Library / Foundations: Chemistry / FCH-13
Foundations: Chemistry
Gas released in an open flask
Volume of gas lost, found from the fall in mass of a reaction flask.
- Ref
- FCH-13
- Rev
- v1
- Field
- Foundations: Chemistry
- Result
- V [cm³]
Mass lost equals the mass of gas that escaped. Divide by Mr of the gas to get moles, then multiply by the molar volume (24 dm³/mol at RTP).
If a gas escapes from an open flask, the balance reading falls by exactly the gas’s mass, so you can work out its volume.
Calculator
- Mass of gas lost0.54 g
- Moles of gas lost0.01227 mol
What drives the result
Each variable is moved across its typical range while the others stay at the values above. The longest bar is the key driver.
- Plotting mass against time gives a rate curve without needing a gas syringe.
- Light gases such as hydrogen are hard to measure this way because their mass loss is tiny.
Chart data as tables
Ranked variables
| Variable | Typical range | Lowest result | Highest result | Swing (cm³) | Local step | Change in result |
|---|---|---|---|---|---|---|
| mstart Starting mass | 1 to 1,000 g | −82,290 | 462,600 | 544,900 | +10% | +2,820 % |
| mend Final mass | 1 to 1,000 g | −462,300 | 82,580 | 544,900 | +10% | −2,810 % |
| Mr Mr of the gas | 2 to 200 g/mol | 64.8 | 6,480 | 6,415 | +10% | −9.09 % |
Sweep of Final mass
| Final mass (g) | Gas volume (cm³) |
|---|---|
| 1 | 82,580 |
| 125.9 | 14,470 |
| 250.8 | −53,650 |
| 375.6 | −121,800 |
| 500.5 | −189,900 |
| 625.4 | −258,000 |
| 750.3 | −326,100 |
| 875.1 | −394,200 |
| 1,000 | −462,300 |
Sweep one variable
Variables and how each one contributes
Gas volume
What the equation gives you. Everything else feeds it.
Starting mass
Mass of flask and contents at the start. Raises the gas volume one-for-one.
Final mass
Mass of flask and contents after the reaction. Lowers the gas volume one-for-one.
Mr of the gas
Molar mass of the gas lost (CO₂ = 44). Heavier gas means fewer moles for the same mass.
Worked example with the values above
- Mass lost: 152.4 − 151.9 = 0.54 g.
- Moles: 0.54 / 44 = 0.012273 mol.
- Volume at 24 dm³/mol = 294.5 cm³.
Assumptions and limits
- Only the gas escapes.
- Molar volume of 24 dm³/mol at room conditions.
Common mistakes
- Using the total mass instead of the mass lost.
- Mixing dm³ and cm³.
Statistics and uncertainty
Balance resolution of ±0.01 g on a 0.54 g mass loss is about 2%, which carries into the volume.
Where it is used
- Rate experiments with carbonates and acids.
- Checking gas evolution without a syringe.
Maths you need
- Moles: n = m / Mr
- Gas volume: V = n × Vm