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I am given $20 \mathrm {mL}$ of the unknown mixture of Barium Nitrate and Sodium Acetate dissolved in water. I am given the total mass of the two solids before they were put into the mixture. I am given $0.200 \mathrm{M}$ of $\ce{NaOH}$, $0.200 \mathrm{M}$ of $\ce{Na2SO4}$, $0.200 \mathrm{M}$ of $\ce{NaCl}$ and ethanol to separate a precipitate. I am learning about selective precipitation.

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    $\begingroup$ Please include what you have tried. $\endgroup$ – user8016 Oct 6 '14 at 1:27
  • $\begingroup$ I plan to add Sodium Sulfate to the unknown mixture to obtain the precipitate of Barium Sulfate. But I am unsure of what to do after. $\endgroup$ – Kristen Carboni Oct 6 '14 at 1:32
  • $\begingroup$ I have not yet conducted the experiment but I must come up with a laboratory procedure to follow. $\endgroup$ – Kristen Carboni Oct 6 '14 at 1:33
  • $\begingroup$ This is not electrochemistry either but I could not come up with a more suitable tag. $\endgroup$ – Kristen Carboni Oct 6 '14 at 1:35
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    $\begingroup$ It sounds like you have a good idea of what to do already. Once you've separated out the barium sulfate, dried it and know its mass, you've got the mass of the sodium acetate as well, right? (Difference between total and original barium salt should be sodium salt, no?) There aren't many options for precipitating sodium salts. $\endgroup$ – Jason Patterson Oct 6 '14 at 2:23
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It occurred to me that you might have been asking how to complete this separation experimentally rather than conceptually, which is what the comments and previous answer all were pointing toward. If that is the case, then once you've precipitated out the barium sulfate, you have to isolate it from all other substances in order to find its true mass.

The precipitate is a solid, so you could separate it from most of its contaminants by pouring the solution through a pre-weighed filter paper, making sure to wash the walls and bottom of the reaction vessel with distilled water and pouring this wash through the filter as well in order to collect all of the precipitate.

The filtrate would contain the bulk of the sodium acetate and sodium sulfate in the reaction mixture. What remains in the filter paper would be barium sulfate and a small amount of reaction mixture, including the sodium salts. To eliminate the sodium salts, add additional distilled water to the filter to wash the precipitate (and filter paper) clean.

Once you have a filter paper with clean barium sulfate, dry it thoroughly and weigh. I don't believe that barium sulfate forms a hydrate, so the mass of the precipitate should all be barium sulfate.

Note: Barium salts are quite toxic, particularly highly soluble ones like barium nitrate. Be careful using them and dispose of the product appropriately.

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Start with your chemical equations: What's going to react and how; what will precipitate out of what sort of solution.

When get one component fully precipitated (as Jason Patterson points out) you can use the stoichiometry of the exchange reaction to determine the initial mass of one of the components.

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