Question 4.2: Pipes made of polyvinyl chloride (PVC; Figure 4.5) are widel...
Pipes made of polyvinyl chloride (PVC; Figure 4.5) are widely used in homes and office buildings, though their use is restricted because vinyl chloride (VC) may leach from them. The maximum concentration of vinyl chloride (CH_{2}CHCl) allowed in drinking water in the United States is 0.002 mg CH_{2}CHCl/L solution. What molarity is this?
Collect, Organize, and Analyze We are given a concentration in milligrams of solute per liter of solution, and we are asked to convert it to units of molarity (moles of solute per liter of solution). The molar mass of a substance relates the mass and number of moles of a given quantity of the substance. Because the solute mass is given in milligrams, we need to convert this mass first to grams and then to moles. Because the number of grams of vinyl chloride is very small, we can expect that the molarity will be smaller still .
Solve The conversion factors we need are the molar mass of CH_{2}CHCl and the equality 1 g =10^{3} mg. The molar mass of vinyl chloride is:
\mathscr{M}=2(12.01 g C/mol) + 3(1.008 g H/mol) + 35.45 g Cl/mol = 62.49 g/mol
and the molarity is
\frac{0.002 \sout{mg}}{L} \times \frac{1 \sout{g}}{10^{3} \sout{mg}} \times \frac{1 mol}{62.49 \sout{g}} = \frac{3.2 \times 10^{-8} mol}{L} =3 \times 10^{-8} M,or 0.03\mu M
Think About It This concentration may seem very low, but remember that the amount of vinyl chloride (0.002 mg, or 2 × 10^{-6} g) in 1 L of water is very small. The extremely low limit of only 0.002 mg/L or 3 × 10^{-8} M reflects recognition of the danger vinyl chloride poses to human health.