Lab Skills

Barometer & Manometer

Switch between barometer, open-tube, and closed-tube manometer modes. Adjust mercury heights and solve 12 randomized pressure calculation problems.

BarometerOpen TubeClosed TubePgas Calculation
Topic L.5

Gas Pressure Measurement

Describe the components of and quantitative information from models and representations that illustrate both particulate-level and macroscopic-level properties.

Barometer: measures atmospheric pressure as the height of a mercury column supported by the atmosphere. Standard atmospheric pressure supports 760 mm Hg.

Manometer: measures the pressure of an enclosed gas relative to the atmosphere.

  • Open-end manometer: Pgas = Patm + Δh if the mercury is higher on the open side (gas pressure exceeds atmospheric), and Pgas = Patm − Δh if higher on the closed side.
  • Closed-end manometer: Pgas = Δh directly, since the sealed arm is a vacuum.

Unit conversions from the equation sheet: 1 atm = 760 mm Hg = 760 torr.

Collecting gas over water. Gas collected by water displacement is saturated with water vapor, so it is a mixture. By Dalton's law:

Pgas = Ptotal − PH₂O

The vapor pressure of water depends only on temperature and is supplied in a table. Forgetting to subtract it overstates the amount of gas collected — one of the most frequently tested lab errors in Unit 3.

Levelling the water. Before reading the volume, raise or lower the collection tube so the water level inside matches the level outside. Only then does the total pressure inside equal atmospheric pressure.

Key points

  • Gas collected over water is a mixture: Pgas = Ptotal − PH₂O.
  • Level the water inside and outside before reading the volume.
  • 1 atm = 760 mm Hg = 760 torr.
  • Convert temperature to kelvin before any gas-law calculation.

Equations

  • not on the sheetDalton’s law applied to a gas collected over water.

Common mistakes

  • Forgetting to subtract water vapor pressure.
  • Using Celsius in PV = nRT.
  • Reading a manometer with the wrong sign. Ask which side the mercury is higher on.

Worked example

A student collects 45.2 mL of hydrogen over water at 22.0 °C. The barometric pressure is 748 torr and the vapor pressure of water at 22.0 °C is 19.8 torr. Calculate the moles of H₂ collected.

Correct for water vapor:
P(H₂) = Ptotal − P(H₂O) = 748 − 19.8 = 728 torr
Convert to atm: 728 ÷ 760 = 0.958 atm

Convert the other quantities:
V = 45.2 mL = 0.0452 L
T = 22.0 + 273.15 = 295.2 K

Apply PV = nRT:
n = PV/RT = (0.958)(0.0452) / [(0.08206)(295.2)]
n = 0.04330 / 24.22 = 1.79 × 10⁻³ mol H₂

Skipping the vapor-pressure correction would have given 1.84 × 10⁻³ mol — about 3% too high. At higher temperatures the error grows rapidly.

Full notes for topic L.5 →