Chemistry Department, Florida State University

Determining the Concentration of an Ionic Solution

Experiment 6 The Conductivity of Ionic Solutions

Introduction/Background

The basis for conductivity properties of solution was first identified by Svante Arrhenius (1859-1927), who thought that the conductivity of solutions arose from the presence of ions. He also postulated that the extent to which a solution can conduct an electric current depends directly on the number of ions present. Some species, such as sodium chloride (NaCl), readily produce ions in aqueous solutions and are thus strong electrolytes (they readily conduct current). Other substances, such as acetic acid, produce relatively few ions when dissolved in water and are weak electrolytes (barely conducts current). A third class of species, such as sugars, forms virtually no ions when dissolved in water and are nonelectrolytes (does not conduct a current).

Electrolytes are in our blood and other bodily fluids that carry an electric charge. It is extremely important for the balance of electrolytes to be maintained, because they affect the amount of water in your body, blood pH levels, and muscle action, as well as many other things. When you sweat, these electrolytes are lost, and must be replenished by drinking fluids containing electrolytes.

Key Concepts

If an ionic compound is dissolved in water, it dissociates into ions and the resulting solution will conduct electricity. Dissolving sodium chloride in water releases ions according to the equation:

NaCl_(s) → Na⁺_(aq) + Cl^-_(aq)

Note: Adding more solvent to a solution does not change the amount of solute in the solution: just the solution concentration. In this experiment we will study the effect of decreasing the concentration of an ionic compound on conductivity. Conductivity will be measured as concentration of the solution is gradually increased. So now we want to know how to calculate concentration in order to figure out the correct dilution amounts. The process of preparing more dilute solutions by adding solvent to a more concentrated one is called dilution. Let's let M_conc and M_dil represent the concentrations and V_conc and V_dil represent the volumes of a concentrated and dilute solution, respectively. If we multiply M x V the product will be the number of moles of solute in a solution. Remember that the amount of solute does not change when diluting a solution. Therefore we can write the equation: M_conc x V_conc = M_dil x V_dil

Example 1: How many milliliters of a 2.00 M NaCl stock solution are needed to prepare 250 mL of 0.400 M NaCl?

Answer: M_conc = 2.00 M NaCl; V_conc = ??; M_dil = 0.400 M NaCl; V_dil = 250 mL
(2.00 M NaCl) x V_conc = (0.400 M NaCl) x (250 mL)
(2.00 M NaCl) x V_conc = 100 mmoles of NaCl
V_conc = (100 m moles of NaCl)/ (2.00 M NaCl) = 50 mL of 2.00 M NaCl stock solution are needed to prepare 250 mL of 0.400 M NaCl.

Example 2:How many liters of a 10.15 M NaOH stock solution are needed to prepare 15.0 L of 0.315 M NaOH?

Answer: click to reveal

Glossary

Conductivity = the quality or power of conducting or transmitting.

Electrolyte = a substance that when dissolved in a suitable solvent or when fused becomes an ionic conductor.

Non-Electrolyte = a substance that does not readily ionize when dissolved or melted and is a poor conductor of electricity.

Solvent = a liquid substance capable of dissolving other substances; "the solvent does not change its state in forming a solution"

Solute = a dissolved substance.

Dilute = reduce in strength or concentration or quality or purity

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