Question

The maximum allowable concentration of Pb2+ ions in water in a local community is 1.51 x 10-8 M. Could a chemist use 2.00 mL of 0.12 M KBr solution on a 5.00 mL sample of tap water to prove through a precipitation reaction that the water supply is safe? Show all work and reasoning

Answer 1

The water supply is not safe.

Step-by-step explanation:

Ionically, the reaction is;

Pb^2+(aq) + 2Br^-(aq) -------> PbBr2(s)

Amount of Br^- reacted = 2/1000 * 0.12 = 2.4 * 10^-4 moles

From the reaction equation;

1 mole of Pb^2+ requires 2 moles of Br^- for complete precipitation

x moles of Pb^2+ requires 2.4 * 10^-4 moles of Br^-

x = 1 * 2.4 * 10^-4 /2

x = 1.2 * 10^-4 moles of Pb^2+

Concentration of Pb^2+ precipitated from 5ml of tap water = 1.2 * 10^-4 * 1000/5

Concentration = 2.4 * 10^-2 M

The amount of Pb^2+ found in the water is greater than the maximum allowable concentration of Pb2+ ions in water in a local community which is 1.51 x 10-8 M. Therefore, the water supply is not safe.