[Michiyo]

Hello, I hope someone can help me with these two questions. I have an idea of how to solve it but I'm just a little lost on how to go about doing them.

For the first question I need to calculate the partial pressure of a contaminant. The only problem I have with this is that I don't have enough information:

In a closed container, the water contains 500mg/L vinyl chloride. Please estimate the partial pressure of vinyl chloride in the air above the water.

I want to use Henry's Law but I don't know Henry's constant for vinyl chloride. Should I use Dalton's Law? But my professor didn't give us the chemical formula for vinyl chloride and usually if required he will provide it for us. So, should I just look it up and make my life easier or is there any other way to calculate this question?

The second question I'm having trouble with is below:

Anaerobic microbes can gradually degrade contaminant A in groundwater. Variation of concentration A with time is shown on the left side. (There is a table with time vs. concentration that I did not include here.)

(1). Please determine the reaction order and rate constant for A degradation by microbes:

(2). If the cleanup goal is [A] < 0.05mM, how many days are totally needed to achieve the target.[/i]


I figured out the first part, I plotted a graph and using the concentration and time I calculated the change in concentration of [A] over the change in time for Zero, First and Second order reactions until I got the correct answer.
 
The second part of the question I'm having trouble with. Do I use the half-life for this part? Or do I use the rate of the reaction which I calculated from the following equation:

rate = Change in [A] / Change in time


For example for the data given at a time of 60 min the concentration of [A] was 3 mM. So substituting 0.05 mM and the rate of the reaction 1.5 mM/min I got:


1.5 mM/min = - (0.05 mM - 3 mM) / (x - 60 min)

1.5 (x - 60) = - (-2.95)

1.5x - 90 = - (-2.95)

1.5x = - (-2.95 + 90)

x = 87.05 / 1.5

x = 58.03 min

What confuses me with this solution is that it isn't in days as the question asks for. That is why I think it is wrong. Can someone point me in the right direction? Thank you!

 

[rabolisk]

For the second problem, unless the degradation of A is zero-order, the rate is not constant. It varies with time, so whatever instantaneous rate you calculated is only valid at that specific time. You cannot use your method. If it is first-order, then you should use the half-life method.