Respuesta :
Formula used for determining the molarity is:
[tex]Molarity = \frac{number of moles of solute}{volume of solution in Liters)}[/tex] -(1)
Number of moles of solute that is [tex]C_{20}H_{42}[/tex] is determined as:
[tex]number of moles = \frac{given weight}{Molar mass}[/tex] -(2)
Mass of the solute can be calculated as:
[tex]Density = \frac{mass}{volume}[/tex]
[tex]mass = density\times volume[/tex] - (3)
density of rain water = 1.00 g/mL (given)
Concentration of [tex]C_{20}H_{42}[/tex] = 0.2 ppb (given)
Since, [tex]1 ppb =[/tex] [tex]10^{-3} mg/L[/tex]
So, 0.2 ppb = [tex]0.2\times 10^{-3} mg/L = 2\times 10^{-4} mg/L[/tex]
Substituting the values in formula (3):
Mass = [tex]2\times 10^{-4} mg = 2\times 10^{-7} g[/tex] (As 1 mg = [tex]10^{-3}[/tex] g).
Now, substituting the values of mass and formula mass in formula (2):
Number of moles = [tex]\frac{2\times 10^{-7}}{282.56} = 7.078\times 10^{-10} mole[/tex]
Volume of solution = 1 L
Substituting the values of number of moles and volume of solution in formula (1):
Molarity = [tex]\frac{7.078\times 10^{-10} mole}{1 L} = 7.078\times 10^{-10} M[/tex]
