Question

An environmental laboratory analyzes a 25.98 gram oil sample by performing an extraction to 500 ml of water. The chromium concentration in the 500.0 ml volumetric flask is 0.81 parts per billion. What is the concentration of the chromium in the soil sample?

Answer 1

We can use the following formula to calculate the concentration of the chromium in the soil sample:

Concentration of Chromium in the soil sample = (Concentration of Chromium in the water sample) x (Volume of water used for extraction) / (Mass of soil sample)

First, we need to convert the concentration of chromium in the water sample from parts per billion (ppb) to molarity (M). We can use the following formula for this:

1 ppb = 1 microgram per liter (µg/L) = 1 x 10^-9 g/L
Molarity (M) = (Mass of solute in grams) / (Molar mass of solute in grams/mol) / (Volume of solution in liters)

The molar mass of Cr is 52 g/mol.

Concentration of Chromium in the water sample = (0.81 ppb) x (1 x 10^-9 g/µg) / (52 g/mol) / (0.5 L) = 1.5 x 10^-11 M

Now, we can use the formula above to calculate the concentration of chromium in the soil sample:

Concentration of Chromium in the soil sample = (1.5 x 10^-11 M) x (0.5 L) / (25.98 g) = 2.9 x 10^-13 M

Therefore, the concentration of the chromium in the soil sample is 2.9 x 10^-13 M.