47 8.4 Quantitative Chemical Analysis
Learning Objectives
By the end of this section, you will be able to:
- Perform Gravimetric Analysis
- Relate Gravimetric Analysis to Reaction Stoichiometry
- Perform a Combustion Analysis
Gravimetric Analysis
Gravimetric Analysis
A gravimetric analysis is one in which a sample is subjected to some treatment that causes a change in the physical state of the analyte that permits its separation from the other components of the sample. Mass measurements of the sample, the isolated analyte, or some other component of the analysis system, used along with the known stoichiometry of the compounds involved, permit calculation of the analyte concentration. Gravimetric methods were the first techniques used for quantitative chemical analysis, and they remain important tools in the modern chemistry laboratory.
The required change of state in a gravimetric analysis may be achieved by various physical and chemical processes. For example, the moisture (water) content of a sample is routinely determined by measuring the mass of a sample before and after it is subjected to a controlled heating process that evaporates the water. Also common are gravimetric techniques in which the analyte is subjected to a precipitation reaction of the sort described earlier in this chapter. The precipitate is typically isolated from the reaction mixture by filtration, carefully dried, and then weighed (Figure 2). The mass of the precipitate may then be used, along with relevant stoichiometric relationships, to calculate analyte concentration.
Example 2
Gravimetric Analysis
A 0.4550-g solid mixture containing MgSO4 is dissolved in water and treated with an excess of Ba(NO3)2, resulting in the precipitation of 0.6168 g of BaSO4.
What is the concentration (percent) of MgSO4 in the mixture?
Solution
The plan for this calculation is similar to others used in stoichiometric calculations, the central step being the connection between the moles of BaSO4 and MgSO4 through their stoichiometric factor. Once the mass of MgSO4 is computed, it may be used along with the mass of the sample mixture to calculate the requested percentage concentration.
The mass of MgSO4 that would yield the provided precipitate mass is
[latex]0.6168 \;\rule[0.5ex]{3.75em}{0.1ex}\hspace{-3.75em}\text{g BaSO}_4 \times \frac{1 \;\rule[0.25ex]{3.5em}{0.1ex}\hspace{-3.5em}\text{mol BaSO}_4}{233.43 \;\rule[0.25ex]{2.75em}{0.1ex}\hspace{-2.75em}\text{g BaSO}_4} \times \frac{1 \;\rule[0.25ex]{3.5em}{0.1ex}\hspace{-3.5em}\text{mol MgSO}_4}{1\;\rule[0.25ex]{3.5em}{0.1ex}\hspace{-3.5em}\text{mol BaSO}_4} \times \frac{120.37 \;\text{g MgSO}_4}{1 \;\rule[0.25ex]{3.5em}{0.1ex}\hspace{-3.5em}\text{mol MgSO}_4} = 0.3181 \;\text{g MgSO}_4[/latex]
The concentration of MgSO4 in the sample mixture is then calculated to be:
[latex]\begin{array}{rl @{{}={}} l}\text{percent MgSO}_4 & = \frac{\text{mass MgSO}_4}{\text{mass sample}} \times 100\% \\[0.5em] & = \frac{0.3181 \;\text{g}}{0.4550 \;\text{g}}