Page 44 - Materials Chemistry, Second Edition

Site Assessment and Remedial Investigation 27

To determine mass fractions of components in gasoline, the following pro-
cedure can be used:

Step 1: Determine the mass of the mixture (i.e., TPH) and mass of
each COC.
Step 2: Determine the mass fraction by dividing the mass of each COC
with the mass of TPH.
To determine the mole fractions of components in gasoline, the following
procedure can be used:

Step 1: Determine the mass of TPH and mass of each COC in the
impacted soil.
Step 2: Determine the molecular weight of each COC.
Step 3: Determine the molecular weight of gasoline from the composi-
tion and molecular weights of all constituents. This procedure
is tedious, and data may not be readily available. Assuming the
molecular weight of gasoline to be 100, which is equivalent to
that of heptane (C H ), is relatively reasonable.
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Step 4: Calculate the number of moles of each COC by dividing its mass
with its molecular weight.
Step 5: Calculate the mole fraction of each COC by dividing the num-
ber of moles of each COC with the number of moles of TPH.

Information needed for this calculation

• Mass of the impacted soil
• Concentrations of the COCs
• Molecular weights of the COCs
Example 2.15: Mass and Mole Fractions of COCs in Gasoline

Three samples were taken from a soil pile (110 yd ) and analyzed for TPH
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(EPA method 8015) and for B/T/E/X (EPA method 8020). The average con-
centration of TPH is 1,000 mg/kg, and those of B/T/E/X are 20, 20, 20, and 20
mg/kg, respectively. Determine the mass and mole fractions of B/T/E/X in
the gasoline. The total bulk density of the soil in the stockpile is 1.65 g/cm .
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Solution:
(a) Mass of the impacted soil
= (volume of soil)(total bulk density)