Subjects and lessons to explore.

Law

General Chemistry I · Gases

Gas Pressure

4 min read
SourcesReport an error
Want it in plain words first? Jump to Eli explains — the same idea, no jargon.
On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Key takeaway
  6. Study tools
  7. Sources & references

In 30 seconds

Pressure is force per unit area. A gas exerts pressure because its particles collide with the walls of the container. The standard atmosphere (atm), the millimeter of mercury (mmHg), the torr, the pascal (Pa), and the bar are all units of pressure connected by exact conversion factors. Gas pressure is measured directly with barometers and manometers.

Why this matters

Pressure units appear everywhere in medicine and daily life: blood pressure (mmHg), tire pressure (psi), scuba-tank gauges, and weather forecasts (bar, hPa). The same pressure is reported in different units in different fields, so fluent unit conversion is essential for reading lab data, dosages, and weather reports.

The college version

Key Ideas

  • Pressure = force ÷ area (P = F/A). SI unit = pascal (Pa = N/m²).
  • Gases push on container walls via countless molecular collisions each second.
  • Atmospheric pressure at sea level supports a mercury column about 760 mm high.
  • 1 atm = 760 mmHg = 760 torr = 101.325 kPa = 101,325 Pa.
  • 1 bar = 100,000 Pa = 100 kPa, which is slightly less than 1 atm (1 atm ≈ 1.01325 bar).
  • Barometer measures atmospheric pressure from a mercury-column height.
  • Manometer measures a gas pressure against either a vacuum (closed-tube) or the atmosphere (open-tube).

Equations and Variables

  • P = F/A — pressure (P, Pa), force (F, N), area (A, m²).
  • P = h·ρ·g — hydrostatic pressure of a liquid column; h = column height (m), ρ = liquid density (kg/m³), g = 9.81 m/s².
  • Conversions: 1 atm = 760 mmHg = 760 torr = 101.325 kPa = 101,325 Pa; 1 bar = 100 kPa.
  • 1 mmHg ≈ 1 torr (equal within experimental precision for this course).

How It Works

  1. Gas particles move rapidly in straight lines and collide with every surface they touch.
  2. Each collision delivers a tiny impulse; the sum of billions of collisions per second is a steady push — pressure.
  3. A barometer (Torricelli, 1643): a tube filled with mercury is inverted into a mercury dish. The mercury falls until its weight balances the atmosphere pushing on the dish surface, leaving a column about 760 mm tall at sea level.
  4. The height of the mercury column is a direct reading of atmospheric pressure.
  5. A manometer: one arm connects to the gas sample; the mercury-level difference between the two arms tells you the gas pressure relative to the reference (vacuum or atmosphere).

Worked Example

Convert 745 mmHg to atm, kPa, and Pa.

  • To atm: 745 mmHg × (1 atm / 760 mmHg) = 0.980 atm.
  • To kPa: 745 mmHg × (101.325 kPa / 760 mmHg) = 99.3 kPa.
  • To Pa: 99.3 kPa × 1000 = 9.93 × 10⁴ Pa. A barometer reading of 734 mmHg means the atmospheric pressure is 734 mmHg = 734 torr = 0.966 atm = 97.9 kPa.

Common Confusions

  • "torr and mmHg are different units" — they are numerically equal for this course (1 torr = 1 mmHg to within a negligible correction).
  • "1 bar and 1 atm are the same" — 1 bar = 100 kPa but 1 atm = 101.325 kPa, so 1 atm is slightly larger than 1 bar.
  • "Gauge pressure equals absolute pressure" — a tire gauge reads pressure above atmosphere; absolute pressure = gauge pressure + atmospheric pressure.
  • "Pressure is only about the weight of air" — the weight of the air column sets atmospheric pressure, but a gas in a sealed container pushes because of collisions, not weight.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Imagine a room full of kids bouncing rubber balls off the walls. The walls "feel" the balls only when they hit. A gas is the same: trillions of tiny, invisible balls are smashing into the walls every instant. "Pressure" is just how hard all those little hits add up over a square meter of wall. Push more balls in (more gas) or make them move faster (heat them), and the walls get pounded harder. The analogy's limit: real gas particles don't literally bounce off wall atoms the way balls do — the collisions are electromagnetic repulsions — but the "many small pushes adding up" picture is exactly right.

Key takeaways

  • P = F/A; SI unit is the pascal (Pa).
  • 1 atm = 760 mmHg = 760 torr = 101.325 kPa = 101,325 Pa.
  • 1 bar = 100 kPa = 100,000 Pa; 1 atm ≈ 1.013 bar.
  • mmHg and torr are effectively equal.
  • A barometer measures atmospheric pressure; a manometer measures gas pressure.
  • Higher altitude → lower atmospheric pressure (shorter mercury column).
  • Pressure = force/area; gas pressure comes from molecular collisions.
  • 1 atm = 760 mmHg = 760 torr = 101.325 kPa = 101,325 Pa.
  • 1 bar = 100 kPa ≈ 0.987 atm.
  • Barometer → atmospheric pressure; manometer → gas pressure.
  • Convert units by multiplying by conversion factors equal to 1 (e.g., 760 mmHg / 1 atm).

Keep learning

Ready to build on this? Continue to the next lesson.

Practice General Chemistry I

This lesson has no separate scored set. Practice draws from the subject’s question bank.

Study tools & related lessonsYou’ll learn to · Related

You’ll learn to

  • Define pressure and state its SI unit.
  • Convert among atmospheres, mmHg, torr, kilopascals, and pascals.
  • Explain how a barometer and a manometer measure pressure.
  • Relate gas pressure to molecular collisions.

Sources & references

  1. OpenStax, "9.1 Gas Pressure," Chemistry 2e.
  2. Petrucci et al., "6.1 Properties of Gases: Gas Pressure," Chemistry LibreTexts.
  3. NIST Chemistry WebBook.

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.