink in grams per litre.

9. Each 100mL of pond water in a tidal area contained 0.034g of salt. Calculate the salt concentration of the pond in ppm.

10. Saline solutions for intravenous drips have a NaCl concentration of 0.90%w/w. Calculate the mass of salt needed to make 500g of saline fluid.

8.2 Solubility and precipitation

11. Use table 8.1 to predict whether the following substances should have a high or a low solubility in water.

(a) aluminium hydroxide, Al(OH)3

(b) lead(II) sulfate, PbSO4

(d) sodium sulfide, Na2S

(e) magnesium carbonate, MgCO3

12. The pairs of solutions listed in (a) to (d) below are mixed. In each case:

(i) determine whether a precipitate will form

(ii) write balanced molecular, ionic and net ionic equations for any reaction that results in a precipitate. If no reaction occurs, write NR for the product of the molecular equation.

(a) Barium chloride and magnesium sulfate solutions

(b) Lead(II) nitrate and sodium hydroxide solutions

(d) Potassium bromide and copper(II) sulfate solutions

13. Which tests are used to confirm the presence of:

(a) carbonate ions

(b) chloride ions

(d) aluminium ions?

14. What would happen to your analysis if you did not remove Pb2+ from the solution before adding NH3 to precipitate Al(OH)3?

15. Draw a flowchart to show how you could separate and test for Cl–, Br– and I–.

8.3 Solubility and mineral deposits

16. Look back at the solubility curves in figure8.4. How, in general, does the solubility of a solid vary with temperature?

17. Look at the general solubility rules in table8.1. What kinds of salts would you expect to precipitate when the concentrations of hydroxide, oxide, carbonate and sulfide ions are high?

Review questions

Use figure8.4 to answer questions 1 and 2.

1. Suppose you stirred 45g of sodium sulfate (Na2SO4) in 100g of water at 50°C and continued stirring as the solution gradually cooled to 25°C.

(a) Describe the changes that you would see in the container as the solution cooled.

(b) Draw models of what the materials in the beaker would look like at 40°C and 25°C.

2. Suppose you dissolved 20g of potassium chlorate (KClO3) in 100g of water at 60°C, boiled the solution to evaporate 25% of the solvent and then allowed the solution to cool to 40°C.

(a) After the required amount of solvent had been evaporated, at what temperature would the first crystals of KClO3 begin to form?

(b) What mass of precipitate will be present in the mixture at 40°C?

3. Convert:

(a) 2.2g kg–1 to concentration in ppm

(b) 345 ppm to a concentration ing kg–1.

4. Samples of laundry materials contain the following information about the concentrations of their active ingredients. (Both chlorine and oxygen act as bleaching agents.) Compare the type, clarity and usefulness of the information provided. Can you tell which has the greater mass of active ingredient?

(a) Liquid chlorine bleach — 40 g L–1 available chlorine present as sodium hypochlorite

(b) Pre-wash powder — 1.7%w/w available oxygen

5. Quaternary ammonium compounds, which act as antiseptics, are similar to ammonium compounds, but the hydrogen atoms have been replaced with organic groups containing C and H. Which product is more concentrated with respect to the active ingredient? Does the information provided give sufficient detail for you to answer this question unambiguously?

(a) Antiseptic powder — 0.8%w/w quaternary ammonium compounds

(b) Liquid antiseptic — 3.5% quaternary ammonium compounds

6. The active ingredients in dog-wash solutions vary, but are usually anti-bacterial and anti-fungal organic compounds. Based solely on the information provided, which brand would you buy? Justify your choice.

(a) Medicinal dog wash, Brand 1 — 1mL of active ingredients per 10mL of solution

(b) Medicinal dog wash, Brand 2 — 3 mg of active ingredients permL of solution

7. Chemical analysis found 0.0003g of insecticide in a 1000g sample of river water. Express the concentration of the insecticide in ppm and in ppb.

8. A particular drug is said to be toxic if its concentration in the human body exceeds 20 ppb. For safety purposes, a danger limit is set at 50% of the toxic limit. What is the maximum amount of the drug that could be present in an 80 kg man without exceeding the danger limit?

9. The salinity of samples taken from the mouth, the middle estuary and the upper estuary of the Brisbane River were recorded as 30 ppt, 13 ppt and 0.8 ppt respectively, where ‘t’ represents thousand.

(a) Rewrite these values in pph and in ppm.

(b) Why do you think the data was recorded in ppt?

(c) Assume that 430 mg L–1 is considered an acceptable concentration of NaCl in treated drinking water. Do any of these river water samples pass the test for NaCl content? (A litre of water has a mass of 1000 g.)

(d) Why do you think the water samples from various parts of the river contain such different amounts of NaCl?

10. Write dissociation equations for sodium hydroxide (NaOH) and sodium phosphate (Na3PO4) in water.

11. Hard water can be caused by the presence of calcium ions. These can be removed by adding sodium carbonate to form a precipitate of calcium carbonate. Write a balanced equation for the reaction between calcium ions in solution and sodium carbonate.

12. Predict whether a precipitate will form when solutions of the following chemicals are mixed. If a precipitate does form, write a net ionic equation for the reaction. If no reaction occurs, write NR.

(a) AgNO3 and KCl

(b) (NH4)2S and Pb(NO3)2

(d) (NH4)2S and NaNO3

(e) H2SO4 and NaCl

(f) Na2SO4 and Hg(NO3)2

(g) NaNO3 and KCl

(h) CaCl2 and NaBr

(i) (NH4)3PO4 and CaCl2

(j) CoSO4 and Ba(OH)2

13. What cations could be present in a solution that gives a precipitate with:

(a) sodium sulfate solution, but not with sodium chloride

(b) sodium sulfate and sodium chloride solutions

(d) sodium carbonate, but not with sodium hydroxide?

14. Describe simple solubility tests that would allow you to distinguish between solutions of:

(a) silver nitrate and zinc nitrate

(b) barium chloride and magnesium chloride

(d) potassium sulfate and sodium chloride

(e) sodium sulfate and sodium sulfite

(f) magnesium nitrate and zinc nitrate

(g) aluminium chloride and iron(III) chloride.

15. Preliminary reports from scientific studies in Australia and Canada in the mid 1990s suggested that regular consumption of drinking water that contained more than 100 ppb of Al3+ could lead to memory loss and, in extreme cases, to severe neurological disorders such as Alzheimer’s disease. There were also reports that using aluminium saucepans could contribute to increased consumption of Al3+ and that this also could lead to mental health problems. Has further research supported these studies?