[zuriel]

Hey folks, I'm a bit confused at a question I have to do and I'm wondering if anyone can offer a guide in the right direction to help me work it out? The part in question is how to adapt the equation to determine a numerical value for k1.

See attached images for the entire question (for clarity) and the part in question. I appreciate all advice, thanks!! 

[Dan]

The part in question is how to adapt the equation to determine a numerical value for k1.

Hint: Consider the magnitude of [PhCO₂^-] during the reaction. There is an approximation you can make.