Question

Finely ground mineral (0.6324g)was dissolved in 25 mLof boiling 4M HCland diluted with $\mathbf{175}\mathbf{mL}\mathbf{}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}$containing two drops of methyl red indicator. The solution was heated to${\mathbf{100}}^{\mathbf{o}}\mathbf{C}\mathbf{,}\mathbf{}\mathbf{and}\mathbf{}\mathbf{50}\mathbf{mL}$ of warm solution containing$\mathbf{2}\mathbf{.}\mathbf{0}\mathbf{g}\mathbf{(}{\mathbf{NH}}_{\mathbf{4}}{\mathbf{)}}_{\mathbf{2}}{\mathbf{C}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{4}}$ were slowly added to precipitate${\mathbf{CaC}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{4}}\mathbf{.}\mathbf{Then}\mathbf{}\mathbf{6}{\mathbf{MNH}}_{\mathbf{3}}$ was added until the indicator changed from red to yellow, showing that the liquid was neutral or slightly basic. After slow cooling for 1 h, the liquid was decanted and the solid transferred to a filter crucible and washed with cold$\mathbf{10}\mathbf{.}\mathbf{1}\mathbf{wt}\mathbf{\%}\mathbf{(}{\mathbf{NH}}_{\mathbf{4}}{\mathbf{)}}_{\mathbf{2}}{\mathbf{C}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{4}}$ solution five times until no${\mathbf{Cl}}^{\mathbf{-}}$ was detected in the filtrate upon addition of${\mathbf{AgNO}}_{\mathbf{3}}$ solution. The crucible was dried at 1 h and then at$\mathbf{105}\mathbf{°}\mathbf{C}$ in a furnace for 2 h.

${\mathbf{Ca}}^{\mathbf{2}\mathbf{+}}\mathbf{+}{\mathbf{C}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{4}}^{\mathbf{2}\mathbf{-}}\stackrel{\mathbf{105}\mathbf{°}\mathbf{C}}{\mathbf{\to }}{\mathbf{CaC}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{4}}\mathbf{+}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\mathbf{\left(}\mathbf{s}\mathbf{\right)}\stackrel{{\mathbf{500}}^{\mathbf{o}}\mathbf{C}}{\mathbf{\to }}{\mathbf{CaCO}}_{\mathbf{3}}\mathbf{\left(}\mathbf{s}\mathbf{\right)}$

FM 40.078 18.5467 g

The mass of the empty crucible was 18.2311 g and the mass of the crucible with ${\mathbf{CaCO}}_{\mathbf{3}}$was 18.5467 g .

(a) Find the wt% Ca in the mineral.

(b) Why is the unknown solution heated to boiling and the precipitant solution, $\mathbf{(}{\mathbf{NH}}_{\mathbf{4}}{\mathbf{)}}_{\mathbf{2}}{\mathbf{C}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{4}}$ also heated before slowly mixing the two?

(c) What is the purpose of washing the precipitate with0.1wt%$\mathbf{(}{\mathbf{NH}}_{\mathbf{4}}{\mathbf{)}}_{\mathbf{2}}{\mathbf{C}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{4}}$?

(d) What is the purpose of testing the filtrate with${\mathbf{AgNO}}_{\mathbf{3}}$solution?

Short answer

The solutions for the given statements are

(a) The mass percentage of calcium in given mineral is 19.98 %.

(b)The solution and the precipitant solution are heated in order to increase the solubility of product which precipitates. In case the solution is minimal supersaturated in precipitation, formation of crystal is slow and large. These are easy for filtration.

(c)To explain: The benefits of cleaning precipitate with ammonium oxalate

• Prevention of re-dissolve of calcium oxalate by oxalate ions.
• Both ammonium and oxalate ion create ionic atmosphere which stops precipitation of peptizing.
• These are benefits of cleaning precipitate with ammonium oxalate.

(d)To explain: The filtrate testing benefits with silver nitrate

• The presence of chloride ion form silver chloride precipitate.
• A chloride ion comes from hydrochloric acid to dissolve mineral. All the remaining solution should be washed and there will be no unnecessary solutions that raise mass of final precipitate $\left({\mathrm{CaCO}}_3\right)$.These are benefits of filtrate testing with silver nitrate.

Step-by-step solution

Calculate the mass percentage of calcium in given mineral

(a)To calculate: The mass percentage of calcium in given mineral.

Mass of calcium carbonate$=18.5467g-18.2311g=0.3156g$

Number of moles of calcium carbonate is calculated as

$=\left(\frac{0.3156g}{100.087g/mol}\right)=3.153\times {10}^{-3}mol$

The mass of calcium is calculated using molecular weight

$=(3.153\times {10}^{-3})(40.078g/mol)=0.1264gCa$

The mass percent of calcium is calculated as

$Wt\%=\frac{0.1264gCa}{0.6324gmineral}\times 100=19.98\%$

Explain the benefits of ammonium and silver nitrate

(b)The solution and the precipitant solution are heated in order to increase the solubility of product which precipitates. In case the solution is minimal supersaturated in precipitation, formation of crystal is slow and large. These are easy for filtration.

(c)To explain: The benefits of cleaning precipitate with ammonium oxalate

• Prevention of re-dissolve of calcium oxalate by oxalate ions.
• Both ammonium and oxalate ion create ionic atmosphere which stops precipitation of peptizing.
• These are benefits of cleaning precipitate with ammonium oxalate.

(d)To explain: The filtrate testing benefits with silver nitrate

• The presence of chloride ion form silver chloride precipitate.
• A chloride ion comes from hydrochloric acid to dissolve mineral. Silver nitrate will precipitate all the chloride ions. All the remaining solution should be washed and there will be no unnecessary solutions that raise mass of final precipitate $\left({\mathrm{CaCO}}_3\right)$.These are benefits of filtrate testing with silver nitrate.