How to teach stoichiometry

Why this matters

Many chemistry students can turn liters of gas into moles and moles into grams, then skip the mole ratio between them. They carry the moles of nitrogen straight over as moles of sodium azide, because that's the number already on their paper. On a worksheet 174 g looks as finished as 116 g, so the teacher finds out on test day.

Stoichiometry in three steps

Stoichiometry is the math of how much of each substance a reaction uses and makes. The coefficients in the balanced equation give the mole ratio between any two substances in it (OpenStax Chemistry 2e). To get from the mass of one substance to the mass of another, students take three steps:

1. Convert grams of the known substance to moles, using its molar mass.
2. Convert moles of the known substance to moles of the one they want, using the mole ratio.
3. Convert moles of the wanted substance to grams, using its molar mass.

The airbag problem starts from liters of nitrogen gas, so step 1 turns liters into moles.

In the Flint activity, students work out how much sodium azide fills a 60.0-liter airbag, then type their answer into an airbag Sparky builds. A student who skips the mole ratio watches the bag overfill, which no answer key on a worksheet can show, and Sparky sends them back to the coefficients in the balanced equation to find the ratio themselves. A right answer gets a harder reaction: how much potassium nitrate uses up the leftover sodium.

Sofia's chat: a 60-liter airbag at STP

Sofia just got her learner's permit. She can balance an equation and convert between grams and moles, but she has never used a mole ratio to get from one substance to another. Her class works at STP, where one mole of any gas takes up 22.4 liters. Early airbags inflated with nitrogen from sodium azide (C&EN, 2022), so the activity asks how much sodium azide fills a 60.0-liter bag.

Every student in the class gets the same activity: the same greeting, the same airbag to fill and the same first problem. Each chat is that student's own. Sparky ties the airbag to what the student is into, and what it says next depends on the student's last message.

Sparky uses the student's interestSparky has the student test her own answer, then sends her back to the equationA right answer gets a harder question

S

Sofia

Interest: Just got her learner's permitStarting point: Converts grams and moles, has never used a mole ratio

Sparky

Hi! I'm Sparky. Today you'll work out how much sodium azide fills a car airbag without bursting it. First, what are you into: driving, cars, games, something else? And how do mole ratios feel right now: new, shaky or solid?

Sofia

I just got my learner's permit, and mole ratios feel shaky

Sparky

A new driver is exactly the right person to care about an airbag working. Too little gas and it won't protect you. Too much and it bursts. I built you an airbag: type grams of sodium azide and press Fill. How many grams of sodium azide (NaN₃) fill a 60.0 L airbag at STP? Show me your steps, then type your answer into the airbag.

Sofia

60.0 / 22.4 = 2.68 mol of N2. then 2.68 x 65.01 = 174 g of sodium azide

Sparky

Good start on the first step. Before anything else, type 174 g into the airbag and press Fill. What does the airbag do?

Sofia

it went to 89.9 L and says overfilled, the bag could burst

Sparky

Right: 90 L in a 60 L bag would be bad news in a crash. Something went wrong between moles of N₂ and grams of NaN₃. Go back to the balanced equation, 2 NaN₃ → 2 Na + 3 N₂. How many moles of N₂ come from every 2 moles of NaN₃?

Sofia

3 moles of N2 for every 2 NaN3. oh so i need 2/3 of the moles. 2.68 x 2/3 = 1.79 mol NaN3, then 1.79 x 65.01 = 116 g

Understanding check

That's right: 116 g of NaN₃. Type 116 into the airbag to check it. Then a harder one. The reaction also leaves sodium metal, which is dangerously reactive, so airbags held potassium nitrate to use it up: 10 Na + 2 KNO₃ → K₂O + 5 Na₂O + N₂. How many grams of KNO₃ use up all the sodium your sodium azide made?

Sofia

116 says 60.0 L just right. for the KNO3: 1.79 mol NaN3 makes 1.79 mol Na. 10 Na to 2 KNO3 so 1.79 x 2/10 = 0.358 mol KNO3. 0.358 x 101.10 = 36.2 g

Sparky

36.2 g of KNO₃, and every step is right. Last question: in one sentence, why did 174 g overfill the bag?

Sofia

i used 1 to 1 so i made 1.5 times too much sodium azide, because 2 NaN3 already give 3 N2

Sparky

That's it. Each pair of NaN₃ gives three N₂, so you need fewer moles of NaN₃ than moles of N₂.

Watched her 1 to 1 answer burst the airbag, fixed the ratio, then balanced the leftover sodium

Sofia had 2.68 moles of nitrogen and used that number as moles of sodium azide. Sparky didn't give her 116 g or say "2 to 3". It asked her to put her own answer in the airbag first, and she saw the bag swell to 89.9 liters before she looked at the equation again. She found the ratio there herself, and she used the same move on the potassium nitrate question without help.

Sofia is an example student, and Sparky's replies are shortened.

What Flint did for Sofia

• Sparky built an airbag Sofia could fill by typing grams of sodium azide, with a dashed line at 60 liters.
• When Sofia used 1 to 1, Sparky asked her to try 174 g in the airbag before it said anything about the equation.
• After the bag overfilled, Sparky pointed her to the coefficients in the balanced equation and let her find the ratio.
• Sparky checked 89.9 L, 116 g and 36.2 g with its calculator before it replied.
• Once Sofia had 116 g, Sparky gave her the potassium nitrate reaction that uses up the leftover sodium.

A teacher can't do this by hand for 30 students, and Sparky does it in every student's chat.

Who used a 1 to 1 ratio

After the class finishes, Sparky reads every student's chat and writes a summary on the activity page. The teacher sees which student is stuck right now, the mistake most of the class made, and what to practice next.

Set up the activity for free

The airbag activity is in Flint's public library, with Sparky's guidelines, greeting and greeting suggestions already written. Open it and select Use activity to copy it into a free Flint account, then move it to a class. To test it as a student before you assign it, open the activity's Preview tab and select Try session myself.

For more science activities, see Flint for science.