500 tons of LPG how much of ethyl mercaptan is to be added to bulk tank to reach the desirred level of 6 to 12 ppm?

Note: edited for correctness

Assuming that you meant ppmw (parts per million by weight), and not ppmv (parts per million by volume); and also that you meant short tons (2000 lbs) and not metric tonnes (1000 kg):

x /(500 tons * 2000 lbs/ton + x) = 6/1,000,000

Where x is the amount of odorant in pounds.

Solving for x yields:

x= 6.000036 lbs for a 6 ppmw (parts per million weight) concentration

This shows we can neglect the amount of odorant added since it is insignificant (0.00036 lbs) in the total, and the equation reduces to:

x/(500*2000) = 6x10-6

yielding: x=6 lbs for 6ppmw

likewise:

x=12.001 lbs of odorant for 12ppmw

or

x=12 lbs for the simple calculation.

Important conversion: ppm/1x106= fraction in total