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“HOW DO I LOVE THEE LET ME COUNT THE WAYS...” DETERMINATION OF AVOGADRO’S CONSTANT

Techniques

Calibration drop counting

MSDS available for

• • stearic acid, CH3(CH2)16COOH cyclohexane, C6H12

Principles

molar volume molecular structures surface areas and volumes Avogadro’s constant percent error

Recommended Advanced Reading

Chapter 3 in Petrucci, Herring, Madura, & Bissonnette’s General Chemistry,10th Ed.

Avogadro Constant...1

INTRODUCTION

The beginning When you begin this experiment, you should have a lab work area, a TA (or demonstrator) and a partner. In today's session you will work with your partner to: Calibrate a dropper and determine the number of drops of a stearic acid in cyclohexane solution required to create a monolayer of the solution on a water surface Calculate the mass of stearic acid required to form the monolayer, use it to estimate the thickness of the monolayer (which is related to the length of the stearic acid molecule), and then use the number of carbon atoms in stearic acid to approximate the diameter and then the volume of a carbon atom calculate a value for the Avogadro constant compare your calculated value for the Avogadro constant with a known value and determine the percent error in your value

This is a general overview of what you will be accomplishing in this experiment.

EXPERIMENT 1:

Introduction

Determination of the Avogadro Constant

“In a diamond shaped like a cube of 15 mm per side, there

are between 500 and 1000 times more carbon atoms than there are stars in the entire visible Universe!”

2....Experiment 1

The Avogadro constant is one of the most important constants used in chemistry. It allows chemists to relate a directly measurable quantity, such as the mass of a substance in grams, to a quantity that cannot be measured directly, such as the number of atoms in the mass of a substance. For instance, the Avogadro constant could be used to determine how many atoms are present in 1 g of carbon. Since atoms are very small compared to the things we normally measure, the units we normally use to measure quantities are very awkward to use. We would have a hard time determining the mass of a dozen atoms, since their collective mass is still too small to measure, even on an analytical balance. For this reason, a unit called the mole is very important. It is a counting unit. Just as we can say that one dozen indicates that we have 12 of a particular item, one mole indicates that we have 6.023 x 1023 of that particular item. While a mole is not very convenient for measuring doughnuts (it would be a HUGE pile!), it is very convenient for measuring small things like atoms. The Avogadro constant has a value of 6.022 x 1023 and is given the symbol NA. It is defined as the number of atoms contained in exactly 12 g of the 126C isotope. Since atomic weights are based on this carbon isotope, and since the Avogadro constant is also based on this carbon isotope, the mass of one mole of any ion or molecule is equal to the sum of the atomic weights of all the atoms in the ion or molecule. The three equations written below are equivalent. The first gives the stoichiometry in terms of moles, the second in terms of mass, and the third in terms of molecules. This example illustrates how we can relate measurable or macroscopic quantities, such as mass, to the stoichiometry which occurs at a molecular level. 2 moles H2 + 1 mole O2 2 moles H2O [1]

2 (2 x 1.0080) g H2 + (2 x 15.999) g O2 2 (18.015) g H2O [2] Avogadro Constant...3

2 (6.022x1023) molecules H2 + 6.022x1023 molecules O2 2 (6.022x1023 molecules) H2O [3] While many sophisticated experimental methods, including electrochemical and x-ray diffraction methods, have been used to determine the Avogadro constant quite accurately, the simple technique you will be using in this laboratory experiment demonstrates that a reasonable value of the Avogadro...