Queensland Chemistry

CHAPTER 11 CHEMICAL ANALYSIS

QUESTIONS

11.1 How much?

22 1. If a sample of CO2 contains 2 × 10 molecules, determine the amount of: (b) carbon atoms in the sample

(c) oxygen atoms in the sample.

2. What amount of CuCO3 is present in a 100 g sample of the salt?

3. What is the mass of 0.25 moles of FeO?

4. What is the molar mass of a compound if 0.15 mole has a mass of 6.6 g?

5. What mass of CO2 would be produced in the reaction discussed in the previous sample problem?

6. In one of the steps in the extraction of zinc, ZnS is converted to ZnO by the following reaction.

2ZnS + 3O2  2ZnO + 2SO2

(b) What amount of oxygen is needed to change 1000 g of ZnS to ZnO?

(c) What mass of oxygen does this represent?

(d) What mass of SO2 is formed in the process?

7. The extraction of lead from lead(II) oxide occurs according to the following equation.

2PbO + 2C + O2  2Pb + 2CO2

(b) What mass of lead will be produced if 100 kg of lead(II) oxide are reduced?

(c) What amount of carbon would be needed in this reaction?

(d) What mass of oxygen would be required?

11.2 Gravimetric analysis

8. Barium ion reacts with sulfate ion to form insoluble barium sulfate. What mass of barium ion was originally present in solution if the addition of excess sulfate ion results in the formation of 0.165 g of BaSO4?

9. When hydrogen sulfide gas is bubbled through a solution of lead(II) ion, 4.268 g of lead(II) sulfide is formed. (b) Write a balanced equation for the reaction.

2+ (c) Calculate the mass of Pb present in the original solution.

10. A solution containing 0.584 g of Ba(NO3)2 is mixed with 0.266 g of Na2SO4. What mass of BaSO4 precipitate will be formed?

Review questions

1. For two moles of (NH4)3PO4 fertiliser, calculate the: (b) amount (in mol) of nitrogen atoms

(c) amount (in mol) of hydrogen atoms

(d) amount (in mol) of oxygen atoms

(e) amount (in mol) of phosphorus atoms

(f) total number of nitrogen atoms

(g) mass of phosphate ion

(h) mass of nitrogen atoms.

2. For 16.2 g of acetic acid (CH3COOH), calculate: (b) the amount (in mol) of acetic acid molecules

(c) the number of molecules of the acid

(d) the amount (in mol) of oxygen atoms

(e) the number of oxygen atoms.

3. Find the number of moles and the number of atoms present in the following samples.

(b) 14.6 g acetylene (ethyne, C2H2)

(c) 0.48 g propane (C3H8)

(d) 85 g ethanol (C2H5OH)

(e) 8.6 g carbon dioxide

(f) 67 g iodine (I2).

4. Calculate the mass of each of the following: (b) 160 mol Fe

(c) 0.075 mol SO2

(d) 4.23 mol NO2

(e) 0.0015 mol Cu(NO3)2.

5. Which of the following substances has the greatest mass? A 200 g of magnesium

B 5 mol of sulfur atoms

C 1.2 × 1024 atoms of helium

6. Octane (C8H18) is a major component of petrol. (b) What is the molar mass of octane?

(c) What is the mass of 3.2 mol of octane?

(d) How many molecules are there in 2.5 g of octane?

(e) How many individual atoms are there in 5 g of octane?

(f) What is the percentage of hydrogen in octane?

(g) What mass of carbon is present in 150 g of octane?

7. Sodium fluoride is thought to reduce tooth decay, especially in children. It is therefore added to some brands of toothpaste. If a tube of toothpaste contains 0.013 g of sodium fluoride: (b) what amount (in mol) of sodium fluoride does this represent

(c) how many fluoride ions does this represent?

8. To prevent a gum disease called scurvy, the minimum daily requirement of vitamin C (C6H8O6) is 60 mg. (b) What amount (in mol) of vitamin C is this?

(c) How many molecules is this?

(d) If 10.0 g of spinach is found to contain 1.2 × 10–5 g of vitamin C, how much spinach must be eaten to provide the minimum daily requirement?

9. Too much cholesterol (C27H46O) is thought to be linked to heart disease. The human body itself manufactures 250 g of cholesterol in the liver. (b) What amount (in mol) of cholesterol is this?

(c) What amount (in mol) of carbon atoms does this represent?

(d) What mass of carbon atoms is contained in 250 g of cholesterol?

10. An oxide of copper is heated in a stream of hydrogen until only the copper remains. The data for the experiment is given below. Calculate the empirical formula for this oxide of copper.

Mass of crucible = 27.002 g Mass of crucible plus contents before heating = 27.128 g Mass of crucible plus contents after heating = 27.114 g

11. During an experiment, 4.60 g of anhydrous zinc sulfate with a molar mass of 161 g mol–1 is obtained by driving the water from 8.20 g of the crystalline hydrated salt of empirical formula ZnSO4·xH2O. Calculate the value of x.

12. Coal contains 0.01% iron pyrite (FeS2) as an impurity. Combustion of coal containing iron pyrite causes sulfur dioxide pollution due to the following reaction:

4FeS2(s) + 11O2(g)  2Fe2O3(s) + 8SO2(g)

Calculate the mass of sulfur dioxide produced by the complete combustion of one tonne of coal.

13. Silver used in tableware and jewellery becomes tarnished when exposed to air if the air contains small amounts of hydrogen sulfide. The tarnish is a layer of silver sulfide.

4Ag(s) + 2H2S(g) + O2 (g)  2Ag2S(s) + 2H2O(g)

Calculate the mass of silver lost from a spoon if 0.300 g of tarnish is produced and then removed with an abrasive polish.

14. Copper dissolves in concentrated nitric acid to produce nitrogen monoxide gas, according to the equation:

3Cu(s) + 8HNO3(aq)  3Cu(NO3)2(aq) + 2NO(g) + 4H2O(l)

In an experiment 2.50 g of copper are completely consumed in the reaction, and the solution is evaporated to dryness. Calculate the mass of copper(II) nitrate produced.

15. Copper is extracted from chalcopyrite (CuFeS2) according to the equation:

2CuFeS2(s) + 5O2(g) + 2SiO2(s)  2Cu(l) + 4SO2(g) + 2FeSiO3(l)

If 1.00 kg of chalcopyrite and 1.00 kg of silica (SiO2) are heated in an abundant supply of oxygen, determine:

(b) which of the reactants (chalcopyrite or silica) is present in excess

(c) the maximum mass of copper that can be extracted

(d) the mass of sulfur dioxide produced.

16. A sample of 1.072 g of a metal carbonate (MCO3) is heated. It is not known whether the carbonate is nickel carbonate, zinc carbonate or barium carbonate. When it finishes reacting, it produces 0.376 g of carbon dioxide according to the equation:

MCO3(s)  MO(s) + CO2(g)

Deduce the identity of M.

17. To test the percentage of sodium chloride in a sample of rock salt, a carefully weighed mass (0.997 g) is first dissolved in water. Silver nitrate solution is then added until no

further precipitation is observed. The precipitate is filtered and then dried in an oven to constant weight. The mass of precipitate obtained is 2.359 g.

(b) What precipitate formed in this process?

(c) Calculate the percentage of sodium chloride in the rock salt, assuming it is the only source of chloride ions.

18. The sulfur content of azalea fertiliser is present in the form of soluble sulfates. To check the percentage of sulfur in such a product, a 2.322 g sample is weighed out and dissolved in water. Barium nitrate is then added under carefully controlled conditions in order to precipitate out all the sulfate present as barium sulfate. After filtering, washing with water and drying, the mass of precipitate obtained is 0.564 g. (b) Calculate the mass of sulfur in the precipitate, and hence the percentage of sulfur in the sample analysed.

(c) If the precipitate is not washed with water prior to drying, explain the effect this would have on the final result.

19. A student on work experience is given a $1 coin that is suspected to be counterfeit. She analyses the coin for its copper content and then its nickel content. The results are as follows.

Copper analysis: A sample of 1.42 g of filings from the coin is dissolved using concentrated hydrochloric acid. Ammonium thiocyanate is then added to precipitate the copper as copper(I) thiocyanate (CuSCN). After the precipitate is collected in a filtering crucible, it is dried in an oven at 115 °C to a constant mass of 2.54 g.

Nickel analysis: A sample of 4.89 g of filings is dissolved, as above. In a carefully controlled procedure, the nickel present is then precipitated as nickel dimethyl glyoxime (Ni(C4H7O2N2) 2). This precipitate is collected and dried as above, yielding a final mass of 0.96 g. (b) Calculate the percentage of copper in the coin.

(c) Calculate the percentage of nickel in the coin.

(d) Calculate the percentage of aluminium present, assuming this to be the only other metal present.

(e) If the alloy should contain 92% Cu, 6% Al and 2% Ni, is the coin analysed likely to be a counterfeit one?

20. The foil wrapper on an Alka-Seltzer tablet states that it contains the following ingredients: aspirin: 0.324 g, calcium monohydrogen phosphate: 0.2 g, sodium hydrogen carbonate: 1.9 g and citric acid: 1.05 g. To check the above ingredients, a gravimetric procedure has been designed whereby the calcium is precipitated as calcium oxalate and

then converted to calcium oxide by heating. Calculate the mass of calcium oxide that would be expected to be obtained per tablet.

21. In earlier times, paints often contained lead ions. Today, many countries, including Australia, no longer permit the sale of lead-based paints. To test whether a heritage-listed house, first painted over a century ago, is contaminated with lead, a 1.50 g sample of paint is taken and dissolved in water. Dilute sulfuric acid is added to precipitate the lead ions as PbSO4. The mass of the dry precipitate is 0.0806 g. Calculate the percentage by weight of lead(II) ions in the paint.

22. Iron salts present in a 100 mL sample of bore water are precipitated as Fe(OH)3, which is then heated to drive off water and form Fe2O3. If the precipitate formed weighs 89.5 mg, what is the original concentration of iron ions in: (b) g mL–1

(c) mol L–1?

23. Copper reacts with sulfur to form copper(I) sulfide. What is the maximum mass of Cu2S that can be formed when 80.0 g of copper react with 25.0 g of sulfur?

24. Aluminium chloride is formed by the reaction of aluminium with chlorine gas. What mass of the salt can be formed from the reaction of 2.50 kg of aluminium with 6.50 kg of Cl2?

25. The first step in the Ostwald process for making nitric acid is:

4NH3(g) + 5O2(g)  4NO(g) + 6H2O(l)

The reaction has a maximum yield of 80%. What is the maximum mass of NO that can be formed when 1000 kg of ammonia react with 3000 kg of oxygen?

Notes: