Question

One analyzed an unknown solution containing some of Analytical Group 1 cations (Ag+, Hg2+2, and Pb+2) according to the procedure presented in this experiment. Following was observed: A white precipitate results when an unknown solution is treated with 6M HCl. After washing, 1 mL of distilled water is added, the precipitate is stirred, and the tube is heated in near-boiling water. After centrifuging, the solution is drawn off and set aside. Then the 6M NH3 is added to the precipitate, nearly all of which appears to dissolve.

a) Which ion(s) are definitely present?

b) Which ion(s) are definitely absent?

c) Which ion(s) are uncertain (can't tell from data given)?

Answer 1

Steps:

Ag, Hg2+2; Pb+2 will ALL preciptiate in presence of HCl in acidic media

Ag+ + Cl --ª AgCl(s) white solid

Hg2+2 + Cl --ª HgCl2(s) white solid

Pb+2 + Cl- ---> PbCl2(s) white solid

all are white solids so we can't be sure which one is present

then, after washing, diluting and precipitating ---> no effects onf precipitant

then when almost boiling, and set aside

after addition of NH3; nearly all precipitate dissolves

then

NH3;

AgCl + NH3 = Ag(NH3)Cl (aq)

HgCl2 + NH3 = Hg(s) + Hg(NH2)Cl(s)

Since all actually disolves; then you are sure Ag and Hg are NOTpresent

a)

Pb+2 is for sure there, since

b)

Ag+ and Cl- are NOT present, since they did not remain in solution after addition of NH3

c)

none