12. Perfluoroalkyl substances (PFAS) are a class of \"forever\" chemicals that are very hard to break down in the
environment, and instead accumulate in organisms such as us. It is uncertain how much PFAS is a safe level of
this contaminant, or if we can even detect such a low quantity.
The Massachusetts Water Resources Authority (MWRA) monitors our water supply for PFAS. They can
reliably detect any concentrations > 1 ppt (parts per trillion, i.e. 1 g per $10^{12}$ g). Let's make a starting
assumption that just one molecule of a particular PFAS, perfluorooctanoic acid (PFOA), in a drop of water
(1.00 µL) is too high a concentration to be safe. Let's work step by step to determine if the MWRA can detect
this low concentration:
a.
One molecule of PFOA has a mass of 414.07 amu. If 1 g = $6.022 \times 10^{23}$ amu, what is the mass of a
single molecule of PFOA?
b. For 1 molecule of PFOA in 1.00 µL of water, what is the concentration in g/L?
c.
Since water has a density of 1.00 g/mL, we can express the above concentration (mass/volume) in a
unitless ratio (mass/mass). What is the above concentration in units of ppt? Can the MWRA detect this
concentration?
d. The current Environmental Protection Agency (EPA) standard requires drinking water to have < 4 ppt
of PFOA. Given this concentration, how many molecules of PFOA might a person consume in their
daily 3 L of water?
