Practical Chemistry
and Qualitative
Analysis

Singapore Junior Chemistry Olympiad

By the end of this chapter, you can answer

1

You need exactly 25.0 cm3 of acid. Which piece of glassware do you reach for?

2

How do you collect ammonia, and why can you not collect it over water?

3

Why do you swirl the flask and stand it on a white tile in a titration?

4

Two solutions both give a white precipitate with sodium hydroxide. How do you tell them apart?

5

Why must you never add hydrochloric acid before testing for chloride ions?

6

Which measurement in an experiment is the biggest source of error, and what do you change to reduce it?

Recall: Chapters 5 and 6

1Name two oxides that react with both acids and alkalis.

Zinc oxide and aluminium oxide (amphoteric)

2Write the ionic equation for the precipitation of barium sulfate.

Ba2+(aq) + SO42−(aq) → BaSO4(s)

3What does acidified KMnO4 going colourless show?

A reducing agent is present.
1

Measuring Physical Quantities

quantitySI unitapparatus and accuracy
timesecond (s)digital stopwatch ±0.01 s, analogue ±0.1 s
temperaturekelvin (K)thermometer, temperature probe with data logger
lengthmetre (m)metre rule ±0.1 cm
masskilogram (kg)electronic balance ±0.01 g
volumecubic metre (m3)pipette, volumetric flask, measuring cylinder, burette, gas syringe
  • K = °C + 273.   1 dm3 = 1000 cm3 = 1 litre.
1

Measuring Physical Quantities

Measuring volume

pipetteone accurate fixed volume, such as 25.0 cm3
volumetric flaskone larger fixed volume, such as 250 cm3
measuring cylindera range, to the nearest 0.5 cm3
burettea range, to the nearest 0.05 cm3
gas syringea gas, up to 100 cm3

Eye level with the meniscus. From above or below: a parallax error.

1

Measuring Physical Quantities

Your turn(a) Give one advantage of a burette over a measuring cylinder. (b) Give one disadvantage of a pipette.

(a)
More accurate (0.05 cm3 against 0.5 cm3), and it measures a range of volumes.[1]
(b)
It measures only one fixed volume.[1]

Quick checkWhich apparatus measures exactly 25.0 cm3 of acid most accurately?

Ameasuring cylinder
Bpipette
Cbeaker
Dgas syringe
NoteB: a pipette delivers one accurate fixed volume, such as 25.0 cm3.
2

Collecting and Drying Gases

Choose from two properties: solubility in water and density compared with air.

Water displacementinsoluble or slightly soluble gases
Upward deliverygases less dense than air
Downward deliverygases denser than air
  • Water displacement: insoluble or slightly soluble gases, such as carbon dioxide.
  • Upward delivery: less dense than air (ammonia).
  • Downward delivery: denser than air (chlorine, HCl, SO2).
2

Collecting and Drying Gases

Drying a gas

A wet gas is dried by passing it through a substance that absorbs water.

2

Collecting and Drying Gases

Drying gases

The drying agent must not react with the gas. Think acid and base (Chapter 5).

drying agentuse fornot for
conc. sulfuric acid (acid)most gases, including Cl2, HClammonia (a base)
quicklime, CaO (basic oxide)ammoniaCO2 (acidic oxide)
calcium chloridegases that do not react with itammonia

Your turn(a) Gas X is less dense than air and insoluble in water. Suggest a method to collect it. (b) Gas Y reacts with acids. Name a drying agent for it and explain.

(a)
Water displacement (insoluble), or upward delivery (less dense than air)[1]
(b)
Quicklime: concentrated sulfuric acid would react with it.[1]
3

Titration Technique

  • The titrant, of known concentration, goes in the burette.
  • Pipette 25.0 cm3 of the analyte into the flask and add a few drops of indicator. Stand it on a white tile, so the colour change is easy to see.
  • Run in the titrant, swirling so it mixes at once, and near the end add it drop by drop. Stop at a permanent colour change.
  • Titre = final − start reading, to 0.05 cm3. Repeat until two titres agree to within 0.10 cm3, then average them.
3

Titration Technique

Your turn(a) Why is the flask swirled during a titration? (b) A burette reads 1.20 cm3 at the start and 24.65 cm3 at the end-point. Find the titre.

(a)
To mix the titrant in, so the colour changes exactly when enough has been added.[1]
(b)
24.65 − 1.20 = 23.45 cm3[1]

Quick checkWhich burette reading is recorded correctly?

A23 cm3
B23.4 cm3
C23.45 cm3
D23.453 cm3
NoteC: a burette reads to the nearest 0.05 cm3, so readings have two decimal places.
4

Testing for Cations

Add NaOH(aq) or NH3(aq) a few drops at a time, then in excess. Most metal hydroxides are insoluble, so a metal cation gives a precipitate. Look for its colour, whether it dissolves in excess, and ammonia gas.

cationNaOH(aq)NH3(aq)
Al3+white ppt., soluble in excesswhite ppt., insoluble
Zn2+white ppt., soluble in excesswhite ppt., soluble in excess
Mg2+white ppt., insolublewhite ppt., insoluble
Ca2+white ppt. only if [Ca2+] is highno ppt.
NH4+, Ba2+no ppt. (NH4+: NH3 on warming)no ppt.
Cu2+pale blue ppt., insolubleblue ppt., dissolves to a dark blue solution
Fe2+ / Fe3+green, browns in air / red-brown, insolublethe same
Mn2+off-white, quickly browns in air, insolublethe same
Cr3+grey-green, dissolves to a dark green solutiongrey-green, insoluble

Mg2+, Ba2+, Cr3+ and Mn2+ are the A Level additions (Data Booklet).

4

Testing for Cations

  • Zn, Al and Cr(III) hydroxides are amphoteric: they react with excess NaOH.
  • Excess NH3 forms soluble compounds with Cu2+ and Zn2+ (Chapter 16).
  • Fe(OH)2 and Mn(OH)2 brown as air oxidises the metal (Chapter 6).
  • Na+, K+, NH4+ and Ba2+ hydroxides are soluble: no ppt.
4

Testing for Cations

One test is often not enough. A white ppt. with NaOH could be any of four ions, and “no ppt.” could be NH4+ or Ba2+. Follow the branch, then use the other alkali.

4

Testing for Cations

4

Testing for Cations

still possibledo thisresult
Al3+ or Zn2+excess NH3(aq)Zn2+ dissolves, Al3+ does not
Mg2+ or Ca2+NH3(aq)Mg2+ white ppt., Ca2+ none
Al3+ or Mg2+excess NaOH(aq)Al3+ dissolves, Mg2+ does not
NH4+ or Ba2+warm with NaOH(aq)NH4+ gives NH3, Ba2+ none

AnalogyA sorting hat. NaOH sorts the cations into houses by what you see. Where a house still holds several ions, NH3 splits them.

4

Testing for Cations

Your turnCompound X contains aluminium, calcium or zinc ions. How would you find out which?

Add NaOH(aq) to excess: insoluble white precipitate shows Ca2+.[1]
If it dissolves, add NH3(aq) to excess: insoluble shows Al3+, soluble shows Zn2+.[1]

Your turnA light blue precipitate with aqueous ammonia dissolves in excess. Give the final colour and the cation.

Dark blue. Cu2+[1]

Quick checkWhich cation gives a white precipitate with both aqueous sodium hydroxide and aqueous ammonia, soluble in excess of both?

AAl3+
BCa2+
CZn2+
DMg2+
NoteC: Al(OH)3 and Mg(OH)2 do not dissolve in excess ammonia, and Ca2+ gives nothing with ammonia.
5

Testing for Anions

aniontestpositive result
CO32−dilute acidCO2: limewater milky
SO32−dilute acidSO2: acidified KMnO4 purple to colourless. With Ba2+: white ppt., dissolves in acid
NO2−dilute acidcolourless NO, turns brown in air (NO2)
Cl− / Br− / I−dilute HNO3, then AgNO3(aq), then NH3(aq)white, soluble / pale cream, partly soluble / yellow, insoluble
SO42−dilute HNO3, then Ba(NO3)2(aq)white BaSO4, insoluble in excess acid (BaSO3 is also white, but dissolves)
NO3−NaOH(aq), then Al foil, warmNH3: damp red litmus turns blue

Br−, NO2− and SO32− are the A Level additions. Al reduces N from +5 to −3, and SO2 reduces MnO4− (Chapter 6).

5

Testing for Anions

5

Testing for Anions

The order and the acid matter

  • Acidify with nitric acid first: it removes carbonate, sulfite and hydroxide, which would also precipitate with Ag+ or Ba2+.
  • Never HCl before a chloride test or H2SO4 before a sulfate test: you would add the ion you are testing for.
  • NH4+ and NO2− also give NH3. Rule them out first: warm with NaOH alone, and test with acid.
  • BaSO3 and BaSO4 are both white. Only BaSO3 dissolves in dilute strong acid.
5

Testing for Anions

Your turn(a) Describe the test for sulfate ions. (b) A solution contains NaCl or NaI. How would you tell which? (c) How would you tell a sulfite from a sulfate?

(a)
Dilute HNO3, then Ba(NO3)2(aq): white precipitate[1]
(b)
Dilute HNO3, then AgNO3(aq): white means chloride, yellow means iodide[1]
(c)
Dilute acid: sulfite gives SO2, which decolourises acidified KMnO4. Sulfate gives no gas.[1]
5

Testing for Anions

Quick checkA student adds dilute hydrochloric acid to a solution, then aqueous silver nitrate, and sees a white precipitate. Why does this not prove the solution contained chloride ions?

ASilver nitrate does not react with chloride ions.
BThe hydrochloric acid itself added chloride ions.
CSilver chloride is soluble in acid.
DThe precipitate should be yellow.
NoteB: HCl supplies Cl−, so a precipitate would form anyway. Acidify with dilute nitric acid instead.
6

Testing for Gases

Tests for gases

6

Testing for Gases

gastestpositive result
H2 (colourless, odourless)burning splintgoes out with a ‘pop’
O2 (colourless, odourless)glowing splintrelights
CO2 (colourless, odourless)limewaterwhite precipitate, dissolves on further bubbling
Cl2 (pungent)damp blue litmusred, then bleached
SO2 (pungent)filter paper with acidified KMnO4purple to colourless
NH3 (pungent)damp red litmusturns blue

NH3 is the only common alkaline gas. SO2 is a reducing agent. Cl2 makes an acid with the water on the paper, then bleaches it.

6

Testing for Gases

Worked exampleSolid K gives off a vapour that condenses to a liquid boiling at 100 °C. Its solution: blue precipitate with NaOH and a gas that turns red litmus blue on warming. The precipitate dissolves in excess NH3. With HNO3 then Ba(NO3)2: white precipitate. What does K contain?

water of crystallisation
liquid boiling at 100 °C
Cu2+ and NH4+
blue, dark blue in excess NH3, and NH3 gas
SO42−
white BaSO4
6

Testing for Gases

Your turn(a) A reaction gives off a colourless, odourless gas. Which gases could it be, and how would you tell them apart? (b) How would you tell chlorine, ammonia and hydrogen apart?

(a)
H2 (pop with a burning splint), O2 (relights a glowing splint) or CO2 (limewater milky)[2]
(b)
NH3 turns damp red litmus blue, Cl2 turns damp blue litmus red then bleaches it, H2 pops[2]
7

Accuracy, Precision and ErrorCore · A Level

Accuracy: how close a result is to the true value. Precision: how close repeated results are to each other.

A systematic error shifts every reading the same way, so repeating does not remove it. A random error scatters readings around their mean, so averaging reduces it.

AnalogyDarts. A player who always hits the same spot left of centre is precise but not accurate. Fix the aim (the instrument or the reading) to gain accuracy. Throw again (repeat) to check the precision.

7

Accuracy, Precision and ErrorCore · A Level

percentage uncertainty = uncertainty of the instrumentmeasured value × 100%

Worked example5.0 cm3 of acid in a measuring cylinder (±0.5 cm3) and 2.00 g of solid on a balance (±0.01 g). Which is the larger source of error, and what change reduces it?

cylinder: 0.5 ÷ 5.0 × 100% = 10%
uncertainty ÷ reading
balance: 0.01 ÷ 2.00 × 100% = 0.5%
the cylinder is the larger source
burette (±0.05 cm3): 1%
a smaller instrument uncertainty
7

Accuracy, Precision and ErrorCore · A Level

Your turn(a) Which has the larger percentage uncertainty: 20.0 cm3 in a measuring cylinder (±0.5 cm3) or 2.00 g on a balance (±0.01 g)? (b) Five readings of a 5.00 g mass are 5.21, 5.20, 5.22, 5.21 and 5.20 g. Accurate, precise, both or neither?

(a)
The cylinder: 2.5% against 0.5%[1]
(b)
Precise but not accurate: close together, all about 0.2 g too high[1]

Quick checkRepeated titres agree closely but are all larger than the true value. Which change would make them more accurate?

ARepeat the titration more times.
BRead the burette with the eye level with the meniscus.
CUse a larger conical flask.
DAverage all the titres.
NoteB: the error is the same every time, so repeating or averaging does not remove it. A correct reading removes a parallax error.

Exit ticket

1How do you collect a gas that is denser than air and soluble in water?

Downward delivery[1]

2Which cation gives a red-brown precipitate with aqueous sodium hydroxide?

Iron(III), Fe3+[1]

3What is the test for oxygen?

A glowing splint relights.[1]

4Define precision.

How close repeated measurements are to each other[1]