Final answer:
The concentration of PCl3 at equilibrium is 24.01 M.
Step-by-step explanation:
To calculate the concentration of PCl3 at equilibrium, we can use the equilibrium constant expression:
Kc = [PCl5] / ([PCl3] * [Cl2])
Given that Kc = 49 and the initial concentrations of PCl3 and Cl2 are both 0.70 M, we can set up the equation as:
49 = [PCl5] / (0.70 * 0.70)
Solving for [PCl5], we find:
[PCl5] = 0.70 * 0.70 * 49
[PCl5] = 24.01 M
Therefore, the concentration of PCl3 at equilibrium is 24.01 M.