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# A sample of drinking water was found to be severely contaminated with chloroform $(CHCl_3$) supposed to be a carcinogen. The level of contamination was 15 ppm (by mass): express this in percent by mass

$\begin{array}{1 1}1.5\times 10^{-4}\\2.5\times 10^{-4}\\4.5\times 10^{-4}\\5.5\times 10^{-4}\end{array}$

15 ppm means 15 parts in million $(10^6)$ parts by mass in the solution
% by mass =$\large\frac{15}{10^6}$$\times 100=1.5\times 10^{-4}$