D values

[spottedlizard20]

I'm having some serious issues with my microbiology homework :s

We have to calculate the decimal reduction time and I'm completely confused as to how to do this. If, say, we start with 800,000 cells initially, and after 60 minutes we have 4,000 cells remaining, how am I supposed to figure out the time at which 90% of the cells are dead??

I'm also having issues remembering how to calculate the number of moles of a compound from general chem. The formula I am using requires a mass when all we have is a volume and no density to convert it. I'm trying to calculate the number of moles in a 2mL solution of 25% NaCl. I need this value in order to determine the aw (available water).

If anyone could give me any pointers, I would really appreciate it

[fafalone]

If 796,000 died in 60 minutes, then (assuming the death rate is constant), 13266.67 died per minute. So, that number * the number of minutes = 720000 (90% of 800000). So working it out it comes to 54.27 minutes. I'm not quite sure how they do it in your class... i.e. the answer will be different if the death rate is nonlinear (exponential, etc).

 

As for the second part, molarity = moles / liters,

[spottedlizard20]

Hey thanks faf!

The molarity formula really helps out a lot I don't know how come I couldn't find that in my text, grrr...

And about the bacterial death rate... it actually occurs exponentially. Should I be plotting data on a logarithmic sheet of graph paper and then determining the D-value?? That seems like a lot of work though. My prof was using a calculation whereby he was moving around decimal places (it was supposed to be a simplified calculation!). The general idea is that if you initially have 10(12) cells, and then you heat them for a minute and have 10(-1) cells, the D-value is 13 minutes because you move the decimal by 13 places. But what confuses me is the fact that I'm starting with 8x10(5) cells and ending up with 4x10(3) cells. It's the damn 8 and 4 that are screwing me up and I don't know how they affect the calculation. Going by the prof's method, the D-value would seem to be 2 minutes... but I really think that is wrong...... That doesn't seem to fit at all :s