Question

(2.4pts) Discussion- Question 2 Suppose you want to test for the presence of lead coming from two different sinks in your home. Consider the techniques used in this and previous experiments to answer the following questions.. (0.6pts) a. How could you determine whether lead, specifically, was or was not present in each sink? X X Normal B IU EE (1pts) b. Assuming both sinks tested positive for the presence of lead, how could you determine which sink has a higher concentration of lead? Clearly and specifically describe the procedures you would use Normal BIU x, X T (0.8pts) c. Suppose the concentration of lead in your sink water is very low, making it difficult to accurately test. How could you increase its concentration in your samples so that it becomes easier to detect? x, x Normal BIU

Answer 1

a) One of the easier methods of detecting lead in water is by treating the water with dilute hydrochloric acid. Treating the water with dilute hydrochloric acid leads to precipitation of lead as a white precipitate. This precipitate is soluble in hot water.

Pb²⁺ + 2Cl^- ---> PbCl₂ (ppt)

b)

Water sample containing lead when treated with potassium chromate solution, insoluble lead(II) chromate forms (yellow colored precipitate) and precipitates out.

Pb²⁺(aq) + K₂CrO₄(aq) ==> PbCrO₄(s) + 2 K⁺

This compound has very low solubility and has a K_sp = 2.8 x 10^-13. This precipitate is filtered through a sintered glass funnel , dried in an oven and weighed. From the weight of the precipitate, the amount of lead can be determined. For instance, 323.19 g of Lead chromate will contain 207.2

Weight of lead in the precipitate = 207.2*weight of precipitate/323.19

c)

By increasing the concentration of Potassium chromate, the sensitivity of the method improves. As already mentioned,

K_sp of lead chromate = [Pb²⁺][CrO₄^2-] =  2.8 x 10^-13

=> [Pb2+] = 2.8 x 10^-13/[CrO₄^2-]

When concentration of concentration of [CrO₄^2-] increases, lower levels of [Pb²⁺] is required to precipitate as lead chromate. So even water samples having low levels of lead can be measured.