Quick answer: Reacting masses come from a balanced equation. This Chemistry 9-10 lab on reacting masses gives the aim, the equipment, the safety points, a step-by-step method, sample results and 8 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
When baking soda meets vinegar, the fizzing stops when one reactant has run out. This lab measures the carbon dioxide lost for different masses of baking soda, so you can see the limiting reactant appear in the data and use it to find the concentration of the vinegar.
Level: Chemistry grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: HS-PS1-7, GCSE C3
Aim
To show that the mass of carbon dioxide released stops rising once the acid is used up, and to use the plateau to find the concentration of vinegar.
Equipment
- Top-pan balance (2 decimal places)
- 100 cm³ conical flask and cotton wool
- 25.0 cm³ measuring cylinder or pipette
- Vinegar (ethanoic acid solution)
- Sodium hydrogencarbonate (baking soda), weighed into six small pots
- Spatula, stopwatch
Safety
Wear safety goggles. Add the powder slowly to avoid froth overflowing. Wipe spills at once. Never seal the flask: carbon dioxide must be able to escape.
Method
- Measure 25.0 cm³ of vinegar into the conical flask and plug it loosely with cotton wool.
- Place the flask and one pot of baking soda on the balance and record the total mass.
- Tip the powder into the flask, replace the plug and wait until the fizzing stops.
- Record the final mass of flask, pot and contents. The mass loss is the carbon dioxide released.
- Repeat with fresh vinegar and each of the six masses of baking soda.
Results
Sample data for NaHCO₃ + CH₃COOH → CH₃COONa + H₂O + CO₂ (Ar: Na = 23, H = 1, C = 12, O = 16):
| Trial | Mass of NaHCO₃ (g) | Mass lost (g) |
|---|---|---|
| 1 | 0.42 | 0.21 |
| 2 | 0.84 | 0.43 |
| 3 | 1.26 | 0.65 |
| 4 | 1.68 | 0.86 |
| 5 | 2.10 | 0.87 |
| 6 | 2.52 | 0.87 |
Analysis questions
Question 1 Plot the graph
Plot mass lost (y) against mass of NaHCO₃ (x) and describe its two parts.Question 2 Find the turning point
Estimate the mass of NaHCO₃ at the point where the line becomes horizontal.Question 3 Explain the plateau
Explain why trials 5 and 6 lose the same mass as trial 4.Question 4 Moles of carbon dioxide
Calculate the moles of CO₂ released at the plateau (M of CO₂ = 44 g/mol).Question 5 Concentration of the vinegar
The ratio acid : CO₂ is 1 : 1. Calculate the concentration of ethanoic acid in the 25.0 cm³ sample.Question 6 Percentage yield of CO₂
The expected plateau for 0.80 mol/dm³ acid is 0.88 g. Calculate the percentage yield.Question 7 Sources of error
State one reason why the mass lost could be a little too small and one reason it could be too large.Question 8 Improve the lab
Suggest two changes that would make the plateau easier to locate.
