In the previous problems the the magnitude of
K was such that the
extent of reaction was significant, and
equilibrium concentrations of all species were
significantly different to initial pressures.
For reactions having smaller equilibrium constants, the
extent of reaction is less, and the amount of reactants consumed is small. In these cases the equilibrium concentrations may be able to be be approximated by the inital concentrations.
Thus if x is small, expressions like 3.00 - x can be simplified to 3.00.
The validity of such approximations must be always checked at the end of the calculation to assess for example if x is REALLY small in comparison to 3.00. However, this is easy to do.
It is always worthwhile to consider making an approximation before trying to solve a complicated equation (like the one in the last two problems in this module).