Difficulty: Intermediate | Prerequisites: Basic understanding of moles, atomic mass, and simple stoichiometry. Review density concepts from the earlier lab.
These two labs sit at the intersection of measurement and theory. The Avogadro's number experiment uses electrolysis to deposit a known metal, then works backward from the mass deposited to estimate the number of atoms in a mole. The chemical formula lab takes that same quantitative thinking and applies it to figuring out what a compound actually is, atom by atom. Both labs ask you to connect physical measurements (mass, current, time) to abstract chemical quantities (moles, ratios, formulas). If you are comfortable with mole conversions and balancing charges, you are ready.
Avogadro's number (6.022 x 10²³) tells you how many particles are in one mole. In the lab, you estimate it by running electrolysis, measuring the mass of metal deposited, and calculating back to the number of atoms. The chemical formula lab uses the Law of Definite Proportions and mass data to find the mole ratio of elements in a compound such as copper chloride hydrate (CuₓClᵧ · zH₂O).
Avogadro's number
6.022 x 10²³. The number of atoms, molecules, or formula units in exactly one mole of a substance. Think of it as the bridge between the scale of individual atoms and the scale of grams you can weigh on a balance.
Electrolysis
An electrochemical process driven by an external power supply that forces a non-spontaneous chemical reaction to occur. In the lab, you pass current through a solution to deposit metal onto an electrode.
Electrode identification (MSU lab)
Zinc = black lead. Copper = red lead. Mixing these up will give you wrong results and possibly damage equipment.
Law of Definite Proportions
The mole ratios of elements in a given compound are always fixed, small whole numbers. In simple terms, water is always H₂O, never H₃O or HO₂.
Law of Multiple Proportions
Some pairs of elements can form more than one compound, each with a different but still whole-number mole ratio. Carbon and oxygen, for instance, form both CO and CO₂.
Hydrate
A compound that includes water molecules locked into its crystal structure. The general formula is written as CuₓClᵧ · zH₂O, where z tells you how many water molecules are present per formula unit.
Mole ratio
The ratio of moles of one element to moles of another in a compound. This is what you calculate from your mass data to determine the empirical formula.
6.022 x 10²³ particles per mole.
This constant lets you convert between the number of individual atoms (or molecules) and the number of moles, which is the scale you actually work at in the lab.
An external power supply drives a non-spontaneous reaction.
Two electrodes are submerged in a solution:
Zinc electrode = connected to the black lead.
Copper electrode = connected to the red lead.
Current flows through the solution, causing metal ions to gain electrons and deposit as solid metal on one electrode.
You weigh the electrode before and after electrolysis.
The mass gained (or lost) tells you how much metal was deposited (or dissolved).
From the mass of metal, you calculate the number of moles, and from the current and time, you calculate the number of electrons transferred.
Combining these gives you an experimental estimate of Avogadro's number.
John Dalton's atomic theory provides the framework: elements combine in fixed, whole-number ratios.
The Law of Definite Proportions says that a given compound always contains the same elements in the same mass ratio.
The Law of Multiple Proportions says that the same pair of elements can form different compounds, each with its own distinct ratio.
The target compound is copper chloride hydrate, written as CuₓClᵧ · zH₂O.
You determine x, y, and z by:
Heating the hydrate to drive off all water. The mass lost equals the mass of water (gives you z).
Dissolving the anhydrous salt and using a reaction to separate copper from chloride.
Measuring the mass of each component.
Converting masses to moles.
Dividing by the smallest number of moles to get the simplest whole-number ratio.
The subscripts x, y, and z in CuₓClᵧ · zH₂O.
These must be small whole numbers (or very close to them, within rounding from experimental error).
N_A = 6.022 \times 10^{23} \text{ mol}^{-1}Avogadro's number: the number of particles in one mole.
\text{moles} = \frac{\text{mass (g)}}{\text{molar mass (g/mol)}}The fundamental conversion between grams and moles.
\text{mole ratio} = \frac{\text{moles of element A}}{\text{moles of element B}}Divide each by the smallest to get whole-number subscripts for the empirical formula.
General formula for copper chloride hydrate:
\text{Cu}_x\text{Cl}_y \cdot z\text{H}_2\text{O}Electrolysis is the industrial process behind electroplating (coating jewellery or car parts with a thin layer of metal), aluminium refining, and the production of chlorine gas from brine. Every time you see a chrome-plated tap or a gold-plated connector, electrolysis made it happen.
Determining chemical formulas is essential in pharmaceutical manufacturing, where the exact composition of a drug must be verified, and in geological surveying, where mineral samples are analysed to identify new deposits.
Students sometimes think electrolysis and a galvanic (voltaic) cell are the same thing. They are not. Electrolysis requires an external power supply to force a non-spontaneous reaction. A galvanic cell produces electricity from a spontaneous reaction.
Students often round mole ratios too aggressively. If your ratio comes out to 1 : 1.5, multiply everything by 2 to get 2 : 3. Only round when the values are very close to whole numbers (e.g. 1.98 rounds to 2).
Confusing the Law of Definite Proportions with the Law of Multiple Proportions is common. Definite Proportions is about one compound having fixed ratios. Multiple Proportions is about the same two elements forming different compounds with different ratios.
Forgetting to account for water of hydration. If you skip the heating step or do not heat long enough, your z value will be wrong.
⚠️ Know which lead connects to which electrode: zinc = black, copper = red.
⚠️ Be able to convert between mass, moles, and number of atoms using Avogadro's number and molar mass.
⚠️ Expect a problem where you are given mass data for each element in a compound and asked to determine the empirical formula.
⚠️ Understand the difference between the Law of Definite Proportions and the Law of Multiple Proportions. Exam questions often test whether you can distinguish them.
Fill in the blank: Avogadro's number is ______ particles per mole.
6.022 x 10²³.
True or false: Electrolysis is a spontaneous process.
False. It requires an external power supply.
Fill in the blank: In the MSU lab, the zinc electrode connects to the ______ lead.
Black.
True or false: The Law of Definite Proportions states that elements can combine in multiple different ratios.
False. That describes the Law of Multiple Proportions. Definite Proportions means a single fixed ratio.
Fill in the blank: To find z in CuₓClᵧ · zH₂O, you heat the sample and measure the ______ lost.
Mass (of water).
Q: A sample contains 1.27 g of copper and 0.71 g of chlorine. What is the mole ratio of Cu to Cl?
A: Moles of Cu = 1.27 / 63.55 = 0.0200 mol. Moles of Cl = 0.71 / 35.45 = 0.0200 mol. The ratio is 1 : 1, so the empirical formula for the copper-chlorine part is CuCl.
Q: Why is an external power supply needed for electrolysis?
A: The reaction is non-spontaneous, meaning it will not proceed on its own. The power supply provides the energy (as electrical current) to force the reaction to occur.
Q: A hydrated sample weighs 3.50 g before heating and 2.13 g after heating. How many grams of water were in the sample?
A: 3.50 - 2.13 = 1.37 g of water.
Q: You calculate a mole ratio of 1 : 1.5 for elements A and B. What is the empirical formula?
A: Multiply both by 2 to get the smallest whole numbers: A₂B₃.
Q: State the Law of Definite Proportions in one sentence.
A: A given chemical compound always contains the same elements in the same fixed mass (and therefore mole) ratio.
The concept of moles and Avogadro's number underpins everything in this course. The density lab used mass and volume; here you extend that to counting atoms. The fertiliser lab (next set of notes) also relies on mole-to-mass conversions when you calculate the percentage of phosphorus in a sample.
The Law of Definite Proportions connects to stoichiometry in lecture: if you can determine a formula, you can predict how much product a reaction will yield.
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