Video: Identifying the Substance That Completely Dissociates in Aqueous Solution in a Set of Chemical Formulas

Which of the following dissociates completely in aqueous solution? [A] MgCl₂ [B] N₂ [C] C₁₂H₂₂O₁₁ [D] H₂O [E] P₂O₅

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Video Transcript

Which of the following dissociates completely in aqueous solution? (A) MgCl2, (B) N2, (C) C12H22O11, (D) H2O, or (E) P2O5.

Before we discuss this question, let’s briefly look at the names of the compounds given. MgCl2 is magnesium chloride. And it is an ionic compound because it is composed of the ions Mg2+ as well as two Cl− ions or chloride ions. N2 is nitrogen. And this is a covalent compound where electrons are shared between the two nitrogen atoms. There are many compounds with the molecular formula C12H22O11, so for now we will assume that the question is talking about sucrose or table sugar. And this is a covalent compound or a molecular compound. H2O, we know, is water. And this is also molecular or covalent. And lastly, P2O5 is diphosphorus pentoxide, a covalent compound.

Let’s now recap what we know about dissociation. Dissociation, very simply, could be defined as the breaking or splitting apart of a compound into smaller parts. This is a reversible process. And it is not a reaction or does not involve a reaction. Dissociation is common in ionic compounds, where ions already exist. When a solid ionic solute is placed in an aqueous solution, which is where water is the solvent, the positive and negative ions separate or dissociate from each other and dissolve in solution.

Complete dissociation is when all the positive and negative ions totally separate from each other. Magnesium chloride undergoes complete dissociation in aqueous solution. All the solid magnesium chloride particles dissociate completely to give magnesium two plus ions and chloride minus ions. The reason why ionic compounds dissolve so well or dissociate so completely in water is because like dissolves like.

Polar solvents such as water have 𝛿+ and 𝛿− charges, which attract the positive and negative charges on the ionic compound, pulling them out of the lattice. All the covalent compounds listed do not consist of ions and so cannot dissociate in aqueous solution. Finally, which of the following dissociates completely in aqueous solution? The answer is MgCl2.

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