atm

Meaning: The atmosphere (atm) is a non-SI unit of pressure, which is force per unit area. One atm is the average pressure of air at sea level. It is the usual pressure unit in gas law, partial pressure, vapor pressure and Kp problems. The MCAT defines STP as 273 K and 1 atm.

Equation: 1 atm = 760 mmHg = 760 torr = 1.01325 × 10⁵ Pa ≈ 101.3 kPa, where Pa is the pascal (N/m²), the SI unit of pressure, and mmHg and torr are the same size.

On the MCAT: Passages often give pressure in mmHg, torr or kPa, and you convert before using PV = nRT. With P in atm, V in L, n in mol and T in K, use R = 0.0821 L·atm/(mol·K). With Pa and m³, use R = 8.314 J/(mol·K). Dalton's law questions ask for a partial pressure, which equals mole fraction × total pressure. A liquid's normal boiling point is the temperature at which its vapor pressure equals 1 atm. In fluids, pressure at depth is P = P₀ + ρgh, and P₀ is about 1 atm at the surface of an open container.

Common confusion: Students plug mmHg or kPa straight into a formula whose R is written with atm. Convert first. Also, gauge pressure is measured relative to the atmosphere, so absolute pressure = gauge pressure + 1 atm (at sea level). Finally, 1 bar is 10⁵ Pa, close to but not exactly 1 atm.