Question

hello, i'm writing a lab report on gravimetric sulfate experiment. Ba2+ + SO4 2- -> BaSO4. disolved in HCL. Can someone please kindly help twith the experiment introduction and background by answerig these questions?? it's instruction from my instructor. I greatly appreciate.

" Explain the theory of gravimetric analysis and ionic precipitation, as it applies to this sulfate experiment. Be sure to include the reaction equation, a mention of Ksp and solubility of the precipitate, excess precipitant & Le Chatalier's principlem crytal growth vs nucleation, digestion, coprecipitation (adsorption, inclusions, occlusions), quantitative wash/rinse/transfer, peptization and why you are able to use hot water to rinse instead of an ionic solution, filtration, decantation, ignition to constant mass, effect of ahless vs. regular filter paper, and how the calculation for weight percen analyte is made."

Thank you very much!!!!

Answer 1

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Gravimetric analysis: By definition, gravimetric analyses are all analytical methods in which the change in mass is the most important measurement. You need an accurate and well calibrated balance to make these types of analyses. Historically mass was one of the first quantities that grew in importance as the analytical techniques develope. In the case of your reaction you are employing the precipitation method of gravimetric analysis, where you start with a solution that contains the ion that you are about to determine and a solution of a reagent that precipitates that ion. For Barium ion the reaction with sulfate forms an insoluble salt that can be collected, washed and dried before you weigh it.

Ksp is the solubility product constant. It derives from the equilibrium constant, but in this case the equilibrium is about the amount of substance that dissolves in water. Take for example your equation:

You start with a solution where barium ions are dissolved and add a solution with sulfate ions. Both react and barium sulfate precipitates. Solubility product constant for this reaction will be:

The Ksp represents the level at which a solute dissolves in aqueous solution. The higher the Ksp, the more soluble the substance is, therefore it is very convenient that the Ksp is small. The smaller the Ksp, the more accurate the result because you can precipitate all off the ion that you want to quantify. Ksp for barium sulfate is 1.08 x 10^-10 which is a very small number, so gravimetric analyses for barium are accurate. How can we make it even better? Using Le Chatelier’s principle. From a textbook you can find it defined like this: “If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change”. But, what does that mean? It means that if, after a system or reaction has reached the equilibrium and you introduce a change, there will be a change in the system that tries to restore the original condition. In this case if you have the reaction at equilibrium an you add one of the ions, for example you will be raising the concentration of it over the equilibrium value, so the system tends to shift to the side of the precipitate to lower the concentration of the ions until it reaches the equilibrium again. How can we use this in our advantage? Because if we add more sulfate ion, the reaction will tend to form more precipitate to reach equilibrium, and as a consequence more barium will precipitate and we will be very sure that we catch it all.

To ensure the precipitation you can help it by scratching the walls of the beaker. This event starts a nucleation stage, where a small nucleus containing the newly forming crystal is created. After this first nucleus is created, the rest of the solid grows rapidly in the crystal growth stage. The solution becomes cloudy at sight and you can see the solid precipitate. Frequently, when a precipitate forms very fast, the size of the individual crystals are very small which can lead to some losses because it does not get retained by the filter paper. To correct this, digestion is carried out by heating the precipitate with the mother liquor for some time.

After we allow the precipitate to settle down it can become contaminated with other ions from the solution around. The process by which other ions precipitate with the one of interest is called coprecipitation. Adsorption, inclusions and occlusions are ways of other ions to co-precipitate. In the experiment you are carrying out this co-precipitate is more soluble so it can be washed off.

Quantitative procedures are those that you perform while ensuring all off the material has been thoroughly washed or transferred from one container to the other.

For the question: “why you are able to use hot water to rinse instead of an ionic solution” You can use hot water because Barium sulfate has very little solubility in water (remember Ksp) and it is better instead of ionic solution because we do not want to contaminate the solid with other ions.

The process of filtration has to be done with a quantitative filter paper. These papers have small porous to retain the higher amount of crystals. You transfer the entire product including the paper to a crucible. Then you have to ignite it in a hot furnace to constant weight. During this process the filter paper burns out so it should be very interesting that it has the ash less quality. Organic material burns cleanly to CO2 and Water, but when some minerals are present you can end up with carbonates (other salts) and those can contaminate your product. Ash less quality paper reduces this problem to a minimum.

Calculation:

With the reaction you can see that 1 mol of barium sulfate comes from one mol of barium ions. So after you have the weight of barium sulfate you divide this mass between the molecular weight and the number of mol will be the number of mol of the barium. If you know what barium salt is present it would be very easy to convert this number of mol to the weight of the salt present in the original sample. You can express that as % or concentration depending if it is a solid or a liquid

Ksp is the solubility product constant. It derives from the equilibrium constant, but in this case the equilibrium is about the amount of substance that dissolves in water. Take for example your equation: