Chemistry IA · Structure 1 · Models of the particulate nature of matter · topic 3 of 25

Counting atoms: Avogadro's constant by electrolysis · IB Chemistry IA idea: guide, sample graph and step-by-step plan

a classic, with a number you can check

Avogadro's number is just handed to you. Now count atoms yourself, with a current, a clock and a balance.

Difficulty●●○ Medium
KitStandard: DC supply, variable resistor, ammeter, balance to 0.001 g, copper electrodes
TimeAbout 10 hours
FitsSL and HL (charge-to-moles link: introduce it yourself)
  • ✓ you like careful weighing
  • ✓ you can keep a current steady
  • ✓ you enjoy getting a famous number yourself

The big idea

← this is your RQ hook!

When copper deposits at the cathode, every Cu²⁺ ion needs two electrons. So the charge passed (current × time) and the mass gained tell you how many atoms that charge corresponds to. Bring in the charge on one electron and Avogadro's constant drops out. The idea is simple, so the marks are in the control: a steady current, clean dry electrodes, and a deposit that stays on. Predict whether the value should depend on the current at all, then test whether it does.

Make it yours: pick one twist

Copying a classic won't show your choices. Pick one of these (or invent your own) and your IA becomes yours.

Swipe for more twists →

Plot mass against charge

Run at several currents and times, and plot mass gained against charge passed.

→ One straight line through the origin, whose gradient gives your constant with an uncertainty from max and min lines. Stronger than one calculated value per run.
Vary the time, not the current

Fix the current and run for 5, 10, 15, 20 and 25 minutes.

→ A different independent variable. Check the current stays the same throughout every run.
Use the anode as a cross-check

Weigh both electrodes and compare the mass lost with the mass gained.

→ A second estimate and a test of how efficient the cell is. Your evaluation can explain any difference.
Change the anode

Repeat a series with a graphite anode instead of copper.

→ Gas forms at the anode and the solution changes, so you lose the cross-check. The cathode still gives your constant.
Change the concentration

Repeat at two or three copper(II) sulfate concentrations.

→ Tests whether the quality of the deposit, and so your value, depends on concentration. Adds a control you must justify.
Measure the charge better

Use a data logger or current probe instead of an ammeter reading.

→ Current is recorded over the whole run, so drift stops being your biggest uncertainty.

Your Chemistry IA research question: run the check

Write your own question first. Then make sure it ticks every box:

  • Names the independent variable, current in amperes, with its range and how many values you will use.
  • Names the dependent variable, the calculated constant, and says which formula, which measured quantities and which constants you treat as known.
  • Names the controls (time, concentration, volume, electrode area and gap, temperature) and how you hold each one steady.
  • Says what you compare your result with, and where that value comes from.
  • Is narrow enough: five or six currents, repeats at each, and a clear rule for rejecting a poor deposit.

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The set-up

One way to set it up. Draw your own version, with your real measurements, for the Method section.

DC+−Adcathode (−):dry the same way,weigh before + afteranode (+): coppertoo. Its mass loss isa cross-checkammeter in series:read it oftenCuSO₄(aq): samevolume andconcentration

How your Chemistry IA could be laid out

The usual order for a topic like this, what each part needs to show, and roughly how much of your 3,000 words it takes. The words are all yours.

Research question and context~250
Your question; why a macroscopic measurement can give a count of atoms; what you predict about the effect of current.
Background chemistry~400
The half-equation at the cathode, how charge, moles of electrons and number of atoms are linked, and the constants you take as known. Where the formula comes from, with a source.
Variablestable
Independent, dependent and controlled variables, each with how you set, measure or keep it constant, and why it matters.
Method~450
Apparatus with resolutions; how the solution was made (volume, concentration); how electrodes were prepared, rinsed and dried; how the current was held and logged; how time was measured; repeats.
Safety, ethics, environment~80
Copper(II) sulfate: harmful if swallowed, damaging to eyes, very toxic to aquatic life. Goggles and gloves, low voltage, used solution collected for heavy-metal waste, never down the sink. No flames if you rinse with ethanol or propanone.
Raw datatable
One clear table: current readings, time, electrode masses before and after, with units and the resolution of each instrument.
Processing~250
Mass gained, charge, the constant for each run, and how each uncertainty (especially the current) reaches your final value. One worked example.
Graphs and analysis~300
Your graph of the calculated constant against current, with error bars; what the shape says; what you saw on the electrodes.
Conclusion~350
Answer with numbers: what value did you get, how does it depend on current, and how does it compare with the accepted value, with its source.
Evaluation~600, the biggest part!
Weaknesses ranked by effect on the final value (use numbers), the limits of the range, and improvements linked to each.
Referencesnot counted
Every source for the accepted value, constants and method.

What your Chemistry IA graph could look like

accepted value drawn as a line ✓

Made-up data, to show you the form of a strong graph, not the answer. Yours will look different. example only

0.00.20.40.60.81.01.24.55.05.56.06.57.0Current I / A (± 0.02 A)Calculated Avogadro constant / 10²³ mol⁻¹accepted value, 6.022if powdery copper falls off at high currentyour runs, with error barshigh current: does the deposit stay on?what did you see on the electrode?ILLUSTRATIVE · MADE-UP DATA
  • Axis labels carry the unit and the uncertainty of the quantity you set.
  • The accepted value is a line to compare with, not a target to hit. Never adjust data towards it.
  • Real error bars differ in length: the same ±0.002 g is a bigger fraction of a small mass at low current. Here they are drawn equal to keep the sketch simple.
  • If points drift away only at high current, explain it from what you saw, not from a guess.
  • Another graph to consider: mass gained against charge passed. It must go through the origin (no charge, no copper), and its gradient gives the constant.

Which uncertainty matters most?

rank them: don't just list them

Rank by the effect on your result, the constant, not on the raw readings. Here is one made-up run at 0.60 A for 20 minutes (about 0.24 g of copper). Work out yours.

Current drift (±0.02 A of 0.60 A)3.3%Copper lost on rinsing (0.003 g)1.3%Mass gain (±0.002 g of 0.237 g)0.8%Timing (±2 s of 1200 s)0.2%biggest effect: discuss this first!% uncertainty in the constant for one run (0.60 A, 20 min)ILLUSTRATIVE · MADE-UP NUMBERS

Your game plan

don't skip the trial run!

Tick steps off as you go. We'll remember on this device.

0 of 11 done

  1. Read and predictFind the half-equation and the link between charge and moles of electrons. Write down what you predict: should the calculated value depend on current at all? Why might it?
  2. Trial run (1 lesson)Run one mid-range current for your chosen time. Is the deposit firm or powdery? Does the ammeter drift? Do you weigh the same way twice and get the same mass? Fix these first.
  3. Decide range, time and repeatsChoose five or six currents and a time that gives a mass you can weigh well. Write the reason for each choice; it becomes your method justification.
  4. Fix the electrode routineDecide exactly how you clean, rinse, dry and weigh the cathode, and do it identically every time. Note the electrode area.
  5. CollectLog the current at regular intervals. Weigh before and after. Record what the deposit looks like each time; it is evidence for your evaluation.
  6. ProcessWork out the charge, the moles of copper and the constant for each run. Decide how the uncertainty in current and mass will carry through.
  7. Graph and fitPlot your constant against current with error bars. Decide what a flat line, a drift or a scatter would each mean.
  8. Compare with the accepted valueFind a cited accepted value. How far is yours, compared with your own uncertainty? Which runs do you trust, and what rule says so?
  9. EvaluateWhich uncertainty or systematic loss mattered most? Rank them with numbers. Make each improvement specific to a weakness you found.
  10. Write, then cutWrite in the order of the layout above, within 3,000 words. Tables, calculations and graphs are not counted.
  11. Check before your teacher's draftRun the checklist, then get your RQ or draft checked. Your teacher reads one draft only.

Chemistry you'll need

Traps to avoid in this Chemistry IA

  • Powdery, loose copper at high current falls off or flakes during rinsing. The mass is lost and the constant comes out low. Look at the deposit and say what you saw.
  • Weighing wet or unevenly dried electrodes: water adds mass, so the gain looks too big.
  • Using the first ammeter reading as the current for the whole run. If it drifts, the charge is wrong, so log it.
  • Forgetting that each Cu²⁺ ion takes two electrons, which puts a factor of 2 in the wrong place.
  • Treating the percentage difference from the accepted value as the uncertainty of your result.
  • Cleaning or weighing the electrode a different way each time, so mass differences are partly routine, not chemistry.
  • Quoting the constant to six significant figures from a current known to two.

More Chemistry IA ideas like this

Make sure it's yours

This guide shows how a topic like this is usually approached. It is not an IA. Your question, choices, data, analysis and words must be your own: change something, tell your teacher where the idea came from, and follow your school's rules on outside help.

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