Question

Suppose a group of volunteers is planning on building a park near a local lake. The lake is known to contain low levels of arsenic (As). Therefore, prior to starting construction, the group decides to measure the current level of arsenic in the lake.

a) If a 15.7 cm3 sample of lake water is found to have 164.5 ng As, what is the concentration of arsenic in the sample in parts per billion (ppb), assuming that the density of the lake water is 1.00 g/cm3?

One of the volunteers suggests hiring an on-site water treatment company to remove the arsenic from the lake. The company claims their process takes 2.74 days to remove 41.90 kg of As from a water source.

b) Calculate the total mass (in kg) of arsenic in the lake that the company will have to remove if the total volume of water in the lake is 0.710 km3?

c) Based on the company\'s claim and the concentration of arsenic in the lake, how many years will it take to remove all of the arsenic from the lake, assuming that there are always 365 days in a year?

Answer 1

a) If a 15.7 cm3 sample of lake water is found to have 164.5 ng As, what is the concentration of arsenic in the sample in parts per billion (ppb), assuming that the density of the lake water is 1.00 g/cm3?

Solution :- lets convert the 15.7 cm3 to liter and 164.5 ng to microgram

15.7 cm3 * 1 L / 1000 cm3 = 0.0157 L

164.5 ng * 1 ug / 1000 ng = 0.1645 ug

Now lets calculate the ppb

0.1645 ug / 0.015 L = 10.97 ppb

So the concetration is 10.97 ppb

One of the volunteers suggests hiring an on-site water treatment company to remove the arsenic from the lake. The company claims their process takes 2.74 days to remove 41.90 kg of As from a water source.

b) Calculate the total mass (in kg) of arsenic in the lake that the company will have to remove if the total volume of water in the lake is 0.710 km3?

Solution :-

Lets convert the 0.710 km3 to L

0.710 km3 * 1*10^12 L / 1 km3 = 7.1*10^ 11 L

Now lets calculate the mass of the As in the lake

7.1*10^-11 L * 10.97 ug / L = 7.788*10^12 ug

Now lets convert the microgram to kg

7.788 *10 ^12 ug * 1 kg / 1*10^9 ug = 7788.7 kg As

c) Based on the company\'s claim and the concentration of arsenic in the lake, how many years will it take to remove all of the arsenic from the lake, assuming that there are always 365 days in a year?

Solution :- 41.90 kg = 2.74 days

So 7788.7 kg = ? days

7788.7 kg * 2.74 days / 41.90 kg = 509.33 days

Now lets convert days to years

509.33 days *1 year / 365 days = 1.395 years

So the time needed is 1.395 years