Which of the following solutions would you expect to have the lowest vapor pressure? Assume all solutions are at the same temperature. (A) 0.2-molar solution of urea, (B) 0.2-molar solution of MgSO4, (C) 0.2-molar solution of KCl, (D) 0.2-molar solution of AlCl3, (E) 0.2-molar solution of LiBr.
In this question, we are given five different solutions that are at the same temperature and have the same molar concentration, which is 0.2 molar. Molar concentration expresses the concentration of a solution as the amount of moles of solute particles per liter of solution. Each solution contains a different solute dissolved in water. Solutes that are ionic compounds disassociate into at least two ions when they are dissolved in a solution. For example, when the ionic compound sodium chloride dissolves in water, each sodium chloride unit dissociates into two particles, a chloride ion and a sodium ion. The solutes in answer choices (B) through (E) are ionic compounds because like sodium chloride, they are composed of a positively charged metal cation and a negatively charged nonmetal or polyatomic anion.
Solutes that are covalent compounds excluding acids do not disassociate into more than one particle when dissolving in water. When sucrose, which is composed of three different types of nonmetals, dissolves in water, the sucrose molecules do not dissociate into more than one particle. They remain as intact molecules in the solution. The solute in answer choice (A) is urea. The molecular formula of urea is CH4N2O. We can see that a molecule of urea is composed of only nonmetal atoms, so these atoms will be connected by covalent bonds. Urea is therefore a covalent compound, and we would not expect urea to dissociate into ions when dissolved in water.
It’s important to know that the solutes in all of the answer choices are also nonvolatile, which simply means that once dissolved, these solutes do not evaporate to form a vapor.
First of all, vapor pressure is the equilibrium pressure exerted by a vapor on the surface of the liquid phase in a closed container. The vapor above the liquid is formed by the process of evaporation. When a solute is dissolved into a solvent such as water, the solute particles interfere with the evaporation of solvent particles. This results in less vapor being formed above the solution, which means that the vapor pressure decreases. The more solute particles there are dissolved in a solution, the lower the vapor pressure. Properties of a solution, such as vapor pressure, that are affected by the number of solute particles are called colligative properties.
To determine which of the solutions will have the lowest vapor pressure, we’ll need to figure out which of the solutes produces the most particles when dissolved in the solution. Let’s begin with answer choice (A). We know urea is a covalent compound and will not dissociate into ions when it dissolves in water to form a solution. Therefore, the solution will contain 0.2 moles of urea molecules per liter. The solute in answer choice (B) is the ionic compound magnesium sulfate. Let’s write a chemical equation to represent magnesium sulfate dissolving in water.
When magnesium sulfate dissolves in water, each magnesium sulfate unit dissociates into two ions, the magnesium ion and the sulfate ion. If all the magnesium sulfate units in one liter of solution have dissociated, then there will be 0.2 moles of magnesium ions and 0.2 moles of sulfate ions. Therefore, there will be a total of 0.4 moles of ions dissolved in the solution. Let’s continue to write equations to represent the dissolving of the remaining solutes in water.
As for answer choice (C), when the ionic compound potassium chloride dissolves in water, each unit dissociates into two ions, the potassium ion and the chloride ion. If all of the potassium chloride units disassociate in one liter of the solution, then there will be a total of 0.4 moles of dissolved ions in the solution. As for answer choice (D), when the ionic compound aluminum chloride dissolves in water, each unit dissociates into four total ions, one aluminum ion and three chloride ions. This means that in one liter of solution, there will be 0.2 moles of aluminum ions and three times 0.2 moles of chloride ions. Therefore, the aluminum chloride solution will contain 0.8 moles of dissolved ions per liter.
Finally, in answer choice (E), when the ionic compound lithium bromide dissolves in water, each unit dissociates into two ions, the lithium ion and the bromide ion. If all the lithium bromide units in one liter of solution disassociate, then there will be 0.2 moles of lithium ions and 0.2 moles of bromide ions. Therefore, the lithium bromide solution will contain 0.4 moles of dissolved ions per liter. Because aluminum chloride dissociates to form the greatest number of dissolved solute particles, the aluminum chloride solution will have the lowest vapor pressure.
“Which of the following solutions would you expect to have the lowest vapor pressure?” A 0.2-molar solution of AlCl3, or answer choice (D).