Derive vander Waal gas equation. Explain the correction terms.

From the ideal gas law, we know that
1) PV = nRT -------------(1)
where P is pressure, V is volume, T is temperature, R is the universal gas constant, and n is the number of moles.
Let b = amount of the excluded volume per mole of the gas,
2) V (actual) = V - nb ---------------(2)
Substituting 2 into equation 1 gives us:
3) P(V - nb) = nRT
and P = (P + n²a/V²)
Finally we have
(P + n²a/V²)(V - nb) = nRT
There are two corrective factors in van der Waals equation. The first, n²a/V², alters the pressure in the ideal gas equation. It accounts for the intermolecular attractive forces between gas molecules. The magnitude of a is indicative of the strength of the intermolecular attractive force. a has units of L².atm/mol².
The factor - nb accounts for the volume occupied by the gas molecules. b has units of L/mol. b is generally much smaller in magnitude than a. 
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