General Chemistry I

Laboratory Experiment

At Home Laboratory Techniques and Measurements

Objective

· In this experiment, proper usage of some common laboratory equipment will be learned to make measurements. Accuracy and precision in measurement and their relationship to significant figures will be investigated.

Introduction

Chemistry is an experimental science that relies on measurements made in the laboratory. Both precision and accuracy are important in measurement. Accuracy represents how close the measurement, or an average of repeated measurements, is to the true or accepted value. Accuracy is dependent not only on the instrument used but on the skill of the person taking the measurement. Precision of a series of measurements represents how close the repeated measurements are to each other. Precision is dependent only on the equipment used to make the measurements.

A series of repeated measurements can be accurate but not precise because the average is close to the accepted value but the values are scattered from each other. Or the repeated measurements can be precise but not accurate; the values are close to each other but their average is far from the true value. A series of measurements can be neither precise nor accurate, but, of course, the best situation is to have both. Accuracy and precision will both be strived for in the laboratory.

Poor accuracy, poor precision

Poor accuracy, high precision

High accuracy, high precision

Figure 1: Three different situations of three darts thrown at a dart board explains accuracy versus precision

In Order to Record a Measurement Precisely:

1. Decide the value of each mark on a scale.

2. Count the marks.

3. Estimate ONE (and only one) additional place.

4. Include the units that have just been counted.

Record the measurement by writing the UNITS and ALL the numbers that can accurately be measured and ESTIMATE ONE MORE PLACE (ex 1.625 g )

Balances and volumetric glassware are commonly used in the laboratory to make measurements of mass and volume. Since the physical equipment used in making the measurement determines the precision of the measurement, some balances and volumetric glassware are more precise than others. The number of markings or divisions on the equipment determines the precision of the measurement. For example, time measurements made using a watch marked in seconds can be expected to have better precision (or reproducibility) than those made using a watch marked in 5 minute increments. Generally speaking more the number of decimal places, higher is the precision.

The number of significant digits in a measured value includes all the digits that can be read from the divisions on the equipment plus one estimated digit. By looking at the number of significant digits in a measurement taken, we can determine the uncertainly or error (precision) inherent in the measurement due to the equipment. Digital instruments are read to the final digit (all are significant).

TECHNIQUES LEARNING OBJECTIVES

Measuring length using rulers and Vernier Calipers.

Measuring mass using balances using images

Measuring Volume – Graduated Cylinder using images

Preparing graphs

Equipment and Chemicals (not all equipment will be used in the At Home Setting)

Procedure

Part A: Measuring Length

1.