How is 400 ppm expressed as a percentage by weight?

To convert parts per million (ppm) to a percentage by weight, you can use the relationship where 1 ppm is equivalent to 0.0001%. Therefore, to convert 400 ppm to a percentage:

1. Recognize that 400 ppm represents 400 parts of a million parts.
2. To express this in a percentage, use the formula:

[ \text{Percentage} = \frac{\text{ppm}}{10,000} ]

3. Applying this formula:

   [ \text{Percentage} = \frac{400}{1,000,000} \times 100 = 0.04 % ]

This shows that 400 ppm translates to 0.04% by weight. This calculation is essential for understanding concentrations, especially in fields such as environmental science and public health where lead exposure limits are critical. By expressing concentrations in percentage terms, it's easier to integrate them into risk assessments and regulatory frameworks.