Chapter 27: Gravimetric and Combustion Analysis

A mixture weighing 7.290mgcontained only cyclohexane, ${\mathbf{C}}_{\mathbf{6}}{\mathbf{H}}_{\mathbf{12}}$(FM 84.159), and oxirane, ${\mathbf{C}}_{\mathbf{2}}{\mathbf{H}}_{\mathbf{4}}\mathbf{O}$(FM 44.052). When the mixture was analyzed by combustion analysis, 21.999mg of${\mathbf{CO}}_{\mathbf{2}}$(FM 44.009) were produced. Find the weight percent of oxirane in mixture.

Combustion analysis of an organic compound gave the composition $\mathbf{71}\mathbf{.}\mathbf{17}\mathbf{\pm }\mathbf{0}\mathbf{.}\mathbf{41}\mathbf{wt}\mathbf{\%}\mathbf{C}\mathbf{,}\mathbf{6}\mathbf{.}\mathbf{76}\mathbf{\pm }\mathbf{0}\mathbf{.}\mathbf{12}\mathbf{wt}\mathbf{\%}\mathbf{H}$, and$\mathbf{10}\mathbf{.}\mathbf{34}\mathbf{\pm }\mathbf{0}\mathbf{.}\mathbf{08}\mathbf{wt}\mathbf{\%}\mathbf{N}$ . Find the coefficients and and their uncertainties and in the formula ${\mathbf{C}}_{\mathbf{8}}{\mathbf{H}}_{\mathbf{\pm }\mathbf{\pm }\mathbf{x}}{\mathbf{N}}_{\mathbf{n}\mathbf{\pm }\mathbf{y}}$.

Determination of sulfur by combustion analysis produces a mixture of ${\mathbf{SO}}_{\mathbf{2}}$and ${\mathbf{SO}}_{\mathbf{3}}$ that can be passed through ${\mathbf{H}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{2}}$to convert both into ${\mathbf{H}}_{\mathbf{2}}{\mathbf{SO}}_{\mathbf{4}}$, which is titrated with standard base. When 6.123m g of a substance were burned, the ${\mathbf{H}}_{\mathbf{2}}{\mathbf{SO}}_{\mathbf{4}}$ required 3.01mL of0.010576MNaOHfor titration. Find wt% sulfur in the sample.

Statistics of coprecipitation. 17In Experiment 1,200.0mL of solution containing 10.0mg of ${\mathbf{SO}}_{\mathbf{4}}^{\mathbf{2}\mathbf{-}}$(from ${\mathbf{Na}}_{\mathbf{2}}{\mathbf{SO}}_{\mathbf{4}}$) were treated with excess ${\mathbf{Bacl}}_{\mathbf{2}}$ solution to precipitate ${\mathbf{BaSO}}_{\mathbf{4}}$ containing some coprecipitated ${\mathbf{Cl}}^{\mathbf{-}}$. To find out how much coprecipitated was present, the precipitate was dissolved in 35mL of 98wt%${\mathbf{H}}_{\mathbf{2}}{\mathbf{SO}}_{\mathbf{4}}$ and boiled to liberate , which was removed by bubbling gas through the ${\mathbf{H}}_{\mathbf{2}}{\mathbf{SO}}_{\mathbf{4}}$. The HCI/${\mathbf{N}}_{\mathbf{2}}$ stream was passed into a reagent solution that reacted with to give a color that was measured. Ten replicate trials gave values of 7.8,9.8,7.8,7.8,7.8,7.8,13,7,12.7,13.7, and 12.7. Experiment 2 was identical to the first one, except that the mL solution also contained of from ). Ten replicate trials gave 7.8,10,8,8.8,7.8,6.9, 8.8, 15.7 , 12.7 , 13.7and $14.7{\mathrm{\mu molCl}}^{-}$.

(a) Find the mean, standard deviation, and 95% confidence interval for ${\mathbf{Cl}}^{\mathbf{-}}$in each experiment.

(b) Is there a significant difference between the two experiments? What does your answer mean?

(c) If there were no coprecipitate, what mass of ${\mathbf{BaSO}}_{\mathbf{4}}$(FM 233.39) would be expected?

(d) If the coprecipitate is (FM 208.23), what is the average mass of precipitate$\left({\mathrm{BaSO}}_4+{\mathrm{BaCl}}_2\right)$in Experiment 1. By what percentage is the mass greater than the mass in part (c)?

Why is high relative supersaturation undesirable in a gravimetric precipitation?

Why is high relative supersaturation undesirable in a gravimetric precipitation?

We now examine some examples that illustrate how to relate the mass of a gravimetric precipitate to the quantity of the original analyte. The general approach is to relate the moles of product to the moles of reactant.

What measures can be taken to decrease the relative supersaturation during a precipitation?

An alloy contains$\mathbf{~}\mathbf{2}\mathbf{.}\mathbf{0}$ wt% Ni. What volume of 0.83 wt% DMG should be used to provide a 50%excess of DMG for the analysis of 1.8 g of steel? What mass of$\mathbf{Ni}\mathbf{(}\mathbf{DMG}{\mathbf{)}}_{\mathbf{2}}$ precipitate is expected?

Why are many ionic precipitates washed with electrolyte solution instead of pure water?