A lot of students faces go blank when their teachers tell them their first Year 12 SAC is a Gravimetric Analysis.
“What on earth is that?? I have no idea what that is!! I’m going to fail Chemistry!” I hear you exclaim.I’m here to tell you this:
Relax!! It’s nothing much new. Trust me.Here is the crux of gravimetric analysis which will bring clarity:
(A precipitation reaction is a chemical reaction in which a solid is formed due to its’ insolubility, usually in water).
“Ah!” I hear you exclaim, “It’s nothing much new.”
Not so bad after all, hey? Remember that gravimetric relates to the word gravity, which relates to gravity acting to allow your precipitate to sink to the bottom of your solution.
What’s the point of a gravimetric analysis? Here it is:Imagine you have an unknown amount of a cation or anion dissolved in solution, but you need to know how much is there. A common way to find out how much is there is by gravimetric analysis. Your task is to use another salt (or ionic compound) which will form an insoluble salt containing the unknown cation or anion.
What’s crucial to your ability to get through such a SAC is two important skills which you learned in Year 11.
Let’s use an example here.Say I have a solution containing sulfate (SO42-) ions, but I don’t know what the concentration of those sulfate ions is. What I would do, is to react those sulfate ions with barium ions (Ba2+, usually in the form of barium chloride BaCl2) to form the insoluble salt barium sulfate (BaSO4), according to the below net ionic equation:
Ba2+(aq) + SO42-(aq) → BaSO4 (s)
What’s important is that we add EXCESS Barium ions to ensure that we have reacted all of our sulfate.The barium sulfate solid will appear as a cloudy solution, which, if left on the bench overnight would settle to form a clear solution with a layer of solid barium sulfate sitting at the bottom of the beaker.
If we are to then filter the cloudy solution and separate the solid barium sulfate from the liquid component, dry it and weigh it,
we can use stoichiometry to calculate the amount of sulfate in the original sample.
“What about the reference to dilution calculations skills?” you ask.Sometimes to perform your gravimetric analysis it is necessary to dilute your original sample to a lower concentration so that you don’t use as much of one of the reagents (in this case it would be the barium chloride).
What I recommend to students preparing for a gravimetric analysis SAC, is to do lots of questions before attempting the SAC. You will find some good ones in the relevant section of your textbook, and you can also access exam-style gravimetric analysis at the VCAA website for past chemistry exams. Below I have compiled a list of gravimetric analysis short answer exam questions presented by VCAA to the VCE cohorts of the previous 7 years (the current and previous study design which are relatively similar).
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About the Author
Amelia has been a Chemistry Educator since 2002, and is passionate about making learning simple. She believes that ANYONE can learn ANYTHING they want if they have the drive and interest and are willing to do the hard yards.