FocusUsing stoichiometry to determine the mass of sodium azide required to produce a specified volume of nitrogen gas for an airbag.
Materials & Prep
Balanced equation reference, periodic table, dimensional-analysis organizer, calculator, guided practice sheet, exit ticket, and projected airbag data: 2.00 mol N₂ occupies 44.8 L at the stated conditions. Prepare the balanced decomposition equation: 2 NaN₃(s) → 2 Na(s) + 3 N₂(g).
Opening and Airbag Quantity
7 minContentAirbag inflation requires a defined quantity of nitrogen gas. The balanced equation 2 NaN₃(s) → 2 Na(s) + 3 N₂(g) establishes the mole ratio 2 mol NaN₃:3 mol N₂. The target quantity is 44.8 L N₂, equivalent to 2.00 mol N₂ under the stated conditions.
Student taskRecord a prediction for whether the required sodium azide mass is less than, greater than, or approximately equal to the nitrogen gas mass, with a written reason based on the equation and particle quantities.
EvidencePrediction and written reasoning identifying the gas product and the 2:3 mole relationship.
Units and Mole Conversions
10 minContentStoichiometric calculations move through a sequence of units: given quantity → moles of given substance → moles of desired substance → grams of desired substance. Molar mass converts grams and moles. For sodium azide, NaN₃: 22.99 + 3(14.01) = 65.02 g/mol. Dimensional analysis requires cancellation of units at every step.
Student taskComplete conversion statements for 44.8 L N₂ to 2.00 mol N₂ and for 2.00 mol NaN₃ to 130.04 g NaN₃, labeling every unit before calculator use.
EvidenceCompleted unit pathway with correct conversion factors, canceled units, and intermediate values.
Worked Stoichiometric Example
13 minContentComplete calculation for 44.8 L N₂: 44.8 L N₂ × (1 mol N₂/22.4 L N₂) × (2 mol NaN₃/3 mol N₂) × (65.02 g NaN₃/1 mol NaN₃) = 86.7 g NaN₃. The balanced-equation coefficient ratio is used only for moles, and molar mass is used only to convert between grams and moles.
Student taskAnnotate the calculation by identifying the given quantity, each conversion factor, the canceled units, the mole ratio, and the final significant figures; then write a one-sentence interpretation of 86.7 g.
EvidenceAnnotated dimensional-analysis chain and accurate interpretation stating that 86.7 g NaN₃ produces 44.8 L N₂ under the stated conditions.
Airbag Application Practice
17 minContentIndependent and paired problems apply the same sequence to new gas volumes. Problem A: determine the mass of NaN₃ needed for 33.6 L N₂. Problem B: determine the mass needed for 67.2 L N₂. Expected results are 65.0 g and 130 g NaN₃, respectively, with appropriate significant figures. Solutions must begin with the balanced equation and show units through the full conversion.
Student taskSolve both airbag-volume problems, including the equation, unit pathway, mole ratio, molar mass, answer, and a reasonableness statement relating the result to the 44.8 L reference case.
EvidenceTwo complete calculation sets showing correct proportional reasoning, unit cancellation, mole ratio, molar mass, and final mass.
Exit Ticket and Synthesis
8 minContentStoichiometry converts a macroscopic gas volume into moles of gas, applies the balanced-equation mole ratio, and converts moles of sodium azide into mass. For this reaction, greater nitrogen volume requires proportionally greater sodium azide mass.
Student taskDetermine the mass of NaN₃ needed to produce 11.2 L N₂ and identify the conversion step represented by 2 mol NaN₃/3 mol N₂. Expected mass: 21.7 g NaN₃.
EvidenceIndividual exit ticket with a complete or traceable unit pathway, correct mole ratio, final mass, and identification of the balanced-equation conversion.
Assessment criteriaProficient work begins with the balanced equation, converts volume to moles, uses the 2:3 NaN₃:N₂ mole ratio, converts sodium azide moles to grams using 65.02 g/mol, shows units that cancel, reports an appropriately rounded answer, and connects the result to the specified nitrogen volume.
DifferentiationA step-labeled dimensional-analysis template and reduced problem set support students who need structure; an extension using a non-reference nitrogen volume and significant-figure justification increases complexity.