Question

. The level of mercury in a stream was suspected to be above the minimum considered safe (1 part per billion by weight). And analysis indicated that the concentration was 0.68 parts per billion. Assume a density of 1.0 g/mL and calculate the molarity of mercury in the stream.

Answer 1

3.39x10⁻⁹ M

Step-by-step explanation:

Let's assume we have 1 mL of the stream. With a density of 1.0 g/mL, we would have 1 g of the stream.

Now we calculate the mass of mercury present in 1 g of the stream, using its given concentration (0.68 ppb):

• ppb = ng Mercury / g stream

• 0.68 ppb = ng Hg / 1 g

• Mass of mercury = 0.68 ng

Now we convert that mass into moles, using mercury's atomic mass:

• 0.68 ng = 0.68 x 10⁻⁹ g

• 0.68 x 10⁻⁹ g ÷ 200.59 g/mol = 3.39x10⁻¹² mol

Finally we calculate the molarity of mercury, as we know that there are 3.39x10⁻¹² Hg moles in 1 mL of the stream:

• 1 mL = 0.001 L

• Molarity = 3.39x10⁻¹² mol / 0.001 L = 3.39x10⁻⁹ M