CHEM3120 Environmental and Analytical Chemistry

How CHEM3120 is assessed

What CHEM3120 covers

CHEM3120 Environmental and Analytical Chemistry runs across three lecture blocks — analytical foundations and techniques, atmospheric chemistry, then aqueous and metal-ion chemistry — and this ten-chapter map follows that teaching schedule in order.

The unit is assessed through weekly online quizzes (best 7 count, 10%), a written assignment (12%, around Week 9), a 40% laboratory component and a closed-book final exam (38%), with a theory-versus-laboratory double hurdle: you must pass both the theory and the laboratory to pass the unit.

Use the map to see how each block's calculations — Beer-Lambert, Henry's law, acid-base and carbonate speciation, pE-pH diagrams — build toward Part A short-answer and Part B multiple-choice on the final.

The unit joins two threads — the analytical chemistry of measuring species in the environment (sampling, resolution and interference, gas chromatography, atomic absorption and flame photometry, mass spectrometry and X-ray methods) and the environmental chemistry of the atmosphere, water and soil (the greenhouse effect and the carbon/nitrogen cycles, water hardness and dissolved oxygen, acid-base and carbonate speciation, ocean acidification, metal-ion redox and pE-pH diagrams, and water purification).

It is a calculation-dense unit: the marks live in choosing the right relationship (Beer-Lambert A = εcl, Henry's law, Ka/Kb and Henderson-Hasselbalch, Ksp precipitation tests, pE-pH lines), substituting with correct units, and interpreting the number.

The University of Sydney assesses CHEM3120 through weekly online quizzes (best 7 count, 10%), a written assignment (12%, around Week 9, submitted on Canvas via Turnitin), a 40% laboratory component and a closed-book final exam worth 38% (2 hours writing + 10 minutes reading; Part A short-answer ~80 marks + Part B 20 multiple-choice).

Critically, there is a theory-versus-laboratory double hurdle — you must pass BOTH the theory component and the laboratory to pass the unit — so steady work across the semester matters, and the CHEM3120 result feeds the Weighted Average Mark (WAM) that later chemistry units build on.

Worked example · free

Total water hardness as a CaCO3 equivalent

Key terms

Accuracy vs precision

Accuracy is closeness to the true value; precision is the reproducibility (spread) of repeated measurements. A method can be precise but inaccurate (a systematic bias) or accurate on average but imprecise.

Detection limit (LOD)

The lowest concentration giving a signal reliably distinguishable from baseline noise — commonly the concentration at which the peak height equals about twice the peak-to-peak baseline noise.

Beer-Lambert law

A = -log(I/I0) = εcl: absorbance is proportional to the molar absorption coefficient ε, concentration c and path length l. The quantitative backbone of AAS and molecular absorption.

Henry's law

At fixed temperature the dissolved concentration of a gas is proportional to its partial pressure: [X(aq)] = kH · P(X). Governs dissolved O2, N2 and CO2 in natural waters.

Speciation (distribution) diagram

A plot of the fraction of each acid-base species versus pH. Two species cross over (each 50%) at pH = pKa; one dominates roughly two pH units either side.

pE

A measure of a solution's electron availability, pE = -log[e-], analogous to pH for electrons; pE° = E°/0.0592 at 25 °C. High pE = oxidising, low pE = reducing conditions.

CHEM3120 FAQ

Questions students ask in each CHEM3120 chapter

What is the difference between accuracy and precision?

Accuracy is closeness to the true value; precision is the spread of repeated results. They are independent: you can be precise but consistently wrong (a systematic bias), or accurate on average but scattered (random error). A good method needs both, which is why calibration checks and replicates matter. Chapter 01 · Analytical Chemistry Foundations

What is the difference between AAS and flame photometry?

Both atomise the sample in a flame, but AAS measures ABSORPTION by ground-state atoms of light from a hollow-cathode lamp (it needs an external light source), while flame photometry measures the EMISSION from the small fraction of atoms that the flame thermally excites (no lamp). Emission suits easily excited elements like Na, K and Li; AAS is more general and uses Beer-Lambert, A = εcl, for quantitation. Chapter 02 · Analytical Techniques: GC, AAS & Flame Photometry

What actually makes a clay different from sand?

Chiefly particle size and structure. Clays are the finest fraction — hydrated, layered alumino-silicates from the acid hydrolysis of silicate rock — with very high surface area, so they hold water and exchange cations strongly. Sand is much coarser (roughly 0.7-1.4 mm grains, mostly silica inland or CaCO3 on beaches) with little surface chemistry. Silt sits in between. The clay fraction is where most of a soil's chemistry happens. Chapter 03 · Soil Chemistry & Salinity

What makes a pollutant persistent?

Chemical stability plus low reactivity and low volatility. A persistent organic pollutant resists photochemical and biological breakdown, so it has a long environmental half-life, and because it is fat-soluble it partitions into living tissue rather than being flushed away in water. Those same features let it travel and accumulate, which is why PCBs, DDT and dioxins remain in the environment long after use. Chapter 04 · Persistent Organic Pollutants

Why is CO2 a more important greenhouse gas than water vapour, despite being less abundant?

Because of WHERE it absorbs. Earth's thermal emission peaks in the infrared near 10 µm (Wien's law at ~288 K), and CO2's vibrational modes absorb strongly in that region, so CO2 intercepts a large share of the outgoing longwave radiation. Water vapour absorbs more at shorter IR wavelengths where Earth radiates comparatively little, so per molecule near the emission peak CO2 is disproportionately effective. Chapter 05 · Atmospheric Chemistry & the Carbon/Nitrogen Cycles

How do I express water hardness as a CaCO3 equivalent?

Convert the mass of each divalent ion (Ca2+, Mg2+) to moles, add the moles together, then re-express that total as CaCO3 by multiplying by 100 g/mol and dividing by the sample volume. The key idea is that hardness is a mole count of divalent ions on a common CaCO3 scale — you never simply add the milligrams of Ca2+ and Mg2+ together, because they have different molar masses. Chapter 06 · Aqueous Chemistry: Water Properties & Hardness

When can I use [H+] = √(Ka·C) for a weak acid?

When dissociation is small — the rule of thumb is that x = [H+] should be under about 5% of the initial concentration C. Then Ka = x²/(C - x) simplifies to x²/C and gives [H+] = √(Ka·C). Always check x/C afterwards; if it exceeds 5%, or the acid is very dilute, solve the full quadratic (supplied on the exam formula sheet) or add the water term [H+] = √(Ka[HA] + Kw). Chapter 07 · Acid-Base Speciation & Distribution Diagrams

Which carbonate species dominates in natural water?

Bicarbonate, HCO3-, across most of the natural range. Because HCO3- is the middle species and it dominates between pKa1 (~6.3) and pKa2 (~10.3), and most natural waters sit in that window (seawater ~8.1), bicarbonate is the main form of dissolved inorganic carbon. Carbonic acid dominates only below ~6.3 and carbonate ion only above ~10.3, both unusual in ordinary waters. Chapter 08 · Carbonate & Sulfide Equilibria; Ocean Acidification

What is pE and how does it relate to E°?

pE = -log[e-] is to electrons what pH is to protons: a measure of how oxidising or reducing a solution is. It links directly to electrode potential through pE° = E°/0.0592 at 25 °C, so a couple with a high standard potential has a high pE° and describes oxidising conditions. High pE means electrons are scarce (oxidising); low pE means electrons are abundant (reducing). Chapter 09 · Metal Ions: Hydrolysis, Redox & pE-pH Diagrams

What is the difference between coagulation and flocculation?

They are two stages of the same goal — getting fine particles to settle. Coagulation chemically destabilises the particles, overcoming the surface charge that keeps them apart, usually by adding an electrolyte or coagulant. Flocculation then uses gentle mixing to let those destabilised particles collide and grow into larger aggregates (flocs). Coagulation makes sticking possible; flocculation grows the flocs big enough to remove by sedimentation or filtration. Chapter 10 · Water Purification & Desalination

How to study for the exam

Treat CHEM3120 as a set of recurring calculation types layered on a conceptual spine, and rehearse both weekly rather than cramming through STUVAC.

Each week, take that block's signature calculation end to end — Beer-Lambert or standard addition for the analytical techniques, Wien's law and the energy balance for the atmosphere, hardness and Henry's law for water, Ka/Kb and speciation fractions for acid-base, the carbonate and pE-pH systems for the aqueous-metals block — and write the relationship, substitute with units, compute once, then interpret and sanity-check the result.

Because the weekly quizzes count best-7-of and the assignment falls around Week 9, keep every block warm rather than filing it away. The final is closed-book but supplies a formula sheet, periodic table and reduction-potential table, so drill METHOD and units, not memorised constants; and because there is a theory-versus-laboratory double hurdle, protect your laboratory marks as carefully as your exam preparation.

When a step will not click, ask Sia to re-explain that one step a different way and set you a fresh practice question in the same style — it teaches the method and checks your reasoning, and it never substitutes for your own graded work. Confirm the exam date, room and permitted materials on Canvas and the exam timetable.

Your AI Chemistry tutor for CHEM3120

Stuck on a hard CHEM3120 question? Sia is AskSia’s AI Chemistry tutor — ask any CHEM3120 Environmental and Analytical Chemistry question and get a clear, step-by-step explanation grounded in how the course is actually taught and assessed. Read this whole study guide free, then take your hardest questions to Sia.