A vessel at \( 1000 \, \text{K} \) contains \( \mathrm{CO_2} \) with a pressure of \( 0.5 \, \text{atm} \). Some of \( \mathrm{CO_2} \) is converted into \( \mathrm{CO} \) on addition of graphite. If total pressure at equilibrium is \( 0.8 \, \text{atm} \), then \( K_p \) is:
