GA: Percentage purity Q

[liamh96]

Hi all,

I am having difficulty wrapping my head around how to solve gravimetric analysis questions;
Would someone be able to give me a push in the right direction on how to answer the following question:

Q: The calcium content of a sample can be determined gravimetrically by first precipitating the calcium ions present as calcium oxalate.
When this precipitate is heated, it decomposes completely to produce calcium oxide.
In testing the purity of a calcium chloride sample, a chemist used GA and obtained 8.81g of CaO from 18.0g of CaCl2.
Calculate the percentage purity of calcium chloride:

Thanks in advance.

[lzxnl]

Hi all,

I am having difficulty wrapping my head around how to solve gravimetric analysis questions;
Would someone be able to give me a push in the right direction on how to answer the following question:

Q: The calcium content of a sample can be determined gravimetrically by first precipitating the calcium ions present as calcium oxalate.
When this precipitate is heated, it decomposes completely to produce calcium oxide.
In testing the purity of a calcium chloride sample, a chemist used GA and obtained 8.81g of CaO from 18.0g of CaCl2.
Calculate the percentage purity of calcium chloride:

Thanks in advance.

The assumption is that all calcium ions in calcium chloride precipitate as calcium oxalate and then in calcium oxide. Note that the calcium ions don't change; what changes is the counterion/the anion, and perhaps the crystal structure (which is irrelevant here)
So, you have 8.81 g of CaO. You can work out the number of moles of calcium oxide, and thus work out the number of moles of calcium ions. This figure is equal to the number of moles of calcium chloride, so you have the number of moles of actual calcium chloride. Work out the mass from this number, and you'll have the mass of calcium chloride in your 18 g. I trust the last step is self-evident.