Measuring gas in a graduated cylinder is most accurately accomplished using the water displacement method, a common technique for collecting and quantifying gases that are insoluble or sparingly soluble in water.
The Water Displacement Method
This method involves collecting the gas over water, allowing it to displace a measurable volume of liquid in an inverted graduated cylinder. It's a straightforward and effective technique for various gases.
How It Works
The principle behind this method is that when a gas is introduced into an inverted, water-filled graduated cylinder submerged in a water bath, the gas pushes the water out. The volume of the displaced water is equal to the volume of the collected gas.
Steps for Measuring Gas Volume
Follow these steps to accurately measure gas volume using water displacement:
- Fill the Cylinder: Completely fill a graduated cylinder with water.
- Invert into Water Bath: Carefully invert the water-filled graduated cylinder into a larger container (a trough or beaker) also filled with water, ensuring no air bubbles enter the cylinder. The mouth of the cylinder should be submerged below the water level. You might need to temporarily cover the mouth with a watch glass or stopper during inversion to prevent water loss.
- Connect Gas Source: Position the tube delivering the gas underneath the opening of the inverted graduated cylinder.
- Collect the Gas: Allow the gas to bubble into the cylinder. As the gas fills the cylinder, it will push the water out.
- Equalize Water Levels: Once the desired amount of gas has been collected, or the gas collection is complete, adjust the height of the graduated cylinder until the water level inside the cylinder is the same as the water level outside in the trough. This step helps to equalize the pressure inside and outside the cylinder with atmospheric pressure, ensuring a more accurate volume reading.
- Read the Volume: Read the volume of the gas directly from the markings on the graduated cylinder at eye level. Remember to read the bottom of the meniscus (the curved surface of the water). Learn more about reading the meniscus accurately.
Suitable and Unsuitable Gases
This method is highly effective for gases that do not readily dissolve in water. However, it is not suitable for gases that are highly water-soluble, as they would dissolve into the water rather than displacing it, leading to inaccurate measurements.
| Category | Examples of Gases | Reason |
|---|---|---|
| Suitable | Hydrogen ($\text{H}_2$), Nitrogen ($\text{N}_2$), Oxygen ($\text{O}_2$), Methane ($\text{CH}_4$) | Insoluble or sparingly soluble in water |
| Unsuitable | Ammonia ($\text{NH}_3$), Chlorine ($\text{Cl}_2$), Hydrogen Chloride ($\text{HCl}$) | Highly soluble in water, will dissolve |
Considerations for Accuracy
- Temperature and Pressure: The volume of a gas is highly dependent on temperature and pressure. For precise measurements, especially in quantitative experiments, the temperature of the water (and thus the collected gas) and the atmospheric pressure should be recorded. The gas volume can then be corrected to standard temperature and pressure (STP) using the ideal gas law.
- Meniscus Reading: Always read the bottom of the meniscus at eye level to avoid parallax error.
- Gas Solubility: Always ensure the gas you are measuring has low solubility in water to ensure the water displacement method yields accurate results. For water-soluble gases, other collection methods, such as upward or downward displacement of air, or using a gas syringe, would be more appropriate.
