Quick answer: A chemical equation shows reactants turning into products. This grade 8 science lab on balancing equations 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.
Does the mass change when a reaction makes a gas? In this lab you react vinegar with baking soda in an open flask and in a sealed one, measure the mass before and after, and use atoms to explain why the readings differ.
Level: Grade 8 science (Year 9 · Class 8) · Curriculum: MS-PS1-2, MS-PS1-5
Aim
To measure the mass before and after a reaction that makes a gas, in an open flask and in a sealed flask, and to explain the results using atoms.
Equipment
- 250 mL conical flask, a small test tube and a balloon
- Balance reading to 0.01 g
- Baking soda (sodium hydrogencarbonate), 3.0 g per trial
- Vinegar, 50 mL per trial, and a measuring cylinder
- Safety goggles
Safety
Wear safety goggles. Vinegar is a mild acid and can sting eyes and cuts, so wipe up spills. Do not seal the flask with a hard stopper because the gas could build up pressure; use a balloon, which can expand.
Method
- Pour 50 mL of vinegar into the flask.
- Put 3.0 g of baking soda into the test tube, and stand the test tube upright in the flask without spilling.
- Place the flask on the balance and record the starting mass. (Open version: leave the neck open. Sealed version: fit the balloon over the neck first.)
- Tip the flask so the baking soda falls into the vinegar and wait until the fizzing stops.
- Record the final mass.
- Repeat three times for each version and calculate the change in mass each time.
Results
The reaction is NaHCO₃ + CH₃COOH → CH₃COONa + H₂O + CO₂. A class in Dar es Salaam recorded this sample data.
| Version | Trial | Mass before (g) | Mass after (g) | Change (g) |
|---|---|---|---|---|
| Open | 1 | 186.52 | 185.07 | −1.45 |
| Open | 2 | 186.48 | 185.05 | −1.43 |
| Open | 3 | 186.55 | 185.10 | −1.45 |
| Sealed | 1 | 188.91 | 188.89 | −0.02 |
| Sealed | 2 | 188.87 | 188.86 | −0.01 |
| Sealed | 3 | 188.95 | 188.93 | −0.02 |
Analysis questions
Question 1 Mean mass change
Calculate the mean change in mass for the open flask and for the sealed flask.Question 2 Explaining the open flask
Explain why the open flask lost mass although no atoms were destroyed.Question 3 Explaining the sealed flask
Explain why the sealed flask showed almost no change.Question 4 Checking the equation
Show that NaHCO₃ + CH₃COOH → CH₃COONa + H₂O + CO₂ is balanced by counting the atoms of Na, C, H and O.Question 5 Expected mass of gas
From the equation, 84 g of NaHCO₃ makes 44 g of CO₂. Calculate the expected mass of CO₂ from 3.0 g.Question 6 Percentage escaped
Use your answers to calculate the percentage of the expected CO₂ that was measured as lost from the open flask.Question 7 Why not 100 %?
Suggest two reasons why the lost mass was less than the expected mass.Question 8 Improving the lab
Suggest one way to make the open-flask measurement more reliable.
