A sample cell with volume V contains n1 moles of gas 1 at a given temperature T and pressure p1. We now add n2 moles of a second (different) gas. Once the cell reaches thermal equilibrium, we observe that the pressure has increased by p = p2 - p1. Assume that we can use a virial equation, truncated after the second coefficient, to describe the behavior of the gas mixture. Derive an expression that will allow us to calculate B12 if the pure component coefficients (B11 and B22) are known.