Chapter 8

Aluminum is determined gravimetrically by precipitating \(\mathrm{Al}(\mathrm{OH})_{3}\) and isolating \(\mathrm{Al}_{2} \mathrm{O}_{3} .\) A sample that contains approximately \(0.1 \mathrm{~g}\) of \(\mathrm{Al}\) is dissolved in \(200 \mathrm{~mL}\) of \(\mathrm{H}_{2} \mathrm{O},\) and \(5 \mathrm{~g}\) of \(\mathrm{NH}_{4} \mathrm{Cl}\) and a few drops of methyl red indicator are added (methyl red is red at pH levels below 4 and yellow at \(\mathrm{pH}\) levels above 6 ). The solution is heated to boiling and \(1: 1 \mathrm{NH}_{3}\) is added dropwise until the indicator turns yellow, precipitating \(\mathrm{Al}(\mathrm{OH})_{3} .\) The precipitate is held at the solution's boiling point for several minutes before filtering and rinsing with a hot solution of \(2 \%\) \(\mathrm{w} / \mathrm{v} \mathrm{NH}_{4} \mathrm{NO}_{3} .\) The precipitate is then ignited at \(1000-1100^{\circ} \mathrm{C},\) form- ing \(\mathrm{Al}_{2} \mathrm{O}_{3}\) (a) Cite at least two ways in which this procedure encourages the formation of larger particles of precipitate. (b) The ignition step is carried out carefully to ensure the quantitative conversion of \(\mathrm{Al}(\mathrm{OH})_{3}\) to \(\mathrm{Al}_{2} \mathrm{O}_{3} .\) What is the effect of an incomplete conversion on the \(\% \mathrm{w} / \mathrm{w}\) Al? (c) What is the purpose of adding \(\mathrm{NH}_{4} \mathrm{Cl}\) and methyl red indicator? (d) An alternative procedure foraluminum involves isolating and weighing the precipitate as the 8 -hydroxyquinolate, \(\mathrm{Al}\left(\mathrm{C}_{9} \mathrm{H}_{6} \mathrm{NO}\right)_{3} .\) Why might this be a more advantageous form of Al for a gravimetric analysis? Are there any disadvantages?

Calcium is determined gravimetrically by precipitating \(\mathrm{CaC}_{2} \mathrm{O}_{4} \cdot \mathrm{H}_{2} \mathrm{O}\) and isolating \(\mathrm{CaCO}_{3}\). After dissolving a sample in \(10 \mathrm{~mL}\) of water and \(15 \mathrm{~mL}\) of \(6 \mathrm{M} \mathrm{HCl}\), the resulting solution is heated to boiling and a warm solution of excess ammonium oxalate is added. The solution is maintained at \(80^{\circ} \mathrm{C}\) and \(6 \mathrm{M} \mathrm{NH}_{3}\) is added dropwise, with stirring, until the solution is faintly alkaline. The resulting precipitate and solution are removed from the heat and allowed to stand for at least one hour. After testing the solution for completeness of precipitation, the sample is filtered, rinsed with \(0.1 \% \mathrm{w} / \mathrm{v}\) ammonium oxalate, and dried for one hour at \(100-120^{\circ} \mathrm{C}\). The precipitate is transferred to a muffle furnace where it is converted to \(\mathrm{CaCO}_{3}\) by drying at \(500 \pm 25^{\circ} \mathrm{C}\) until constant weight. (a) Why is the precipitate of \(\mathrm{CaC}_{2} \mathrm{O}_{4} \cdot \mathrm{H}_{2} \mathrm{O}\) converted to \(\mathrm{CaCO}_{3} ?\) (b) In the final step, if the sample is heated at too high of a temperature some \(\mathrm{CaCO}_{3}\) is converted to \(\mathrm{CaO}\). What effect would this have on the reported \(\% \mathrm{w} / \mathrm{w}\) Ca? (c) Why is the precipitant, \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{C}_{2} \mathrm{O}_{4},\) added to a hot, acidic solution instead of a cold, alkaline solution?

A sample of an impure iron ore is approximately \(55 \% \mathrm{w} / \mathrm{w} \mathrm{Fe}\). If the amount of Fe in the sample is determined gravimetrically by isolating it as \(\mathrm{Fe}_{2} \mathrm{O}_{3},\) what mass of sample is needed to ensure that we isolate at least \(1.0 \mathrm{~g}\) of \(\mathrm{Fe}_{2} \mathrm{O}_{3} ?\)

The concentration of arsenic in an insecticide is determined gravimetrically by precipitating it as \(\mathrm{MgNH}_{4} \mathrm{AsO}_{4}\) and isolating it as \(\mathrm{Mg}_{2} \mathrm{As}_{2} \mathrm{O}_{7}\) Determine the \(\% \mathrm{w} / \mathrm{w} \mathrm{As}_{2} \mathrm{O}_{3}\) in a \(1.627-\mathrm{g}\) sample of insecticide if \(\mathrm{it}\) yields \(106.5 \mathrm{mg}\) of \(\mathrm{Mg}_{2} \mathrm{As}_{2} \mathrm{O}_{7}\)

After preparing a sample of alum, \(\mathrm{K}_{2} \mathrm{SO}_{4} \cdot \mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3} \cdot 24 \mathrm{H}_{2} \mathrm{O},\) an ana- lyst determines its purity by dissolving a \(1.2931-\mathrm{g}\) sample and precipitating the aluminum as \(\mathrm{Al}(\mathrm{OH})_{3}\). After filtering, rinsing, and igniting, \(0.1357 \mathrm{~g}\) of \(\mathrm{Al}_{2} \mathrm{O}_{3}\) is obtained. What is the purity of the alum preparation?

A 1.4639 -g sample of limestone is analyzed for \(\mathrm{Fe}, \mathrm{Ca}\), and \(\mathrm{Mg}\). The iron is determined as \(\mathrm{Fe}_{2} \mathrm{O}_{3}\) yielding \(0.0357 \mathrm{~g} .\) Calcium is isolated as \(\mathrm{CaSO}_{4},\) yielding a precipitate of \(1.4058 \mathrm{~g},\) and \(\mathrm{Mg}\) is isolated as \(0.0672 \mathrm{~g}\) of \(\mathrm{Mg}_{2} \mathrm{P}_{2} \mathrm{O}_{7} .\) Report the amount of Fe, \(\mathrm{Ca}\), and \(\mathrm{Mg}\) in the limestone sample as \(\% \mathrm{w} / \mathrm{w} \mathrm{Fe}_{2} \mathrm{O}_{3}, \% \mathrm{w} / \mathrm{w} \mathrm{CaO},\) and \(\% \mathrm{w} / \mathrm{w} \mathrm{MgO} .\)

The number of ethoxy groups \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}-\right)\) in an organic compound is determined by the following two reactions. $$ \begin{array}{c} \mathrm{R}\left(O \mathrm{CH}_{2} \mathrm{CH}_{3}\right)_{x}+x \mathrm{HI} \longrightarrow \mathrm{R}(\mathrm{OH})_{x}+x \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{I} \\ \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{I}+\mathrm{Ag}^{+}+\mathrm{H}_{2} \mathrm{O} \longrightarrow \mathrm{AgI}(s)+\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH} \end{array} $$ A 36.92 -mg sample of an organic compound with an approximate molecular weight of 176 is treated in this fashion, yielding \(0.1478 \mathrm{~g}\) of \(\mathrm{AgI}\). How many ethoxy groups are there in each molecule of the compound?

A \(516.7-\mathrm{mg}\) sample that contains a mixture of \(\mathrm{K}_{2} \mathrm{SO}_{4}\) and \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}\) is dissolved in water and treated with \(\mathrm{BaCl}_{2},\) precipitating the \(\mathrm{SO}_{4}^{2-}\) as \(\mathrm{BaSO}_{4}\). The resulting precipitate is isolated by filtration, rinsed free of impurities, and dried to a constant weight, yielding \(863.5 \mathrm{mg}\) of \(\mathrm{BaSO}_{4} .\) What is the \(\% \mathrm{w} / \mathrm{w} \mathrm{K}_{2} \mathrm{SO}_{4}\) in the sample?

The amount of iron and manganese in an alloy is determined by precipitating the metals with 8 -hydroxyquinoline, \(\mathrm{C}_{9} \mathrm{H}_{7} \mathrm{NO}\). After weighing the mixed precipitate, the precipitate is dissolved and the amount of 8-hydroxyquinoline determined by another method. In a typical analysis a 127.3 -mg sample of an alloy containing iron, manganese, and other metals is dissolved in acid and treated with appropriate masking agents to prevent an interference from other metals. The iron and manganese are precipitated and isolated as \(\mathrm{Fe}\left(\mathrm{C}_{9} \mathrm{H}_{6} \mathrm{NO}\right)_{3}\) and \(\mathrm{Mn}\left(\mathrm{C}_{9} \mathrm{H}_{6} \mathrm{NO}\right)_{2},\) yielding a total mass of \(867.8 \mathrm{mg}\). The amount of 8 -hydroxyquinolate in the mixed precipitate is determined to be \(5.276 \mathrm{mmol}\). Calculate the \(\% \mathrm{w} / \mathrm{w} \mathrm{Fe}\) and \(\% \mathrm{w} / \mathrm{w} \mathrm{Mn}\) in the alloy.

A \(0.8612-\mathrm{g}\) sample of a mixture of \(\mathrm{NaBr}\), \(\mathrm{NaI}\), and \(\mathrm{NaNO}_{3}\) is analyzed by adding \(\mathrm{AgNO}_{3}\) and precipitating a \(1.0186-\mathrm{g}\) mixture of \(\mathrm{AgBr}\) and AgI. The precipitate is then heated in a stream of \(\mathrm{Cl}_{2}\), which converts it to \(0.7125 \mathrm{~g}\) of \(\mathrm{AgCl}\). Calculate the \(\% \mathrm{w} / \mathrm{w} \mathrm{NaNO}_{3}\) in the sample.