Question

Whatis unit of a and b which are van der waals constant?

Answer 1

Part of your confusion is in the formula you have written. According to the Tanner's General Chemistry webpage The van der Waals Equation, if you are considering 1 mole of a substance, then the formula is

(P+a/V2)(V−b)=RT

For n moles

(P+an2/V2)(V−nb)=nRT

The Science HQ website Van der waal’s equation, the constants a and b represent pressure and volume corrections respectively. The units are based on the formula for n moles of substance (the 2nd equation above).

Taking each term on the left hand side individually:

(P+an2/V2) is the van der Waals equation equivalent for P in the ideal gas equation, thus this therm must have the same unit, so using Pa as the unit for pressure would require that the units of a to be PaL2mol−2 (the website uses the unit atm, but the same idea still applies.

In a similar manner, the term (V−nb) is the van der Waals equivalent of V in the ideal gas equation. So to keep the unit as for volume in the second term, the unit for b must be Lmol−1.

One thing to always be careful with, as there are a number of units used for both pressure and volume, it is imperative to keep these constant throughout your calculations.

Answer 2

Answer:

a is the intermolecular forces and

b is the molecular size

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