This week, I had the AP students do some very simple labs focused upon law of conservation of mass, but connecting it to stoichiometry. Students mixed a white powder with a liquid to produce a gaseous product (massing beakers initially, with and after, see image at right), share their data on the board, then graph the class data on a TI calculator (see image below).
There were a couple of points to this. First, the initial share was with total mass before and after and the students see they don’t match — the gas was lost. This means different data is needed, but it still has to come from what they collected. Why not compare the mass of the gas (that lost mass) and the mass of the solid? When this is graphed (solid mass on the x-axis, gas on the y), a linear relationship is evident; further supported by the linear regression done. [I’m reading my notes from this summer, one-finger-typing my lists and such and my students were waiting on me; even made a point of telling me to just ask them what to do next time as they do this all the time in math. They enjoyed that way more than they should have.] We do discuss just what the linear equation means (y=mx+b). In this case, y = 0.502x +.055, the slope tells them that 0.502g of gas is produced for every 1g of solid used and that there’s error because ‘b’ has a value other than zero (the error in the class data).
Then, the second point of the discussion — the part I did not carry off as well as I wanted — determining the ‘ideal’ relationship for this solid and this gas. Enter the chemical reaction (just in case they had not determined it was baking soda and vinegar) and I place the molar masses of each substance underneath, totaling the masses of reactants and products to show the masses are equal, show conservation of mass. This wasn’t exactly a revelation to this group, but I didn’t let them tell me what the ideal equation for the line should be: the total mass of the gas divided by the total mass of the solid. This is the point that gets us to stoichiometry. As soon as you start comparing the masses, ideally seeing that molar ratio behind the mass comparison because molar masses were multiplied by coefficients, you are performing the basics of stoichiometry calculations.
They answer four questions
- How good is your point? Explain. [Their point on their graph has a smiley face above it.]
- How many grams of gas should you lose? Explain how you know.
- ? g of gas if 5g solid used? Show 2 ways.
- ? g of solid if 5g gas produced? Show 2 ways.
All of these are answered by evaluating the graph and their point on the graph
- it’s good because it’s close to the ideal line
- more or less gas should have been lost — based on whether it’s above or below the line (getting to another rabbit trail in the discussion about the source of error: below the line, not enough gas produced; above the line too much; and what could have caused this)
- the first way, find it on the graph; second way, calculate by multiplying masses (factor label)
- the first way, find it on the graph; second way, calculate by multiplying the masses, but inversely as the gas needs ‘canceled’
The goal with this set of labs is to have the students (1) want this ‘proved’ again to see this truth with a different reaction; (2) lead to other questions like what the graph would look like if the amount of solid is varied with the same amount of liquid or vice versa, keep the solid constant and vary the liquid; i.e., predicting limiting reagents. Again, the whole point is to get them to take the data, see the linear relationship, but ask why and want to see more data to prove it — not exactly carried off in this way. Instead, I had them do the lab several times, plug in our data, examine the graphs produced, and see that, “yup, it’s linear, again.”
My problem here was my approach. This would work well for chemistry if I fix the conservation of mass introduction & link to the results — what does conservation of mass mean? reactants should equal products, so we balance the equation, but is there another way?…work to masses of reactants and products and what this means for the lab, compare the line produced to the ideal one they find…ask if this is the case all the time? is it predictable? Push for the questions because it’s new. AP, on the other hand, should be about predicting the results, predicting the graph, predicting the ideal line. This information is not new it’s review, so, I needed to take it to that point instead of getting caught in no-man’s-land between new & review where I lost some of the effectiveness of the enterprise.
[Thanks to Jim Cortez for sharing this during our summer APSI.]