Final answer:
A flammable vapor or fume is considered hazardous when it exceeds 10 percent of its LFL, which is option (A). Percent composition by mass is determined by dividing the mass of each element in a sample by the total mass, and then multiplying by 100.
Step-by-step explanation:
The subject of the original question is safety in chemistry related to the flammability of vapors or fumes. The limit at which a flammable vapor or fume is considered hazardous is when it exceeds 10 percent of its lower flammability limit (LFL), which corresponds to option (A).
To connect this with an example related to percent composition by mass, which is a separate concept in chemistry, let's consider a sample analysis. If you have a 10.0-g sample of a gas containing 2.5 g of hydrogen (H) and 7.5 g of carbon (C), you can determine the percent composition. The percent composition would be 25% hydrogen and 75% carbon because you divide the mass of each element by the total mass of the sample and then multiply by 100 to get the percentage.