Answer
$$P = 12.0 \space atm$$
Work Step by Step
1. Find the concentration:
$$\frac{0.186 \space mol}{2.16 \space L} = 0.0861 \space M$$
2. The exponent of each concentration is equal to its balance coefficient.
$$K_C = \frac{[Products]}{[Reactants]} = \frac{[ NH_3 ][ CO ]}{[ HCONH_2 ]}$$
3. At equilibrium, these are the concentrations of each compound:
$ [ HCONH_2 ] = 0.186 \space M - x$
$ [ NH_3 ] = 0 \space M + x$
$ [ CO ] = 0 \space M + x$
$$4.84 = \frac{( x)( x)}{(0.186 - x)}$$
4. Solve for x:
x = 0.179
$ [ HCONH_2 ] = 0.007 \space M$
$ [ NH_3 ] = 0.179 \space M$
$ [ CO ] = 0.179 \space M$
5. Find the total amount of moles of gas:
$$n_{total} = (0.007 + 0.179 + 0.179) \times 2.16 = 0.788$$
6. Find the pressure using pv = nrt:
$$P = \frac{nRT}{V} = \frac{(0.788)(0.0821)(400)}{2.16} = 12.0 \space atm$$
