Chapter 10

Describe the unit operations commonly employed in gravimetric analysis, and briefly indicate the purpose of each.

What is digestion of a precipitate, and why is it necessary?

What is coprecipitation? List the different types of coprecipitation, and indicate how they may be minimized or treated.

Why must a filtered precipitate be washed?

Why must a wash liquid generally contain an electrolyte? What are the requirements for this electrolyte?

Calculate the weight of sodium present in \(50.0 \mathrm{~g} \mathrm{Na}_{2} \mathrm{SO}_{4}\)

How many grams \(\mathrm{CuO}\) would \(1.00 \mathrm{~g}\) Paris green, \(\mathrm{Cu}_{3}\left(\mathrm{AsO}_{3}\right)_{2} \cdot 2 \mathrm{As}_{2} \mathrm{O}_{3} \cdot \mathrm{Cu}\left(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\right)_{2},\) give? Of \(\mathrm{As}_{2} \mathrm{O}_{3} ?\)

A 523.1 -mg sample of impure \(\mathrm{KBr}\) is treated with excess \(\mathrm{AgNO}_{3}\) and \(814.5 \mathrm{mg} \mathrm{AgBr}\) is obtained. What is the purity of the KBr?

What weight of \(\mathrm{Fe}_{2} \mathrm{O}_{3}\) precipitate would be obtained from a \(0.4823-\mathrm{g}\) sample of iron wire that is \(99.89 \%\) pure?

Iron in an ore is to be analyzed gravimetrically by weighing as \(\mathrm{Fe}_{2} \mathrm{O}_{3} .\) It is desired that the results be obtained to four significant figures. If the iron content ranges between 11 and \(15 \%\), what is the minimum size sample that must be taken to obtain \(100.0 \mathrm{mg}\) of precipitate?

The chloride in a 0.12 -g sample of \(95 \%\) pure \(\mathrm{MgCl}_{2}\) is to be precipitated as \(\mathrm{AgCl}\). Calculate the volume of \(0.100 \mathrm{M} \mathrm{AgNO}_{3}\) solution required to precipitate the chloride and give a \(10 \%\) excess.

Ammonium ions can be analyzed by precipitating with \(\mathrm{H}_{2} \mathrm{PtCl}_{6}\) as \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{PtCl}_{6}\) and then igniting the precipitate to platinum metal, which is weighed \(\left[\left(\mathrm{NH}_{4}\right)_{2} \mathrm{PtCl}_{6} \stackrel{\text { heat }}{\longrightarrow} \mathrm{Pt}+2 \mathrm{NH}_{4} \mathrm{Cl}(\mathrm{g})+\right.\) \(\left.2 \mathrm{Cl}_{2}(\mathrm{~g})\right] .\) Calculate the percent ammonia in a \(1.00-\mathrm{g}\) sample that yields \(0.100 \mathrm{~g} \mathrm{Pt}\) by this method.

A mixture containing only \(\mathrm{BaO}\) and \(\mathrm{CaO}\) weighs \(2.00 \mathrm{~g} .\) The oxides are converted to the corresponding mixed sulfates, which weigh \(4.00 \mathrm{~g}\). Calculate the percent \(\mathrm{Ba}\) and \(\mathrm{Ca}\) in the original mixture.

A mixture containing only \(\mathrm{BaSO}_{4}\) and \(\mathrm{CaSO}_{4}\) contains one-half as much \(\mathrm{Ba}^{2+}\) as \(\mathrm{Ca}^{2+}\) by weight. What is the percentage of \(\mathrm{CaSO}_{4}\) in the mixture?

A mixture containing only \(\mathrm{AgCl}\) and \(\mathrm{AgBr}\) weighs \(2.000 \mathrm{~g}\). It is quantitatively reduced to silver metal, which weighs \(1.300 \mathrm{~g}\). Calculate the weight of \(\mathrm{AgCl}\) and \(\mathrm{AgBr}\) in the original mixture.

Write solubility product expressions for the following: (a) AgSCN, (b) \(\mathrm{La}\left(\mathrm{IO}_{3}\right)_{3}\), (c) \(\mathrm{Hg}_{2} \mathrm{Br}_{2}\), (d) \(\mathrm{Ag}\left[\mathrm{Ag}(\mathrm{CN})_{2}\right] ;\) (e) \(\mathrm{Zn}_{2} \mathrm{Fe}(\mathrm{CN})_{6},\) (f) \(\mathrm{Bi}_{2} \mathrm{~S}_{3}\).

Bismuth iodide, \(\mathrm{BiI}_{3},\) has a solubility of \(7.76 \mathrm{mg} / \mathrm{L}\). What is its \(K_{\mathrm{sp}}\) ?

Calculate the concentration of barium in the solution at equilibrium when \(15.0 \mathrm{~mL}\) of \(0.200 \mathrm{M}\) \(\mathrm{K}_{2} \mathrm{CrO}_{4}\) is added to \(25.0 \mathrm{~mL}\) of \(0.100 \mathrm{M} \mathrm{BaCl}_{2}\)

Compounds \(\mathrm{AB}\) and \(\mathrm{AC}_{2}\) each have solubility products equal to \(4 \times 10^{-18} .\) Which is more soluble, as expressed in moles per liter?

The solubility product of \(\mathrm{Bi}_{2} \mathrm{~S}_{3}\) is \(1 \times 10^{-97}\) and that of \(\mathrm{HgS}\) is \(4 \times 10^{-53} .\) Which is the least soluble?

A student proposes to analyze barium gravimetrically by precipitating \(\mathrm{BaF}_{2}\) with NaF. Assuming a 200 -mg sample of \(\mathrm{Ba}^{2+}\) in \(100 \mathrm{~mL}\) is to be precipitated and that the precipitation must be \(99.9 \%\) complete for quantitative results, comment on the feasibility of the analysis.

Write the thermodynamic solubility product expressions for the following: (a) \(\mathrm{BaSO}_{4} \rightleftharpoons \mathrm{Ba}^{2+}+\mathrm{SO}_{4}^{2-}\) (b) \(\mathrm{Ag}_{2} \mathrm{CrO}_{4} \rightleftharpoons 2 \mathrm{Ag}^{+}+\mathrm{CrO}_{4}^{2-}\)