Gravimetric analysis is a technique through which the amount of the ion being analyzed can be determined through the measurement of mass. Gravimetric analysis depend on comparing the masses of two compounds containing the ion to be analyzed. Here’s an example:
A 3.46 g sample of limestone(CaCO3 ) was dissolved in 0.1M (HCl) solution like this:
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CaCO3 (s) + 2HCl (aq) => Ca2+ (aq) + 2Cl – (aq) + CO2 (g) + H2O (l)
What is the mass (by percentage) of calcium in the original limestone sample?
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That’s a great question. The theoretical percentage of calcium in pure calcium carbonate (CaCO₃) is about 40%, which comes directly from its molecular formula — calcium (40.1 g/mol) out of the total formula mass of 100.1 g/mol. However, the sample analyzed in the lab was limestone, which is not pure calcium carbonate. Natural limestone often contains other materials such as silica, clay, magnesium carbonate, or moisture. These impurities reduce the overall calcium content.
In this experiment, the gravimetric analysis determined that the sample contained 18.3% calcium by mass. This lower percentage indicates that only part of the sample was actually calcium carbonate, while the rest was made up of other substances or possibly water. It’s also possible that small experimental errors occurred, such as incomplete precipitation, filtration loss, or using hydrated calcium oxalate instead of the anhydrous form. So the 40% represents the calcium content in pure CaCO₃, while 18.3% reflects the actual calcium content measured in your specific limestone sample, which likely contained additional non-carbonate materials. Does that help?
Trying to understand … Why wouldn’t the percentage of Ca in CaCO3 just be calculated based on the molecular formula? Like this:
40.1 + 12 + 3(16) = 100.1
100.1 / 40.1 = 0.4 = 40%
The video arrived at an answer of 18.3% via gravimetric analysis, but looking back at the molecular formula, that doesn’t make sense, unless the original sample was tainted or composited with other substances, right?