All calculatorsMolar Mass of Gas Calculator
Determine a gas’s molar mass (g·mol⁻¹) from experimental data using PV = nRT. Enter mass m, pressure P, volume V, and temperature T (optionally a compressibility factor Z)—get M and n with step-by-step details and a visual gauge.
Background
For ideal gases, PV = nRT and M = m / n. Combining gives M = mRT / (ZPV), where Z = 1 for ideal gases. Units are converted automatically.
How to use this calculator
- Step 1: Enter mass, pressure, volume, and temperature with correct units.
- Step 2: Choose °C or K — conversions are automatic (K = °C + 273.15).
- Step 3: (Optional) Include Z if non-ideal; default is Z = 1.
- Step 4: Click Calculate to get M (molar mass) and n (moles).
Formula & Equation Used
Ideal gas law: PV = nRT
Moles: n = PV / (ZRT)
Molar mass: M = mRT / (ZPV)
Example Problems & Step-by-Step Solutions
Example 1 — 1.20 g gas at 1.00 atm, 0.950 L, 298.15 K
Using R = 0.082057 L·atm·mol⁻¹·K⁻¹, n = PV/(RT) = (1.00 × 0.950)/(0.082057 × 298.15) = 0.0389 mol. M = m/n = 1.20 / 0.0389 = 30.8 g·mol⁻¹.
Example 2 — 0.650 g gas at 755 mmHg, 500 mL, 25 °C
Convert: P = 755 mmHg × (1 atm / 760 mmHg) = 0.993 atm; V = 0.500 L; T = 25 + 273.15 = 298.15 K. n = PV/(RT) = (0.993 × 0.500)/(0.082057 × 298.15) = 0.0202 mol. M = 0.650 / 0.0202 = 32.2 g·mol⁻¹.
Example 3 — 0.250 g gas at 100 kPa, 1.00 L, 298 K, Z = 1.05
Convert: P = 100 kPa × (1 atm / 101.325 kPa) = 0.9869 atm; n = PV/(ZRT) = (0.9869 × 1.00)/(1.05 × 0.082057 × 298) = 0.0374 mol. M = 0.250 / 0.0374 = 6.68 g·mol⁻¹.
Frequently Asked Questions
Q: Which R constant do you use?
We auto-select R to match your pressure and volume units (e.g., 0.082057 L·atm·mol⁻¹·K⁻¹).
Q: What if my gas is non-ideal?
Include Z (compressibility). Set Z = 1 if unknown for an ideal-gas approximation.
Q: Can I enter temperature in °C?
Yes. We automatically convert °C to Kelvin (K = °C + 273.15).
