Periodic trends in reactivity lab answers

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General Chemistry I

Laboratory Experiment Eastfield College Logo

cid:image001.png@01D454B5.EBBC7FC0 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)

Equipment:

Chemicals:

NFPA

mg Balance

Metric Ruler

Vernier Calipers

50 mL Graduated Cylinder

Computer with MS Excel

Pen or Pencil

Book

Metal Slug (cylinder)

Metal Weight

Deionized Water

0 0 0

Procedure

Part A: Measuring Length
1. Use the metric ruler to measure the length of a pen or pencil in centimeters (cm). Read the ruler to the nearest one hundredth of a centimeter (+/- 0.01 cm).

2. Record the measurements on the REPORT SHEET.

3. Take an image of the measurement using a cell phone and add to the report sheet.

4. Use the metric ruler to measure the length width and depth of a book in centimeters (cm). Read the ruler to the nearest one hundredth of a centimeter (+/- 0.01 cm).

5. Record the measurements on the REPORT SHEET.

6. Take an image of the measurement using a cell phone and add to the report sheet.

7. Calculate the volume of the book.

8. From the Image, use the metric ruler to measure the length and diameter of the cylindrical metal slug in centimeters (cm). Read the ruler to the nearest one hundredth of a centimeter (+/- 0.01 cm).

9. Record the measurements on the REPORT SHEET.

Copper Slug Measurements

Part B: Measuring Mass
1. Zero the balance at on the lab bench by pressing the 0/T (or Tare) button.

2. Place the metal weight carefully on the balance pan.

3. Record the mass of the metal weight on the report sheet.

4. The mass of the copper and silver metal slugs will be provided on the report sheet.

Image from : https://www.wayfair.com/home/pdp/ktaxon-lab-analytical-balance-digital-precision-scale-ktax1704.html

Part C: Measuring Volume

1. Reading the Volume of Liquids. Read the volume of the colored liquid in the three display cylinders. Be sure to read the bottom of the Meniscus (darkest colored portion) at the front.

2. Use the measurements from Part A to calculate the volume of the metal slugs. Calculate the volume using the πr2 x L.

3. Volume of metal slugs by water displacement. Fill a 50 ml graduated cylinder with water. Read the volume (this is known as the initial volume). Carefully slide the metal slug into the graduated cylinder. Read the volume (this is known as the final volume). Take the difference to calculate the volume of water displaced. Record the change in volume for both metal slugs and record on the data sheet.

Copper Slug

Part D: Measuring Density

1. Calculating the Density for the book measured assuming the mass of the book is 566.98 g

2. Calculating the Density for the Copper slug using the volume calculated in part C by the water displacement method. The Mass of the Copper Slug is 54.387 g.

Part E: Graphing

1. Use the following data set for the graphing exercise:

Volume ( mL )

Mass ( g )

0

0

3.34

2.180

6.54

4.322

9.98

6.617

13.11

8.804

2. To prepare the graph, open an excel worksheet on the laptop.

3. Create two columns – volume in the first column & mass in the second. Be sure to have the heading “volume” and “mass” in each column. Do not put units along with the numbers.

xy scatter with markers only

Figure 3: Excel graph of mass versus volume

4. Using the mouse select/highlight the two columns that contain mass and volume data, and then click the "Insert" tab on the Excel main menu

5. Set the Excel graph to a "Scatter plot with only Markers" type by selecting this option from the drop-down menu on the "Charts: Scatter" section of the Excel Ribbon menu.

6. If the volume is located in the first column of data then it will appear on the x-axis – make sure of this. Mass must be on y-axis (why?)

7. Double-click on the Chart Title to change the default title to “Mass versus volume”. Right click on the graph and click “format axis”. A “+” sign should appear next to the graph – click on it. Make sure axis title is selected. Click on and change the vertical y-axis title to Mass (g) and the horizontal x-axis title to Volume (mL).

8. Draw a straight line -- called a trendline -- through the data by right-clicking on any data point and selecting "Add Trendline" from the resulting pop-up box. A side window will pop up.

9. Select "Linear", under trendline options. Check the “Set Intercept” box, and set the intercept to zero. This action will ensure the trendline is the best fit for the data. Also select the “display equation on chart” and display R-square value”. Excel will display the equation for the data with on the graph as well as its R2 value.

10. Make final adjustments to the density graph, using "Format Axis" and "Format Size" options. These allow changes to the line thickness and color, and increase the size of the graph.

11. Click & select the graph. Use “Print selection” to print the graph only. Be sure to print two copies.

12. Attach the graph to the data sheet and turn it in to the instructor.

13. Return all equipment to the proper place. Put away the cleaned glassware, lock the drawer and return the key.

V. Johnson (7-27-2018) Page 8 of 8 BAKS 5/26/2020