Question

An analytical chemist wants to measure the concentration of lead in drinking water in their building. They dilute 1mL of the sample to 100mL of ultrapure water, then acidified (acidification does not affect concentration. It is just a step to make sure the metals are dissolved in the water) and quantified the metal concentration by Inductively coupled plasma - optical emission spectrometry. The result of the diluted sample measured 0.5ppm Pb. Which of the following represents the concentration of the undiluted sample? Is this water safe to drink? (select one or more)
a. 50 ppb
b. 50 ppm
c. 2 x 10-4 M
d. 2 x 10-4 % mass
e. 5 ppm
f. 5 ppb
g. 5 x 10-4 M
h. 5 x 10-4 % mass
i. Yes
j. No

Answer 1

Answer:

b. 50ppm of Pb

c. 2x10⁻⁴M

j. no

Explanation:

The diluted sample has a concentration of 0.5 ppm of Pb. The sample was diluted from 1mL to 100mL. That means the dilution factor is:

100mL / 1mL = 100

That means, the sample was diluted 100 times from its original concentration.

As diluted sample has a concentration of 0.5ppm, the undiluted sample has a concentration of

0.5ppm×100 = 50ppm of Pb

1 ppm means 1x10⁻⁴ % by mass, 50 ppm are 50x10⁻⁴ % by mass

As molar mass of Pb is 207g/mol, molarity of 0.050g / L a(50 ppm) are:

0.050gₓ (1mol / 207g) / 1L = 2x10⁻⁴M

Based on EPA regulation, the maximum concentration of Lead in drinking water must be 0.015ppm. That means this water is not safe to drink