Quick answer: A mole is 6.022 × 10²³ particles, and its mass in grams is the molar mass M, so n = m/M links mass to amount. This AP Chemistry lab on mole concept gives the aim, the equipment, the safety points, a step-by-step method, sample results and 7 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
A mixture of sodium hydrogencarbonate and sodium chloride loses mass when heated because only the first compound decomposes. You use that mass loss to find the composition of the mixture.
Level: AP Chemistry · Curriculum: AP Chem Unit 1
Aim
To find the mass percent of sodium hydrogencarbonate in a mixture with sodium chloride by measuring the mass lost on heating.
Equipment
- Crucible with lid, tripod, pipe-clay triangle and Bunsen burner
- Balance reading to 0.001 g
- Tongs and heat-proof mat
- Sample of the mixture
Safety
Wear goggles. The crucible stays hot for several minutes, so use tongs and place it on the heat-proof mat. Do not heat strongly at the start or the solid may spit.
Method
- Weigh the empty crucible with its lid.
- Add about 2.5 g of the mixture and reweigh.
- Heat gently for 3 minutes, then strongly for 5 minutes with the lid slightly open.
- Cool on the mat and reweigh.
- Reheat for 2 minutes, cool and reweigh until two readings agree within 0.002 g.
The reaction is 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. Sodium chloride does not change.
Results
| Measurement | Mass (g) |
|---|---|
| Crucible + lid | 24.318 |
| Crucible + lid + mixture | 26.818 |
| After first heating | 26.206 |
| After second heating | 26.198 |
| After third heating | 26.198 |
Analysis questions
Question 1 Mass of mixture and mass lost
Calculate the mass of the mixture and the total mass lost.Question 2 Why constant mass?
Explain why the sample is heated to constant mass.Question 3 Gas lost per mole
State the mass of CO₂ + H₂O lost when 2 mol of NaHCO₃ (M = 84.006 g/mol) decompose, using M(CO₂) = 44.009 and M(H₂O) = 18.015 g/mol.Question 4 Mass of sodium hydrogencarbonate
Use the mass lost to find the mass of NaHCO₃ in the sample.Question 5 Mass percent
Calculate the mass percent of NaHCO₃ in the mixture, and the moles of NaHCO₃.Question 6 Effect of an error
Some solid spat out of the crucible during the first heating. Explain whether the reported percent would be too high or too low.Question 7 Improvement
Suggest one change that would make the result more reliable.
