Do you have your own bathroom? Then you might be familiar with soap scum, which is the result of a combination of having hard water and soap.
Do you have your own bathroom? Then you might be familiar with soap scum, which is the result of a combination of having hard water and soap (0:44). To determine how hard water is, we can use gravimetric analysis, which is an experimental procedure used to determine the amount of a substance, for example an ion, by precipitating it from an aqueous solution (1:25). Basing off of the solubility rules (1:45), the episode describes the steps to precipitate the calcium carbonate with sodium carbonate (3:05) and briefly discusses how to calculate the water hardness (5:08) and calcium ion concentration (5:38). Experimental errors described in the episode discuss how the yield can be higher or lower than expected (6:01).
Question: Why does the sodium carbonate have to be in excess?
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Hi and welcome to the APsolute Recap: Chemistry Edition. Today’s episode will recap Gravimetric Analysis.
Lets Zoom out:
Unit 1 - Atomic Structure and Properties
Topic 1.3 Elemental Composition of Pure Substances
Big idea - Scale, Proportion and Quantity
Are you one of the lucky teenagers that have their own bathroom? Is cleaning your bathroom or a shared bathroom one of your chores? Then you feel my pain: soap scum! Ugh. Soap scum is the result of a combination of having hard water and soap. It is the precipitate of calcium 2+ and magnesium 2+ ions from the hard water with an anion of fatty acids, the stearate, of soap. And it is really hard to clean! Therefore, it can be helpful to know just how hard your water is. An experimental approach to determine this is gravimetric analysis. So let’s take a closer look!
Lets Zoom in:
Gravimetric analysis is an experimental procedure which is used to determine the amount of a substance, such as an ion, by precipitating it from an aqueous solution. The precipitate can then be dried and measured. Then the original mass or concentration can be determined using stoichiometry,
The foundation for gravimetric analysis are precipitation reactions. Quick recap of solubility rules: NAAA - Nitrates, alkali metals, acetate and ammonium salts are always soluble. That’s what you need to know for AP Chemistry. But, that doesn’t help with our soap scum. So let’s add to it and focus specifically on common polyatomic ions. Most sulfate compounds are soluble, most hydroxides are only slightly soluble. Carbonates and phosphates are mostly insoluble. Why do I keep saying “mostly”? because there are always exceptions to the rule. Chemistry - the science of exceptions. Even though there are different ions that contribute to the hardness of water, namely Ca2+, Mg2+ and Fe2+, the Ca2+ is usually the star of the show and for our purposes we assume that it is the only ion present in the hard water sample. To determine the concentration of Ca2+ ions in hard water we can precipitate it as calcium carbonate by adding an aqueous solution of sodium carbonate. As mentioned before, all alkali compounds are soluble in water, therefore, we will have our calcium carbonate precipitating.
Let’s talk about the steps for this gravimetric analysis: Step 1) We prepare a sodium carbonate solution of known concentration, for example 1.0 Molar. Step 2) Add about 12 mL of sodium carbonate to a 50 mL sample of hard water and observe the formation of the precipitate. Step 3) Filter your precipitate. This step depends a bit on the equipment you have available and can be done with traditional filter paper and funnel or with a vacuum filtration system. It is important here to weigh your filter paper beforehand and to get all the precipitate out of the beaker. Use DI water to rinse the precipitate into the filter paper. A rubber spatula can also help. Wash your precipitate with some DI water to remove spectator ions. Step 4) Remove your filter paper, put it on a watch glass, which mass you’ve recorded, and dry it. This can be done overnight or in a drying oven. Step 5) Once the precipitate is dry, you can remove it, wait for it to cool and then weigh the precipitate, watch glass and filter paper.
Time for analysis - you can now determine the hardness of water, which is usually reported in mg/L using dimensional analysis: divide the grams of calcium carbonate obtained by the volume of the sample in liters, in your example 0.05L, and convert your grams to milligrams. Voila! Values over 120 mg/L are considered hard or very hard water. And that means, you have to scrub harder to clean your bathtub!
You can also calculate the concentration of Calcium ions on the original sample using stoichiometry: convert your mass of calcium carbonate to moles of calcium carbonate. The 1:1 relationship between the calcium cations and the carbonate gives you the moles of calcium. Dividing it by the volume in L will give you the concentration of calcium in the hard water sample.
To wrap up this recap, let’s talk about errors and how they affect your data. What if your sample wasn’t completely dry? How would this change the calculated water hardness or molarity? If the sample isn’t dry, we would record a higher mass. This would result in a higher calculated hardness. It would also result in a greater number of moles/liter of calcium ions. What could be explanations for a lower yield than expected? There is always a chance that some of the precipitate gets lost during transfer: stuck in the beaker, even though you rinse it, stuck on the sides of the funnel, some remaining on the rubber spatula. What if your yield is larger than expected? We’ve already discussed that the precipitate might not have been completely dry. Another error could be that the precipitate wasn’t washed thoroughly enough before drying and that spectator ions like sodium will be left on the filter paper and add to the mass. To prevent these, it is important to follow all procedural steps and make sure to rinse as much precipitate from equipment onto the filter paper.
To recap…
Gravimetric Analysis is an experimental procedure to determine the amount of a substance by precipitating it from an aqueous solution. Applying solubility rules, ions can be selectively precipitated and using stoichiometry, the hardness of water and the concentration of ions can be calculated. In the lab, two aqueous solutions are mixed and a precipitate forms. The precipitate is filtered, washed, dried and weighted. If the precipitate is not completely dry, the calculated values would be too high. Other errors that affect yield include not washing the precipitate or losing precipitate during transfer between equipment.
Coming up next on the Apsolute RecAP Chemistry Edition: Lewis Diagrams and Formal Charges.
Today’s Question of the day is about gravimetric analysis.
Question: Why does the sodium carbonate have to be in excess?