Which of the following is soluble in water? QUESTION: Which of the following is soluble in water? A. MgCO3, B. MgF2, C. Mg(OH)2, D. MgSO4 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. Which of the following is soluble in water? Magnesium carbonate… Magnesium fluoride… Magnesium hydroxide… Or Magnesium sulfate PAUSE CLICK To answer this question, use the solubility rules for common ionic compounds…. You’re likely to see different versions of these rules in different textbooks. They are all pretty much equivalent although some textbooks include more ions than others. In general, the rules will tell you CLICK which compounds are always soluble… CLICK which compounds are generally soluble and what the exceptions are…. And that pretty much anything else is insoluble and what the exceptions are. Let’s apply the rules to each of the choices. Let’s start with choice A…. Magnesium carbonate CALLOUT “magnesium carbonate” pointing to MgCO3 Rule 1 does not apply. It doesn’t say anything about magnesium or carbonate ions. The second rule does not apply either. So, we move on to the third rule. The exceptions to group IIA doesn’t deal with carbonates. Therefore, we conclude that magnesium carbonate is insoluble in water. CROSS OUT Choice A Choice A is incorrect. How about choice B…. Magnesium fluoride…. CALLOUT “magnesium fluoride” pointing to MgF2 Again rule 1 does not apply. Rule 2 applies in this case. Rule 2 tells us that fluorides are generally soluble. HIHLIGHT “Fluoride” However, magnesium fluoride is an exception HIGHLIGHT “except with” and “group II(A)” If we refer to a periodic table, we will find that magnesium belongs to group IIA. Flourides of group IIA are insoluble. Therefore, magnesium fluoride is insoluble in water. Choice B is incorrect CROSS OUT Choice B Let’s examine choice C…. Magnesium hydroxide CALLOUT “magnesium hydroxide” pointing to Mg(OH)2 Again, the first rule does not apply. It doesn’t say anything about magnesium or hydroxide . The second rule doesn’t apply either. So, we go on to the last rule. The compound is soluble unless it is one of the exceptions listed. It’s not an exception…. It’s is not a sulfide… HIGHLIGHT “sulfides of …’ It is not a hydroxide, but the cation is not calcium , strontium, or barium HIGHLIGHT “calcium, strontium, and barium” Therefore, we conclude that magnesium hydroxide is insoluble in water. Choice C is also incorrect. CROSS OUT Choice C. Therefore, choice D must be correct. Let’s verify. The compound in choice D is magnesium sulfate… CALLOUT “magnesium sulfate” pointing to MgSO4 Rule 1 does not apply. It doesn’t say anything about magnesium or sulfate. Rule 2 applies. HIGHLIGHT “Sulfate --- except with ….“ Sulfates are generally soluble … unless in combination with lead(II), strontium, calcium, or mercury(I). Magnesium sulfate is not one of the exceptions. Therefore, magnesium sulfate is soluble. PAUSE END RECORDING

Video ID: 5-5-3 © 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