When a Zn-Al alloy dissolves in HCl, the total hydrogen gas collected depends on how much of each metal was present, because zinc and aluminium produce H₂ at different molar ratios. Measure the gas volume, correct the pressure with Dalton's Law, plug into PV = nRT to get total moles of H₂, then solve a pair of simultaneous equations to find the mass (and percentage) of each metal in the alloy.
Difficulty: Introductory to Intermediate
Prerequisites: Basic stoichiometry (mole conversions, balancing equations), the Ideal Gas Law (PV = nRT), and Dalton's Law of Partial Pressures. Familiarity with unit conversions (mmHg to atm, Celsius to Kelvin) is assumed.
This experiment sits at the intersection of gas laws, stoichiometry, and analytical chemistry. The core question is practical: given an alloy of two known metals (zinc and aluminium), how do you figure out how much of each is in there without physically separating them?
The trick is that each metal produces a different amount of hydrogen gas per gram when dissolved in acid. By measuring the total gas evolved and applying the Ideal Gas Law, you can back-calculate the composition. This connects general chemistry concepts (balanced equations, mole ratios, gas behaviour) to a real analytical technique.
You should already be comfortable converting between grams, moles, and litres of gas, and you should know how to correct a gas pressure for water vapour using Dalton's Law.
Ideal Gas Law (PV = nRT)
The equation relating pressure (P), volume (V), number of moles (n), the gas constant (R), and temperature (T) for a gas behaving ideally. In simple terms, if you know three of those four variables, you can solve for the fourth.
Dalton's Law of Partial Pressures
The total pressure of a gas mixture equals the sum of the partial pressures of each individual gas. Think of it as: each gas in a mixture contributes its own slice of the total pressure, independently of the others.
Partial pressure of hydrogen (P_H₂)
The portion of total gas pressure attributable to hydrogen alone. In this experiment, the collected gas is a mixture of H₂ and water vapour, so you must subtract the water vapour pressure (and correct for any height difference in water levels) to isolate the hydrogen pressure.
Vapour pressure of water (P_H₂O)
The pressure exerted by water molecules evaporating into the gas space above liquid water. It depends on temperature and must be subtracted from the barometric pressure when calculating the pressure of dry hydrogen.
Water displacement (gas collection over water)
A technique where the gas produced in a reaction pushes water out of an inverted bottle. The volume of water displaced equals the volume of gas collected. In simple terms, the gas shoves the water aside, and you measure how much water moved.
Height correction pressure (P_ΔH)
A small pressure correction for the difference in water levels between the collection bottle and the beaker. Calculated by dividing the height difference (in mm) by 13.6 (the density of mercury relative to water). Whether you add or subtract it depends on which water level is higher.
Stoichiometric ratio
The mole-to-mole relationship between reactants and products in a balanced chemical equation. For zinc, the ratio of Zn to H₂ is 1 : 1. For aluminium, the ratio of Al to H₂ is 2 : 3 (or equivalently, 1 mol Al produces 1.5 mol H₂).
Simultaneous equations (system of two equations)
A pair of equations with two unknowns that you solve together. Here, one equation says the masses of Zn and Al add up to the total alloy mass, and the other says the moles of H₂ from each metal add up to the total H₂ measured. Solving them gives you the mass of each metal.
Percent composition (% by mass)
The mass of one component divided by the total mass of the mixture, multiplied by 100. This is how the result is reported: what fraction of the alloy is zinc, what fraction is aluminium.
Zinc reacts with hydrochloric acid in a straightforward 1 : 1 mole ratio with hydrogen gas:
Zn(s) + 2 H⁺(aq) -> Zn²⁺(aq) + H₂(g)
1 mol Zn produces 1 mol H₂
Molar mass of Zn = 65.38 g/mol (the lab report uses 63.38, which is incorrect, but follow your course's given value)
Aluminium reacts with a different ratio, producing more hydrogen per mole of metal:
2 Al(s) + 6 H⁺(aq) -> 2 Al³⁺(aq) + 3 H₂(g)
1 mol Al produces 1.5 mol H₂
Molar mass of Al = 26.98 g/mol
The key insight: per gram, aluminium generates far more hydrogen than zinc does. This difference is what makes the experiment work. If the alloy were pure zinc, you would get one specific volume of gas; if pure aluminium, a much larger volume. The actual volume falls somewhere in between, and where it falls tells you the ratio.
Hydrogen gas is collected by displacing water from an inverted bottle. The volume of water pushed out equals the volume of gas collected. But the gas above the water is not pure hydrogen; it is saturated with water vapour.
To find the pressure of dry hydrogen, apply Dalton's Law:
P_bar = P_H₂ + P_H₂O +/- P_ΔH
Rearranged: P_H₂ = P_bar - P_H₂O +/- P_ΔH
The height correction (P_ΔH) accounts for the difference in water levels between the collection bottle and the beaker:
P_ΔH = height difference in mm / 13.6
The factor 13.6 converts mm of water to mm of mercury (mercury is 13.6 times denser than water)
If the water level in the collection bottle is higher at the end, subtract P_ΔH (the water column presses down on the gas)
If the water level in the displaced-water beaker is higher, add P_ΔH
Once you have the pressure of dry hydrogen (in atm), the volume of gas (in litres), and the temperature (in Kelvin), plug into:
n = PV / RT
R = 0.08206 L·atm / (mol·K)
T must be in Kelvin: T(K) = T(°C) + 273.15
This gives you the total moles of hydrogen produced by both metals combined.
You have two unknowns (mass of Zn and mass of Al) and two equations:
Equation 1 (mass balance): mass_Zn + mass_Al = mass_alloy
Equation 2 (mole balance for H₂): (mass_Zn / M_Zn)(1) + (mass_Al / M_Al)(3/2) = n_total
Substitute mass_Al = mass_alloy - mass_Zn into Equation 2, then solve for mass_Zn. The algebra:
(mass_Zn / 65.38)(1) + ((mass_alloy - mass_Zn) / 26.98)(3/2) = n_total
Expand, collect terms in mass_Zn, solve
Once you have mass_Zn, find mass_Al by subtraction, and convert each to a percentage:
% Zn = (mass_Zn / mass_alloy) x 100
% Al = (mass_Al / mass_alloy) x 100
As a check, calculate how many moles of H₂ each metal should produce individually:
Moles H₂ from Zn = (mass_Zn / M_Zn) x 1
Moles H₂ from Al = (mass_Al / M_Al) x (3/2)
These should sum to your total n from the Ideal Gas Law
Ideal Gas Law (rearranged for moles)
n = \frac{PV}{RT}Where P is in atm, V in litres, R = 0.08206 L·atm/(mol·K), and T in Kelvin.
Height correction for water column
P_{\Delta H} = \frac{\text{height difference (mm)}}{13.6}Pressure of dry hydrogen
P_{H_2} = P_{\text{bar}} - P_{H_2O} \pm P_{\Delta H}Add P_ΔH if the displaced-water bottle level is higher; subtract if the starting bottle level is higher at the end.
Simultaneous equations for alloy composition
m_{Zn} + m_{Al} = m_{\text{alloy}}\frac{m_{Zn}}{M_{Zn}} \cdot 1 + \frac{m_{Al}}{M_{Al}} \cdot \frac{3}{2} = n_{\text{total } H_2}Percent composition
\% \text{Zn} = \frac{m_{Zn}}{m_{\text{alloy}}} \times 100Molar mass from Ideal Gas Law (used in post-lab Q7)
MM = \frac{mRT}{PV}Propagation of error (absolute error for a product/quotient)
\frac{\delta_{\text{result}}}{\text{result}} = \frac{\delta_m}{m} + \frac{\delta_T}{T} + \frac{\delta_P}{P} + \frac{\delta_V}{V}This type of analysis is used in metallurgy and quality control to verify alloy compositions without expensive instruments like X-ray fluorescence. Dissolving a sample in acid and measuring evolved gas is a classical wet-chemistry approach.
The broader principle of using gas volume to determine the amount of a reactant comes up in environmental monitoring (measuring CO₂ from combustion), food science (measuring CO₂ from fermentation), and industrial process control.
Students often forget to subtract the vapour pressure of water and use the barometric pressure directly as the hydrogen pressure. The gas collected over water is always a mixture, so this correction is never optional.
Students sometimes confuse which way the height correction goes. The rule: if the displaced-water side is higher, the gas pressure is slightly lower than you would otherwise calculate (add P_ΔH to what you subtract). Drawing a quick diagram of the U-tube effect helps.
A common error is using Celsius instead of Kelvin in PV = nRT. The Ideal Gas Law requires absolute temperature. Using 24°C instead of 297 K will give a wildly wrong answer.
Students occasionally mix up the stoichiometric ratios, applying the 3/2 factor to zinc instead of aluminium. Zinc is the simpler case (1 : 1), aluminium is the one with the 3/2 ratio.
The pressure correction sequence (barometric pressure, minus water vapour, plus or minus height correction) is a multi-step calculation that examiners love because each step is a place to make an error.
Expect questions that change one variable (e.g. "what if 150 mL of gas leaked?") and ask how the percent composition shifts. The direction of the shift depends on the stoichiometric ratios: losing gas means you calculate fewer total moles of H₂, which makes the algebra assign more mass to the metal with the lower H₂-per-gram yield (zinc), so % Zn goes up.
Error propagation using the Ideal Gas Law (post-lab Q7) is a classic exam question: know how to calculate absolute error from relative errors of each measured quantity.
The question of why the acid did not need to be replaced between trials tests whether you can do a limiting-reagent calculation across all three runs combined.
True or False: The gas collected over water is pure hydrogen. (False, it is a mixture of H₂ and water vapour.)
Fill in the blank: 1 mole of aluminium reacts with acid to produce ______ moles of H₂. (1.5)
True or False: You should use temperature in Celsius when applying PV = nRT. (False, use Kelvin.)
Fill in the blank: To convert a height difference in mm of water to mm of mercury, divide by ______. (13.6)
True or False: If gas leaks from the apparatus, the calculated % Zn will appear lower than the true value. (False, it will appear higher.)
Q: Why must you subtract the vapour pressure of water when calculating the pressure of dry hydrogen?
A: The gas collected over water is a mixture of H₂ and water vapour. Dalton's Law says the total pressure equals the sum of partial pressures, so the vapour pressure of water must be removed to isolate the pressure due to hydrogen alone.
Q: An alloy sample of 1.50 g produces 0.065 mol of H₂ when dissolved in excess HCl. What is the percent zinc in the alloy? (Use M_Zn = 65.38 g/mol, M_Al = 26.98 g/mol.)
A: Set up the two equations. mass_Zn + mass_Al = 1.50 g. (mass_Zn / 65.38)(1) + (mass_Al / 26.98)(3/2) = 0.065. Substituting mass_Al = 1.50 - mass_Zn and solving gives mass_Zn ≈ 0.557 g. % Zn = (0.557 / 1.50) x 100 ≈ 37.1%.
Q: If 150 mL of hydrogen gas leaked during the experiment, would the calculated percent zinc be higher, lower, or unchanged? Explain.
A: Higher. Losing gas reduces the measured volume, so the Ideal Gas Law gives fewer total moles of H₂. Because aluminium produces more H₂ per gram than zinc, the simultaneous equations compensate by assigning more mass to the lower-yield metal (zinc), inflating its percentage.
Q: Why was it unnecessary to refill the HCl between trials?
A: The total moles of H⁺ needed for all three trials (about 0.43 mol) was less than the moles available from 75 mL of 6 M HCl (0.45 mol). The acid was in excess throughout.
Q: What problems would arise if the reaction produced H₂S or NH₃ instead of H₂?
A: Both H₂S and NH₃ are polar and soluble in water. They would dissolve rather than displace water, so the water-displacement collection method would not accurately measure the volume of gas produced.
This experiment connects directly to gas law problems throughout general chemistry. The pressure correction step (Dalton's Law, water vapour) reappears whenever gas is collected over water, regardless of the reaction producing it.
The simultaneous-equations approach is a general analytical technique. Any time a mixture of two substances produces a measurable quantity at different rates, you can set up the same pair of equations (mass balance + property balance) to determine composition.
Error propagation (post-lab Q7) links to the broader topic of significant figures and measurement uncertainty, which is tested across all laboratory courses.
Ideal Gas Law, PV = nRT, Dalton's Law of Partial Pressures, gas collection over water, water displacement method, vapour pressure correction, zinc-aluminium alloy, Zn-Al alloy composition, percent composition by mass, stoichiometry of metal-acid reactions, simultaneous equations chemistry, hydrogen gas evolution, molar mass from gas data, error propagation ideal gas law, alloy analysis, wet chemical analysis, general chemistry lab, MSU chemistry