pound per pound-mole
Molar massexact by definition
The pound per pound-mole is exactly 10⁻³ kg/mol, so the numerical value is identical to grams per mole: methane is 16.04 lb/lb-mol and 16.04 g/mol. US process engineering keeps this unit precisely because the number never changes.
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The pound-mole is defined as the amount whose mass in pounds equals the molecular weight, which forces the identity. It is the one place in imperial engineering where nobody has to convert anything, and it survives for that reason alone.
About the pound per pound-mole
The pound per pound-mole is how US gas processing writes molecular weight, and the industry's favourite derived number keeps it hidden in plain sight: gas specific gravity is the molar mass of a gas divided by air's 28.964 lb/lb-mol, so a 0.60-gravity natural gas has a molecular weight near 17.4, a little above pure methane's 16.04 because of the ethane and propane riding along. Wellhead correlations from pseudocritical properties to bottomhole pressure take gas gravity as their first input, which makes this quiet unit the foundation of a great deal of petroleum arithmetic.
For mixtures the working figure is the apparent molecular weight, the mole-fraction-weighted average across the components, and it is the hinge between the two ways a stream is booked: a mass flow in lb/h divided by the apparent molecular weight in lb/lb-mol is a molar flow in lb-mol/h, ready for the ideal-gas solver with the pressures in psia and the temperatures in Rankine.
The origin note above explains the unit's one luxury: nothing ever needs converting, because the digits are identical to g/mol by construction. That identity is fragile in only one direction, the amount itself. A pound-mole is 453.59 moles, and forgetting that while the molar mass stays numerically unchanged is how the factor sneaks into an otherwise clean calculation.
One pound per pound-mole in every molar mass unit
| 1 | gram per mole (g/mol) | |
| 1 | kilogram per kilomole (kg/kmol) | |
| 1 | milligram per millimole (mg/mmol) | |
| 1 | pound per pound-mole (lb/lb-mol)this unit | |
| 1 | dalton (as molar mass) (Da) | |
| 0.001 | kilogram per mole (kg/mol) | |
| 0.001 | kilodalton (as molar mass) (kDa) |