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 11.1 cm3 sample of lake water is found to have 177.1 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 2.32 days to remove 41.40 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 0.350 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

mass of AS = 177.1 ng= 177.1*10^-9 gm, volume of lake water sample = 11.1 cm3, density= 1g/cm3

mass of water= 11.1cm3*1g/cm3= 11.1 gm

concentration of As= 177.1*10^-9 gm/11.1gm= 15.95*10-9 gm/gm= 15.95 ppb ( 1 billion= 10⁹)

2. km3= 10¹⁵cm3, 0.35km3=0.35*10¹⁵cm3 ( 1km= 1000m, 1m= 100cm),1km= 10⁵ cm)

11.1cm³ contains 177.1*10^-9 gm of As

0.35*10¹⁵ cm3 contains 0.35*10¹⁵*177.1*10^-9 gm of As=61985000 gm = 61985 kg of As

41.4 kg is As is removed in 2.32 days

61985 kg of As is removed in 2.32*61985/41.4 =3473.5 days

365 days= 1 year

3473.5 days= 3473.5/365 years=9.52 years.