Precipitation is expected from which mixture? QUESTION: Precipitation is expected from which mixture? A. Na2CO3(aq) + KNO3(aq) B. AgNO3(aq) + CaCl2(aq) C. Neither D. Both Solubility rules (for common ionic compounds): Compounds of the following are always soluble: sodium, potassium, ammonium, nitrate, nitrite, perchlorate, acetate Compounds of the following are generally soluble: chloride, bromide, iodide --- except with silver, lead(II), mercury(I), and copper(I) Sulfate -- except with lead(II), barium, strontium, calcium, and mercury(I) Fluoride --- except with lead(II) and group II(A) The rest are generally insoluble. Exceptions: sulfides of group II(A) are soluble hydroxides of calcium, strontium and barium are moderately soluble. For which of the following mixtures of aqueous solutions do we expect a precipitation reaction to occur? A… sodium carbonate and potassium nitrate B…. Silver nitrate and calcium chloride C…. Neither Or D… both PAUSE CLICK A preciptate is a compound that is insoluble in water. Therefore, to answer this question, we just need to look for a pair of ions that will form a compound that is insoluble in water. We use the solubility rules for common ionic compounds…. CLICK CLICK CLICK Let’s start with choice A…. Sodium carbonate gives us sodium and carbonate ions. CALLOUT “Na+ and CO32-” pointing to Na2CO3 Potassium nitrate gives us potassium and nitrate ions CALLOUT “K+ and NO3-” pointing to KNO3 According to our solubility rules… any compound containing sodium ions are soluble in water. HIGHLIGHT sodium Therefore, if we were to get a precipitate from the mixture in choice A, the precipitate is not going to have any sodium ions CROSS OUT Na in Na2CO3 The precipitate is not going to have any potassium ions. HIGHLIGHT potassium CROSS OUT K in KNO3 The precipitate is not going to have any nitrate ions. HIGHLIGHT nitrate CROSS OUT NO3 in KNO3 Of the four kinds ions in the mixture, three are unable to form an insoluble compound…. Sodium, potassium, and nitrate There is nothing in the mixture that can combine with carbonate to form an insoluble compound. Therefore, choice A is incorrect. We do not expect a precipitate from this mixture. Obviously, choice D is also incorrect since A is incorrect. Let’s examine choice B. Here’s we have a mixture of silver and nitrate ions from silver nitrate CALLOUT “Ag+ and NO3-” pointing to AgNO3 As well as calcium and chloride ions from calcium chloride. CALLOUT “Ca2+ and Cl-” pointing to CaCl2. Let’s examine which of these ions can give us a precipitate. Rule 1 tells us that all nitrates are soluble. So, we can’t expect a precipitate from nitrate ions. HIGHLIGHT “nitrate” in rule 1 CROSS OUT NO3 in AgNO3 Rule #2 tells us that silver chloride is insoluble in water. HIGHLIGHT “chloride” in “chloride, bromide, …” HIGHLIGHT “except with silver” Therefore, we expect a precipitate of silver chloride to form when silver nitrate and calcium chloride are mixed. SHOW TEXTBOX “Ag+(aq) + Cl-(aq)  AgCl(s) “ The silver ions from silver nitrate will combine with chloride ions from calcium chloride to form a precipitate. The correct answer is B. PAUSE END RECORDING

Video ID: 5-7-1 © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Narrator: Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08