ap chemistry
posted by cheri .
The system
CO2(g) + H2(g) *) H2O(g) + CO(g)
is at equilibrium at some temperature. At
equilibrium a 4.00 L vessel contains 1.00 mole
CO2, 1.00 mole H2, 2.40 moles H2O, and 2.40
moles CO. How many moles of CO2 must be
added to the system to bring the equilibrium
CO concentration to 0.731 mol/L?
Answer in units of moles

..........CO2 + H2 ==> H2O + CO
.........0.25.0.25...0.60...0.60
Kc = (H2O)(CO)/((CO2)(H2)
Substitute and solve for Kc.
For the second part, if the final concn for CO = 0.731, it follows final concn H2O must be 0.731; therefore, you must have added 0.7310.600 = 0.131 for both H2O and CO. That means H2 must have contributed the 0.131 so H2 at equilibrium will be 0.250.131 = 0.119
Substitute those values into Kc expression and solve for (CO2). Then you know CO2 initially was 0.25 and final concn is the new calculated value. Find the difference to know how much must be added. That will be in M, convert to mols in 4L. Post your work if you get stuck. 
jk

3.1 moles