Avogadro and all that

Experiment 2: Avogadro and All That

Objective

• To help you become familiar with the layout of the laboratory including safety aids and the equipment that you will be using this year.
• To make an order-of-magnitude estimate of the size of a carbon atom and of the number of atoms in a mole of carbon, based on simple assumptions about the spreading of a thin film of stearic acid on a water surface

Grading

• Lab Report (90%)
• TA points (10%)

Before coming to Lab

Read the following:

• Lab instructions
• Background Information
• Concepts of the experiment

Print out the lab instructions and report form.

Read and sign the equipment responsibility form and the safety rules. Email Ms. Duval, at nduval@rice.edu , to confirm completing this requirement by noon on September 14 ${}^{\text{th}}$ .

Introduction

Since chemistry is an empirical (experimental) quantitative science, most of the experiments you will do involve measurement. Over the two semesters, you will measure many different types of quantities – temperature, pH, absorbance, etc. – but the most common quantity you will measure will be the amount of a substance. The amount may be measured by (1) weight or mass (grams), (2) volume (milliliters or liters), or (3) determining the number of moles. In this experiment we will review the methods of measuring mass and volume and the calculations whereby number of moles are determined.

Experimental Procedure

1. Identification of Apparatus

On the tray (in DBH 214) we have a number of different pieces of common equipment. We will, identify and sketch each - I know this may sound a trivial exercise but it is necessary so that we are all on the same page.

1. beaker
2. Erlenmeyer flask (conical flask)
3. side-arm Erlenmeyer flask
4. graduated (measuring) cylinder
5. pipettes, both types graduated and bulb
6. burette
7. Bunsen burner
8. test tubes
9. watch glass
10. volumetric flask

2. Balance Use

In these general chemistry laboratories, we only use electronic balances – saving you a lot of time. However, it is important that you become adept at using them.

Three aspects of a balance are important:

1. The on/off switch. This is either on the front of the balance or on the back.
2. The "Zero" or "Tare" button. This resets the reading to zero.
3. CLEANLINESS . Before and after using a balance, ensure that the entire assembly is spotless. Dirt on the weighing pan can cause erroneous measurements, and chemicals inside the machine can damage it.

Balance Measurements :

1. Turn the balance on.
2. After the display reads zero, place a piece of weighing paper on the pan.
3. Read and record the mass. (2)
4. With a spatula, weigh approximately 0.2 g of a solid, common salt NaCl. The excess salt is discarded, since returning it may contaminate the rest of the salt.
5. Record the mass (1). To determine how much solid you actually have, simply subtract the mass of the weighing paper (2) from the mass of the weighing paper and solid (1). Record this mass (3). You have just determined the mass of an "unknown amount of solid."
6. Now place another piece of weighing paper on the balance and press the Zero or Tare button then weigh out approximately 0.2 g of the salt (4). Thus, the zero/tare button eliminates the need for subtraction.