DENSITY OF SODIUM CHLORIDE SOLUTIONS

DENSITY OF SODIUM CHLORIDE SOLUTIONS
OBJECTIVES:
Review proper use of balances and volumetric glassware
Review concepts of molarity and density of solutions
Apply concepts of precision and accuracy to experimental data
INTRODUCTION:
The purpose of this experiment is to make a solution of sodium chloride of precisely known
concentration and measure its density using a variety of analytical tools. The laboratory and
analysis skills to be learned and/or reviewed in this experiment include:
- proper use and care of analytical balances
- proper use and care of volumetric flasks
- proper use and care of volumetric pipets
- proper use and care of burets
- definition of molarity and calculations involving molarity
- definition of density and calculations involving density
- definitions of precision and accuracy and their calculation from experimental data
The above skills and techniques are described in the “Review of Techniques” section of the
laboratory manual. You should review these sections of the lab manual before completing
the pre-lab assignment. It is important that you carefully follow the correct procedures for
each device to ensure precise and accurate results. You will be graded on the precision and
accuracy of your results for this experiment.
PRE-LAB QUESTIONS:
1. What is the difference between glassware labeled “TC” and “TD”
2. Which piece of glassware is used to make solutions of known and precise molarity
Why (What features make it useful for this purpose)
3. Describe the process of conditioning a pipet. Why must pipets be conditioned before
use Why don’t we condition volumetric flasks
4. To what level of precision should you record buret readings
5. Write a complete description of how you will prepare 250 mL of your assigned salt
solution (including procedures and glassware to use and calculated values).
6. How will we measure/calculate the density of our prepared sodium chloride solutions
in this experiment
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DENSITY OF SODIUM CHLORIDE SOLUTIONS
CHEMICALS AND MATERIALS
NaCl(s)
water
EQUIPMENT TO BE CHECKED OUT
5-mL graduated pipet
250mL volumetric flask
25mL buret
25mL Erlenmeyer flask with stopper
SAFETY
Goggles must be worn in the laboratory.
WASTE DISPOSAL
All solutions may be rinsed down the sink
EXPERIMENTAL PROCEDURE:
Your instructor will assign you a specific molarity of sodium chloride solution to prepare.
You will prepare 250 mL of this solution and then measure its density by weighing specific
volumes of the solution on the analytical balance. Your instructor may assign one or all of
the methods for determining density from steps 2-4 below.
1. Prepare 250 mL of your assigned concentration of sodium chloride solution. Record
the exact mass of sodium chloride used.
2. Use your 5-mL graduated pipet to transfer 5.00 mL of your prepared solution to the
25-mL volumetric flask (keep stopper on after transferring to avoid evaporation
losses) and measure the mass of this solution on an analytical balance. Repeat this
procedure until you have 5 consecutive trials that agree within 0.2% (or about 0.02 g).
3. Use your 10-mL volumetric pipet to transfer 10.00 mL of your prepared solution to
the 25-mL volumetric flask (keep stopper on after transferring to avoid evaporation
losses) and measure the mass of this solution on an analytical balance. Repeat this
procedure until you have 5 consecutive trials that agree within 0.2% (or about 0.02 g).
4. Use a buret to sequentially transfer approximately 5 mL aliquots of your solution to
the pre-weighed weighing vial and weigh the transferred solution (do not empty the
weighing vial—add each additional 5mL to it). Repeat until you have delivered a
total of 20 mL of solution to your weighing vial. Note that it is not important to
transfer exactly 5.00 mL each time, but it is important to know precisely what volume
of solution was transferred—record the exact buret readings.
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DENSITY OF SODIUM CHLORIDE SOLUTIONS
DATA ANALYSIS:
(Your instructor may assign specific questions from those below depending on which
procedural steps you completed.)
1. Calculate the molarity of your sodium chloride solution from your experimental data.
2. (From step 2 in the procedure) Calculate the density of density of your sodium
chloride solution based on the measurements from the graduated pipet and analytical
balance. Determine a density for each trial and also calculate the average density,
standard deviation and relative standard deviation.
3. (From step 3 in the procedure) Calculate the density of density of your sodium
chloride solution based on the measurements from the volumetric pipet and analytical
balance. Determine a density for each trial and also calculate the average density,
standard deviation and relative standard deviation.
4. (From step 4 in the procedure) Use Excel to make a graph of mass vs. volume for
your measurements from the buret and analytical balance. Perform a linear fit of this
data and include the equation of the line and the corresponding R 2 value in your plot.
Use the equation of the line to find the average density of your sodium chloride
solution from these measurements.
5. (From steps 2 or 3 and 4 in the procedure) Calculate the percent difference between
the average density determined using the graduated pipet or volumetric pipet and the
average density determined using the buret.
6. (From steps 2 or 3 in the procedure) Use the average density determined with the
graduated pipet or volumetric pipet to calculate the % by mass NaCl in your prepared
solution.
7. (From steps 2, 3, 4 in the procedure) Obtain the “true value” for the density of your
solution from the instructor and calculate the percent error for your pipet and buret
determinations.
POST-LAB QUESTIONS:
1. (From steps 2 and/or 3 in the procedure) Does the experimental precision you
obtained with the graduated pipet and/or volumetric pipet agree with the expected
precision of the measurement based on significant figures Discuss.
2. (From steps 2 and 3 in the procedure) Do the density results calculated from the
graduated piper and volumetric pipet and buret agree within the precision of the two
instruments Discuss.
3. (From steps 2 and/or 3 in the procedure) Does the “true value” of the density of your
prepared solution lie within the precision of your measurements for the graduated
and/or volumetric pipet Discuss.
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DENSITY OF SODIUM CHLORIDE SOLUTIONS
REFERENCES:
Written by Janet Truttmann, September 2010, Revised February 22, 2011 (Reviewed by
Jenny Leung, Jody Williams-Tyler, Terri Beam)
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