Asolution contains 15 ppm of benzene. the density of the solution is 1.00 g/ml. this means that

the solution is 15% by mass of benzene
1.0 g of the solution contains 15 * 10^-6 g of benzene
there are 15 mg of benzene in 1.0 g of this solution
100 g of the solution contains 15 g of benzene
1.0 l of the solution contains 15 g of benzene

second choice: 1.0 g of the solution contains 15 × 10⁻⁶ g of benzene.

Explanation:

ppm is a unit of concentration that means parts per million. In grams that is grams of solute per one million (10⁶) grams of solution.

Then, 15 ppm of benzene means that there are 15 grams of benzen in 1,000,000 grams of solution.

That leads to:

1,000,000 g solution / 15 g benzene

Multiplying numerator and denominator by 10⁻⁶ you find:

1,000,000 × 10⁻⁶ g solution / (15 × 10⁻⁶ g benzene)

Simplifying:

1.0 g solution / (15 × 10⁻⁶ g benzene)

Which is read as 1.0 g of the solution contains 15 × 10⁻⁶ g of benzene, i.e. the second answer choice.

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