A sample of drinking water was found to be severely contaminated with chloroform, CHCl3, supposed to be carcinogenic in nature. The level of contamination was 15 ppm (by mass). Express this in percent by mass.

The correct option is 1.5 x 10^-3%.

Step-by-Step Explanation:

1. Understand the meaning of ppm (parts per million):
Parts per million (ppm) is a unit of concentration that expresses the mass of a solute per one million (${10}^6$) parts of the total solution by mass.
Given that the level of contamination of chloroform (${\mathrm{CHCl}}_3$) is $15\text{ppm}$ by mass, it means:
There are $15\text{g}$ of chloroform present in ${10}^6\text{g}$ of the water sample.

2. Recall the formula for percent by mass (mass percent):
The percent by mass of a solute in a solution is calculated using the following formula:

$\text{Percent by mass}=\left(\frac{\text{Mass of solute}}{\text{Total mass of solution}}\right)\times 100$

3. Substitute the values into the formula:
Here, the mass of the solute (chloroform) is $15\text{g}$ and the total mass of the solution is ${10}^6\text{g}$.

$\text{Percent by mass}=\left(\frac{15}{{10}^6}\right)\times {10}^2$

4. Simplify the expression:

$\text{Percent by mass}=15\times {10}^{2-6}=15\times {10}^{-4}\%$

Expressing this in standard scientific notation gives:

$1.5\times {10}^{-3}\%$

Thus, the contamination level expressed in percent by mass is 1.5 x 10^-3%.