Chemistry: Scientific Measurements                                                                               DO NOT WRITE ON THIS

 

Introduction:  One of the objectives of a chemistry course is that you can successfully collect quantitative measurements.  It is important to know that no measurement is absolutely certain. In other words, measurements have some degree of uncertainty.

 

Objective: In order to produce the best quantitative results you, the student/experimenter, will learn how to identify the error in a measure and apply this while measuring mass using the electronic balance and volume using various volumetric instruments.

 

Part I.  Analyzing error of volumetric instruments.

  

Materials & Methods:

 

            1. Obtain a polystyrene cup, a 100 mL beaker, a 125 mL Erlenmeyer flask, a 50 mL graduated   

                cylinder, a 50 mL buret,  a 25 mL pipet and a 2nd 250 mL beaker filled with distilled water. 

           

            2. Obtain the mass of the empty, dried Styrofoam cup. Record the mass of the cup under Mass of

                Cup in the data table below.

           

            3. Measure 50.0 mL of water with the 100 mL beaker then pour it into the cup.  Measure the mass

                of the cup with water and record under Mass Cup/H2O.  Pour out the water and dry the cup with  

                a paper towel. Repeat steps #2 and #3 again for the beaker.

 

            4. Repeat steps #2 & #3 for each of the instruments.  Measure out 50.0 mL for each except for the

                buret which you will measure out 25.0 mL. Be sure that you are doing 2 trials for each. 

 

   Calculations:

 

            1. Mass of Water: subtract the Mass of the Cup from the mass of the Cup & Water.  Record this

                 mass under Mass of Water.

           

            2. Calculated Volume of Water: determine the volume of the water actually delivered to the cup by

                using the density of water, D= 1.00 g/mL.  Density is the ratio of the mass of a substance and the

                volume of the substance. Solve for volume by dividing the mass of water by the density of water.

           

            3. Average Calculated Volume of water:  add the calculated volumes of water from each trial and

                divide by 2. 

           

            4. Percent Error.  Percent error is the error divided by the theoretical yield.  This is calculated by:                                  

Percent Error  =   | Actual yield ñ Theoretical yield |

                       Theoretical yield

Actual yield ý calculated volume

Theoretical yield ý  intended volume (50.0 mL or 25.0 mL for buret)

                                               

 

                                                                                                                   

Chemistry:  Scientific Measurements Lab                                                                                               Names:

 

PART I.  Data Table

Container

Mass of Cup (g)

Mass

Cup/H2O (g)

Mass of H2O (g)

Calculated volume of H20 (mL)

Average calc. vol. of H20 (mL)

Percent Error

Beaker. Trial 1

 

 

 

 

 

 

Beaker. Trial 2

 

 

 

 

 

 

Flask. Trial 1

 

 

 

 

 

 

Flask. Trial 2

 

 

 

 

 

 

Cylinder. Trial 1

 

 

 

 

 

 

Cylinder. Trial 2

 

 

 

 

 

 

Buret. Trial 1

 

 

 

 

 

 

Buret. Trial 2

 

 

 

 

 

 

Pipet. Trial 1

 

 

 

 

 

 

Pipet. Trial 2

 

 

 

 

 

 

 

 

Questions and Analysis:

 

1.      Rate the glassware in their accuracy for delivering 50 mL of water from the most to least accurate.

 

 

2. One instrument has an error of + 1.0 mL and another has an error or + 0.1 mL. 

a. Which instrument is more accurate?  Why? 

 

 

 

 

b. Which instrument is more precision? Why?

 

 

 

 

3.      Explain the relevance of knowing the uncertainty of a measuring device.