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DSE Chemistry: Analytical Chemistry (Elective) — Practice Questions & Answers
Identifying ions by qualitative analysis, finding concentrations by titrimetric analysis, and using instrumental methods such as spectroscopy and chromatography.
376 practice questions available for this topic — here are 10 with full answers and explanations.
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Practice questions with answers
1
Multiple choice · Easy
When a substance is warmed with acidified potassium dichromate and the orange solution turns green, the substance is acting as
a reducing agent (it is oxidised)
an oxidising agent
a base
a catalyst
Tap an answer to check it.
Why The orange dichromate(VI) is reduced to green chromium(III), so the substance is a reducing agent (it is oxidised).
2
Multiple choice · Medium
A compound turns acidified potassium dichromate solution from orange to green. Which TWO functional groups could it contain?
an alcohol group or an aldehyde group
an alkane or a noble gas
a ketone or a carboxylic acid
a halogen or a nitro group
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Why Both a primary/secondary alcohol and an aldehyde are oxidised by acidified dichromate, turning it green.
3
Multiple choice · Hard
The colour change of acidified potassium dichromate from orange to green is due to the change
Cr(+6) in dichromate to Cr(+3) ions
Cr(+3) to Cr(+6)
Mn(+7) to Mn(+2)
Fe(+2) to Fe(+3)
Tap an answer to check it.
Why Dichromate Cr2O7 2- (orange, +6) is reduced to Cr3+ (green, +3).
4
Fill in the blank · Easy
Acidified potassium dichromate changes colour from orange to
Check answer
Answer:
green
Why Green.
5
Fill in the blank · Medium
In the dichromate test, the chromium changes oxidation state from +6 to
Check answer
Answer:
+3 / 3 / plus 3
Why +3.
6
Fill in the blank · Hard
A compound that oxidises dichromate but gives a negative 2,4-DNPH result and no carbonyl is likely a
Check answer
Answer:
carboxylic / carboxylic acid
Why Carboxylic acid.
7
Multiple choice · Easy
In a mass spectrum of a pure organic compound, the peak with the highest m/z (ignoring isotope peaks) usually corresponds to
the molecular ion, giving the relative molecular mass
the smallest fragment
the solvent
a doubly charged ion only
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Why The molecular ion peak (M+) occurs at the m/z equal to the relative molecular mass.
8
Multiple choice · Medium
A compound shows a molecular ion peak at m/z = 108 and a fragment at m/z = 91. The loss of 17 mass units suggests loss of
an OH group
a CH3 group
a C2H5 group
a Cl atom
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Why 108 - 91 = 17, the mass of an OH group (or NH3); loss of OH from a molecular ion is common.
9
Multiple choice · Hard
For a compound containing a -C6H5 group, a stable fragment ion at m/z = 91 is commonly assigned to
the C7H7+ (tropylium/benzyl) cation
the CH3+ cation
the OH- ion
a chloride ion
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Why m/z 91 corresponds to C7H7+ (the tropylium / benzyl cation), a very stable fragment from compounds with a benzyl group.
10
Fill in the blank · Easy
The peak at the highest m/z in a mass spectrum is called the
Check answer
Answer:
molecular / molecular ion / parent
Why Molecular ion (M+).
Key terms in Analytical Chemistry (Elective)
Qualitative analysis: The identification of which substances or ions are present in a sample.
Quantitative analysis: The determination of how much of a substance is present in a sample.
Chemical test: A reaction that gives an observable change used to identify a substance or ion.
Flame test: Using the characteristic colour given to a flame to identify a metal ion.
Precipitation test: Adding a reagent to form an insoluble solid whose colour helps identify an ion.
Gas test: A test that identifies a gas by a characteristic property such as relighting a glowing splint.
Titration: A volumetric technique to find an unknown concentration using a standard solution.
Standard solution: A solution of accurately known concentration used in quantitative analysis.
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