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CH141Lab3Fall2010

# CH141Lab3Fall2010 - Electrical Conductivity of Aqueous...

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Electrical Conductivity of Aqueous Solutions PRE-LAB ASSIGNMENT : Reading: Chapter 4 (sections 1, 2, and 3) in Brown, LeMay, Bursten & Murphy. 1. Using Table 1 in this handout, determine which solution has a higher conductivity, 0.1 M HCl or 0.1 M NaCl? Explain what creates the higher conductivity. 2. Write the balanced complete ionic equation for the titration of aq. NaOH with aq. HCl. INTRODUCTION: In this lab you will explore the nature of aqueous solutions by investigating the relationship between conductivity and strong and weak electrolytes. To do this, you will add increasing amounts of either acid or base to several electrolyte solutions. After each addition you will measure the conductivity of the solution. From the conductivity data, you will work to deduce the nature of the chemical reaction that occurred in the experiment. This experiment is another "discovery" exercise to get you thinking about how the experimental data you have collected relates to the concepts you are learning in class. You will work in small groups to decide how best to work up the data. Plotting your data will allow you to visually follow the chemical reactions that are occurring and deduce what ions that are present. What is conductivity? Conductivity is a measure of the concentration of ions in solution. By completing the circuit shown in Figure 1, we can measure the conductivity of the solution in the beaker. The conductivity is proportional to the current that flows between the electrodes. For current to flow, ions must be present in solution to carry the charge from one electrode to another. Increasing the number of ions in solution will increase the amount of charge that can be carried between electrodes and will increase the conductivity. The units microSiemens/cm (uS/cm) and milliSiemens/cm (mS/cm) are most commonly used to describe the conductivity of aqueous solutions. 1 Figure 1. Schematic of a simple conductivity measurement system . * The Siemen was formerly called mho (pronounced "mo"), which was derived as a unit of conductivity by reversing the letters in "ohm," the unit of resistance.

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-2- CH 141 Lab Exp #3 Another factor in conductivity measurements is that not all ions carry charge (conduct electricity) equally. H + and OH are unique and move through solution very rapidly and are very good charge carriers. Ions such as NH 4 + and Cl move through solution at a slower rate and therefore do not conduct electricity as well. Uncharged species in solution do not carry any charge. Table 1 is a table of molar conductivity for the ions in this exercise. The molar conductivity is the conductivity of a solution for the ion containing one mole of charge per liter. Note that the molar conductivity of H + ions is 5-7 times the conductivity of other small cations. The molar conductivity of OH -
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CH141Lab3Fall2010 - Electrical Conductivity of Aqueous...

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