Limiting Reagent Stoichiometry - Chemistry Stack Exchange most recent 30 from chemistry.stackexchange.com 2019-08-19T02:13:33Z https://chemistry.stackexchange.com/feeds/question/24656 http://www.creativecommons.org/licenses/by-sa/3.0/rdf https://chemistry.stackexchange.com/q/24656 1 Limiting Reagent Stoichiometry A is for Ambition https://chemistry.stackexchange.com/users/13213 2015-02-05T14:21:20Z 2015-02-05T19:52:09Z <p>What mass of precipitate forms when a solution containing 6.24 g of potassium sulfide is reacted with a solution containing 19.2 g of barium nitrate?</p> <p>I have already identified the limiting reagent $\left(\text{K}_2 \text{S}\right)$ as well as the mass of the precipitate.</p> <p>My question, however, is: why is the Barium Sulfide formed in the product a solid and not aqueous? I thought the problem translated into the molecular equation: $$\text{K}_2 \text{S}_{\text{(aq)}} + {\text{Ba}(\text{NO}_3)_2}_{\text{(aq)}} \ce {-&gt;} 2{\text{KNO}_3}_{\text{(aq)}} + \text{BaS}_{\text{(aq)}}$$</p> <p>But according to the answer key I was given, $\text{BaS}$ is a solid precipitate, not aqueous. How can that compound be a solid if, according to solubility rules, all sulfides plus an alkali earth metal are soluble? Shouldn't it be aqueous, not solid? </p> <p>Please help, thanks :)</p> https://chemistry.stackexchange.com/questions/24656/-/24667#24667 0 Answer by DavePhD for Limiting Reagent Stoichiometry DavePhD https://chemistry.stackexchange.com/users/5160 2015-02-05T16:59:17Z 2015-02-05T17:16:05Z <p>Generally speaking, if the problem doesn't give concentration or enough information to find concentration, you can't know whether all or only a portion of the product will dissolve. You need to know the solubility product constant (Ksp), the final volume, and the amount of each ion to calculate the amount that will dissolve.</p> <p>However, this particular question has a complicating issue because: </p> <p>$\ce{2BaS2 + 2H2O -&gt; Ba(OH)2 + Ba(SH)2}$</p> https://chemistry.stackexchange.com/questions/24656/-/24677#24677 1 Answer by wes3449 for Limiting Reagent Stoichiometry wes3449 https://chemistry.stackexchange.com/users/13030 2015-02-05T19:52:09Z 2015-02-05T19:52:09Z <p>So basically this problem is bad and incomplete (they tend to be at this level) in that it doesn't give you the concentrations/volumes of the reagents/solutions, so we end up making several assumptions. First of all, <strong>precipitates cannot be aqueous</strong>, by definition they are solids. So the way you end up solving this question is by assuming one of the products precipitates out completely, and it is the least soluble one that precipitates out (barium sulfide in this case). Unfortunately, this isn't exactly accurate.</p> <p>In a real world scenario, given enough water you could have no precipitate at all, but most likely you would have a mix (not all of the barium sulfide would precipitate out of solution). At higher levels of chemistry (AP/IB -> post-secondary) you'll learn that none of these reactions (including the solubility ones) ever go to completion (some just go so far that you can assume they go to completion), and what you end up doing is using solubility constants to find out how much of each product is dissolved and/or precipitated. If you're interested in more info google "solubility constants" and "common ion effect".</p>