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In a precipitation reaction, how do I know which soluble reactant I should put in excess? E.G. $$\ce{Na2SO4 +Pb(NO3)2 -> PbSO4 + 2NaNO3}$$ In my book, $\ce{Na2SO4}$ is added in excess, but can we add $\ce{Pb(NO3)2}$ in excess?

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Sometimes the excess is the cheap/unlimited reagent you use to get a high yield on your expensive/limited reagent, and sometimes you use the one that you don't mind/can easily extract from the products.

In this case, if you wanted to collect and record the lead(II) sulphate as a precipitate, using an excess of the lead(II) nitrate would be a waste. If your intention was to produce a large quantity of sodium nitrate solution, and you don't mind having a load of the lead nitrate contaminating it, then you could use an excess of the lead nitrate.

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  • $\begingroup$ How will the lead(II)nitrate be a waste? $\endgroup$ – Kazi Mar 5 '16 at 15:16
  • $\begingroup$ In that instance it would be wasted because your intention is to convert lead nitrate into lead sulphate. If you have excess lead nitrate then your % yield would be low and it will not tell you how much lead was present in the solution which may be relevant, even though you produced the maximum amount of sodium nitrate. $\endgroup$ – Spontification Mar 5 '16 at 15:25

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