Experiment 6 - Solutions

Objectives

1. To determine the concentration of a saturated solution of NaCl, sodium chloride, in moles per liter.
2. To compare the relative solubility of two solutes in two solvents.
3. To study double replacement reactions, specifically, the effect of adding solutions containing selected negative ions (anions) to solutions containing positive ions (cations) of metals in group II (Mg, Ca, Sr, and Ba).
4. To observe the energy changes, which occur when a substance dissolves in water.

Introduction

The solubility of a solute is the maximum amount of solute, which can be dissolved in a given quantity of solvent or solution. The solubility of a substance is often expressed in moles of solute per liter of solution. Substances are considered to have a high solubility if more than 0.1 mole can dissolve in one liter of solution.

A saturated solution is a solution, which contains the maximum amount of solute that can dissolve at a given temperature. In general, solubility increases as temperature increases. The amount of solute that is dissolved in a solution is expressed as concentration. The unit of concentration is referred to as molarity, M. We can express molarity in an equation form

The amount of solute, which can dissolve in a given amount of solvent depends on the nature of the solute and solvent. In general, "Like dissolves like", describes the general principle of solubility (i.e. polar solvents dissolve polar solutes and non-polar solvents dissolve non-polar solutes). Thus, mineral acids, bases and salts, which are polar, tend to be much more soluble in water, which is polar, than in solvents such as ether or hexane, which are essentially non-polar. Conversely, non-polar substances tend to be only slightly soluble in water, but very soluble in non-polar solvents.

Liquids that dissolve in one another are said to be miscible. Again, the general principle "Like dissolves like" dictates whether two liquids are miscible. If both liquids are polar then the liquids are miscible. If the two liquids are nonpolar then the liquids are miscible.

However, a polar liquid and a nonpolar liquid are immiscible. They repel one another and separate into two layers. Two liquids will be studied in this lab, water and hexane. Water is a polar liquid. Hexane is a non-polar liquid. The specific gravity of water is 1.0. The specific gravity of hexane is 0.66. Which layer do you think is on top?

A matrix of solutions will be studied in this lab. We will be mixing solutions together and observing whether reactions occur. The type of reaction we will be studying is known as Double Replacement Reactions or Metathesis Reactions. When two solutions are mixed, the ions in the mixture may combine to form a compound, which has a low solubility in water. In such a case, a precipitate will appear.

The dissolving process may be endothermic (with a net absorption of energy) or exothermic (with a net release of energy).