CHAPTER 20 CONSUMER PRODUCTS AND QUALITY CONTROL
Name:
QUESTIONS
20.2 The chemist’s solution — quality control
1.
2.
3.
4.
5.
What is the concentration of each of the following solutions?
(a)
0.256 moles of KNO3 in 1.50 L of solution
(b)
20 mL of solution containing 2.5 × 10–6 moles of KMnO4
What amount of KI is needed to prepare:
(a)
5.0 L of 0.1M solution
(b)
100 mL of 2.0M solution?
What mass of solid is needed to prepare:
(a)
100 mL of 0.200M CuSO4·5H2O
(b)
10 L of 0.5M NaOH?
(a)
What volume of 2.0M CoCl2 is needed to make 100 mL of a 0.10M solution of the
salt?
(b)
200 mL of 0.5M Al(NO3)3 is diluted to a volume of 1 L.
(i)
What is the new concentration of the salt solution?
(ii)
What is the concentration of nitrate ions in that solution?
Write a sentence to show you know the difference between equivalence point and
endpoint.
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20.3 You as an analytical chemist
6.
A student uses 0.1250M NaOH to titrate 25.00 mL aliquots of an acetic acid solution.
The average titre required is 28.61 mL.
(a)
What is the concentration of the acetic acid solution?
(b)
What mass of CH3COOH would be present in 100 mL of the acid solution?
7.
Using the data from titrations 2 and 3, calculate the concentration of NaOH.
8.
Another student made up a standard solution containing 22.98 g of KHP and had
an average titre of 27.30 mL. What value would her calculations give for the molarity of
NaOH?
9.
(a)
Urea is an organic compound. Its formula is CO(NH2) 2. Draw a structural formula
for urea.
(b)
What are the formulas for all of the inorganic fertilisers mentioned previously?
10. Another batch of the fertiliser is analysed using the same concentrations of HCl and
NaOH. The size of the fertiliser sample is 1.500 g, the volume of NaOH added is
50.00 mL and the volume of HCl required is 25.17 mL. Does this sample meet label
specifications?
11. Use the ionic half-equation method of balancing redox equations to confirm the
coefficients in this equation.
12. What volume of I2 should be added if the apricots are to be sold within Australia?
13. Analysis of another sample taken from a freshly opened bottle of the same brand of apple
juice required an average I2 titre of 19.22 mL.
(a)
Does this sample live up to label specifications?
(b)
What impact would using a freshly opened bottle have on the validity of
the analysis?
Review questions
1.
Calculate the molarity of the following solutions.
(a)
0.054 mol of NaNO3 in 150 mL of solution
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2.
3.
4.
5.
(b)
28.6 g of Na2Cr2O7 in 200 mL of solution
(c)
8.5 × 1023 molecules of CH3COOH in 500 mL of solution
What mass of solid is needed to prepare each of the following solutions?
(a)
200 mL of 0.30M Zn(NO3)2
(b)
25 mL of 0.50M Na2C2O4
(c)
10 L of 0.1M NaOH
(d)
1 L of 0.25M CoCl2·6H2O
Arrange the following solutions of KNO3 in order of increasing concentration.
(a)
150 mL containing 0.125 mol
(b)
500 mL containing 0.440 mol
(c)
200 mL containing 28.6 g
(d)
1.5 L containing 40.0 g
For 30 mL of a 0.85M solution of AlCl3, calculate:
(a)
the amount of aluminium chloride
(b)
the mass of aluminium chloride
(c)
the amount of aluminium ion
(d)
the mass of aluminium ion
(e)
the amount of chloride ion
(f)
the mass of chloride ion.
A solution is made by mixing 25 g of CuSO4·5H2O, 20 g of Na2SO4 and 10 g of
Al2(SO4)3. The total volume of the final solution is 200 mL. Calculate the concentrations
of each of the ions in solution.
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6.
What are the concentrations of solutions prepared by diluting:
(a)
20 mL of 0.15M HCl to 100 mL
(b)
50 mL of 10M NaOH to 2.0 L
(c)
75 mL of 18M H2SO4 to 2.5 L?
7.
A 100 mL graduated cylinder contains 86 mL of 1.0M H2SO4. Sufficient concentrated
H2SO4 (18M) is added very slowly and carefully to make the solution up to the 100 mL
mark. The mixture is transferred to a beaker and stirred. What is the concentration of the
final solution?
8.
What volumes of 0.500M HCl and 0.100M HCl must be mixed to give 1.00 L of
0.200M acid?
9.
What volume of 0.250M KOH should be added to 150 mL of 0.125M KOH to make
a solution that has the same hydroxide ion concentration as a solution containing 15.0 g
of Ba(OH)2 per litre?
10. (a)
What are the main requirements for a primary standard?
(b)
Name an acid that can be used as a primary standard.
(c)
Name a solid that can be used as a primary standard to standardise HCl.
11. What is the concentration of the acid if:
(a)
25.00 mL of hydrochloric acid react with 24.02 mL of 0.9846M NaOH
(b)
30.00 mL of acetic acid react with 27.64 mL of 1.025M NaOH
(c)
18.05 mL of sulfuric acid react with 25.00 mL of 0.1455M KOH
(d)
22.63 mL of nitric acid react with 30.00 mL of 0.1296M NH3 solution?
For questions 12 and 13, state any assumptions made.
12. A student wishes to standardise a solution of HCl that she thinks is approximately 0.2M.
She decides to standardise the HCl using Na2CO3. What mass of the solid should she
weigh out:
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(a)
if she is going to titrate individual solid samples
(b)
to make up a standard solution using a 250 mL volumetric flask?
13. A solution of NaOH is prepared by weighing out approximately 10 g of the solid and
dissolving it in 1 L of water. What mass of KHP would be needed if the NaOH solution is
to be standardised against:
(a)
individual weighed samples
(b)
samples taken from 100 mL of a standard solution?
14. (a)
A sample of pure CaCO3 weighing 0.800 g requires 34.87 mL of HCl for complete
reaction. What is the concentration of the acid?
(b)
The same acid is then used to analyse an impure sample of CaCO3. In this
experiment, a 1.208 g sample of the solid requires a titre of 29.60 mL of HCl. What
is the percentage of CaCO3 in the solid?
For questions 15 and 16, state any assumptions made.
15. KMnO4 solution can be standardised using FeSO4·7H2O in the presence of H2SO4.
(a)
Write a balanced equation for the reaction.
(b)
Why must acid be added to the reaction mixture?
(c)
What mass of the iron(II) sulfate should be used to standardise a permanganate
solution that is thought to be 0.02M using:
(d)
(i)
individual weighed samples
(ii)
samples taken from 100 mL of a standard solution?
What is the indicator in this reaction?
16. KMnO4 solution can also be standardised using Na2C2O4 in the presence of H2SO4. What
mass of sodium oxalate should be used to standardise a permanganate solution that is
thought to be 0.05M using:
(a)
individual weighed samples
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(b)
samples taken from 250 mL of a standard solution?
17. A K2Cr2O7 solution contains 25.00 g of the salt per litre of solution. How many mL of
this solution are needed to react with 3.050 g of FeSO4·7H2O that has been dissolved in
water and mixed with a small amount of sulfuric acid?
18. In an analysis of iron tablets, the Fe2+ in the tablet is converted to Fe3+ using KMnO4.
What volume of 0.0400M KMnO4 would be needed if the tablet contained 300 mg of
Fe2+?
19. A 1.400 g sample of steel is dissolved in a minimum amount of sulfuric acid. The
solution is added to some distilled water in a 250 mL volumetric flask. The solution is
then diluted to a total volume of 250 mL. When 25.00 mL aliquots of this solution are
titrated with 0.02120M KMnO4, an average titre of 22.54 mL is needed. What is the
percentage of iron in the steel?
20. A 500 mg vitamin C tablet is dissolved in distilled water and then titrated against
0.05240M I2 solution using starch as an indicator. An average titre of 31.45 mL is
required. The equation for the reaction is:
C6H4O2(OH)4 + I2  C6H6O6 + 2H+ + 2I–
(a)
What is the mass of vitamin C in the tablet?
(b)
What is the percentage of vitamin C in the tablet?
21. An analyst is performing over–under tests on some foodstuffs, using starch indicator and
0.0200M I2 solution. What volume of the iodine solution should he add to:
(a)
a 50 g sample of apricots with a maximum allowable SO2 level of 2000 mg kg–1
(b)
a 50 g sample of sausage mince with a maximum allowable SO2 level of 500 mg
kg–1?
22. To determine the NaOCl concentration in a household bleach, iodide ion in acid solution
is added to the bleach. Hypochlorite anion oxidises the iodide to iodine, which is then
reduced by thiosulfate solution to iodide. These reactions are shown in the following
equations.
OCl– + 2I– + 2H+  I2 + Cl– + H2O
I2 + 2S2O32–  2I– + S4O62–
It is found that 37.98 mL of 0.8482M thiosulfate solution are needed to analyse
a 30.00 mL sample of the bleach.
(a)
What is the concentration of NaOCl solution?
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(b)
What mass of NaOCl would be present in the sample of bleach analysed?
(c)
Assuming that the density of the bleach is 1 g mL–1, what is the percentage by mass
of NaOCl in the bleach?
23. A 1.400 g sample of lawn fertiliser is dissolved in water and made up to 250 mL in
a volumetric flask. 40.00 mL of 0.1050M NaOH are added to 25.00 mL samples of the
solution. The solution is boiled to remove ammonia, and the excess base is then titrated
with 0.09855M HCl. The average HCl titre is 19.56 mL.
(a)
What amount of NaOH is added?
(b)
What amount of HCl is used in the titration?
(c)
What amount of NaOH reacts with the ammonium ion in the fertiliser?
(d)
What amount of NH4+ is present in the fertiliser sample?
(e)
What is the percentage of nitrogen as ammonium ion in the fertiliser?
24. A 0.500 g aspirin tablet containing aspirin and filler is analysed using the back titration
method. When the tablet is dissolved in 40.00 mL of 0.1855M NaOH, the following
reaction occurs.
C6H4(OCOCH3)COOH + 2NaOH  C6H4(OH)COONa + CH3COONa + H2O
20.33 mL of 0.2010M HCl are needed to titrate the excess NaOH. What is the percentage
of aspirin in the tablet?
Notes:
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