The fda limit for levels of toxic methylmercury in fish is 1 ppm. In a 100 g sample of fish, what mass of methylmercury does this equal?

ppm i. e. parts per million is a unit of concentration.

It is mentioned that the FDA limit of methylmercury in fish is 1 ppm

This means that the allowed level is 1 mg of methyl mercury for every 1 kg of the sample of fish

1 ppm = 1 mg/kg

For a 100 g i. e. 0.1 kg sample of fish the allowed levels of methyl mercury would be = 0.1 kg * 1 mg/1 kg = 0.1 mg