Problem 1

A 0.4062 g alloy sample containing tin and lead is dissolved in hot

\ce{HCl}

\ce{HNO3}

(Pb → Pb(II), Sn → Sn(IV)); on cooling a precipitate of tin compounds and a lead compound appears.

25.00\ \mathrm{cm^3}

of 0.2000 M

\ce{Na2H2EDTA}

is added (the precipitate dissolves) and the solution is diluted to

250.0\ \mathrm{cm^3}

. A

25.00\ \mathrm{cm^3}

aliquot, buffered at pH 6 with hexamine and Xylenol Orange indicator, is titrated with standard 0.009970 M

\ce{Pb(NO3)2}

: the colour changes yellow→red at

24.05\ \mathrm{cm^3}

. Then 2.0 g of solid

\ce{NaF}

is added — the solution returns to yellow — and titration resumes to a second endpoint at

15.00\ \mathrm{cm^3}

. (

\ce{F^-}

binds

\ce{Sn^{IV}}

strongly but not Pb(II) at pH 6.) (1.1) What lead compound precipitates in step 2? (1.3–1.8) Give the roles of hexamine, Xylenol Orange and

\ce{NaF}

, and the key ionic equations of the two titrations. (1.10) Calculate the weight percentages of Sn and Pb in the alloy.

Step 1 of 4: Precipitate and its dissolution

← Previous Next →

\ce{PbCl2(s)};\qquad \ce{PbCl2(s) + H2Y^{2-} -> PbY^{2-} + 2H+ + 2Cl^-}

Analysis

The sparingly soluble lead compound is $\ce{PbCl2}$ (or a hydroxo species); excess EDTA dissolves it because $\ce{PbY^{2-}}$ is a very stable complex even at pH 6. Hexamine is the pH buffer, Xylenol Orange the metallochromic indicator (free: yellow; Pb–XO complex: red).