Question

Pure nitrosyl bromide, NOBr at 0.762 M is placed into a sealed vessel and heated up to 400 K. When the system reaches equilibrium, it is found that 65.1% of the nitrosyl bromide has decomposed to nitrogen monoxide and bromine. Determine Kc for this reaction at 400 K:
2 NOBr(g) -> 2 NO(g)+Br2(g)

Answer 1

If you start with 0.762M NOBr then NO will be 0.762 x 0.651 = about 0.496M and Br will be 1/2 that. The un-dissociated NOBr will be 0.762 x 0.349 = ?
Substitute those values into the Kc expression and solve.