Q.In a homogeneous reaction $A \rightarrow B+C+D$, the initial pressure was $P_0$ and after time t it was P. The expression for rate constant in terms of $P_0$, P and t will be

Step 1. Let x = the drop in A's own partial pressure. Then: partial pressure of A remaining $=P_0-x$; B, C and D (each coefficient 1) each have partial pressure $x$. Total pressure $P=(P_0-x)+x+x+x=P_0+2x$.

Step 2. Solving for x: $x=\dfrac{P-P_0}{2}$.

Step 3. Partial pressure of A remaining $=P_0-x=P_0-\dfrac{P-P_0}{2}=\dfrac{2P_0-P+P_0}{2}=\dfrac{3P_0-P}{2}$. …

Track the partial pressure of A remaining in terms of the total pressure change (using the fact that 1 mole of A produces 3 moles of gaseous products) …

• Miscounting the number of product species (B, C, D = three separate species, each with coefficient 1) and getting the wrong total-pressure relation $P=P_0+2x$. …