Q40P - Chemistry: Molecular Nature Of Matter And Change

Question

(a) What is the basis of the approximation that avoids using the quadratic formula to find an equilibrium concentration?

 (b) When should this approximation not be made?

Step-by-Step Solution

Answer

1. When K is small, and the initial concentration is big, the approximation applies when the shift in concentration from initial to equilibrium concentration is so comparable that it is inconsequential.
2. When the change in concentration is higher than 5%, the approximation should not be employed. This can happen when very tiny or when [reactant] changes dramatically due to a big K.

 

Step 1: (a) What is the basis of the approximation?

When performing chemical calculations, we are usually concerned with employing the most precise data and formulae to arrive at a satisfactory result. However, there are situations when approximations are warranted to make computations easier or even practicable.

The approximation applies when the change in concentration from initial to equilibrium concentration is so similar that it is insignificant. This occurs when K is small, and the initial concentration is large.

Step 2: (b) When should this approximation not be made?

The approximation should not be used when the change in concentration is greater than 5%. This can occur when ${\left[\mathrm{Reachant}\right]}_{\mathrm{initial}}$ is very small or when the change in is relatively large due to large K.