Final answer:
To express a concentration of 500 parts per million as a fraction, it is written as 500/1,000,000. This simplifies to 5/10,000.
Step-by-step explanation:
The concentration of iron in a water sample is 500 parts per million. To express this concentration as a fraction, you would use the definition of parts per million. One part per million implies one part of a substance per one million parts of the total. Therefore, 500 parts per million can be expressed as the fraction 500/1,000,000 which can be simplified by dividing both the numerator and the denominator by 100 to get 5/10,000.