Mass & Density Measurements

Revised 22 August 2024

Introduction

The density of a solid or liquid is defined as mass/unit volume, an intensive property of the substance. It can be determined by a number of methods. In this lab, two different methods will be used to determine the density of seawater: The Erlenmeyer flask and the pycnometer.

Safety

DO NOT PIPET BY MOUTH. Use a pipetting aid. Carefully examine all glassware for cracks or chips. Ask for a replacement if you find any. Wear your safety glasses.

Prelab

Prepare data tables for all parts of the experiment. See the Chemistry Guide for more detail.

Procedure

Record, in ink, all measurements directly in your lab notebook. The use of the pipet will be demonstrated by your instructor, as will the use of the analytical balance. Carefully take note of these procedures. You must learn to manipulate a pipet and the balance correctly to get reproducible results.

How to use the analytical balance

Weighing an object on a single-pan analytical balance is a simple matter. However, you must use them very carefully, because of the sensitivity and the expense of the balance (some cost more than $2500). Directions for the operation of single-pan analytical balances vary with the make and model of the balance. Regardless of the balance you use, proper care of the balance requires that you observe the following:

Do not drop an object on the pan.
Center the object on the pan.
Do not place chemicals directly on the pan; use a small beaker, watch glass, weighing paper, or a weighing boat.
Do not weigh hot or warm objects; objects must be at room temperature.
Return all weights to the zero position after weighing (i.e. rezero the balance when leaving)
Clean up any chemical spills in the balance area. If you are unsure how to do this, ask your professor or lab technician for help.
Inform your professor or lab technician if the balance is not operating correctly; do not attempt to repair it yourself, please.

METHOD 1. Density using the Erlenmeyer flask

A. Density of seawater using the Erlenmeyer flask

Note the source of the seawater sample. Measure and record the temperature of the seawater sample being sure to securely clamp the thermometer. Weigh a 125-mL Erlenmeyer flask and record its mass. With a clean pipet, add 10 mL of seawater into the flask and reweigh. Repeat two more times.

Analysis of the results

Using the volume of the pipet, calculate the density of seawater. In your lab report, do not forget to use the correct number of significant figures to deal with the uncertainty in your measurements.

METHOD 2. Pycnometer

A. Volume of pycnometer

Weigh a clean, dry pycnometer with its stopper. Measure and record the temperature of the distilled water being sure to securely clamp the thermometer. Fill the pycnometer to the brim with distilled water. Because of the surface tension of water, the water may appear to bubble at the opening of the pycnometer. This is normal and should happen. Stopper the pycnometer. Upon doing so a small amount of water will spill out. This is also normal and should happen. Wipe the pycnometer dry with a kimwipe and record its mass.

Analysis of the results

Using the equation for density and the density of water at the temperature of your distilled water (ask your professor or lab technician to show you where to find this value), calculate the volume of the pycnometer. This value for the volume of your pycnometer will be used in the next step to calculate the density of the seawater sample using the pyncometer. In your lab report, do not forget to use significant figures and the ± notation to deal with the uncertainty in your measurements.

B. Density of seawater using the pycnometer

Make sure as much water as possible is removed from the pycnometer by gently tapping the opening onto an absorbent piece of paper. Weigh empty pycnometer. Fill the pycnometer with seawater and reweigh. Repeat two more times.

Analysis of the results

Using the volume of the pycnometer determined in the previous step, calculate the density of the seawater. In your lab report, do not forget to use significant figures to deal with the uncertainty in your measurements. Compare to the results obtained from method 1.

Cleanup

Empty the pycnometer and rinse several times with distilled water. Place it where you originally found it. Clean all other glassware with soap and water, and rinse well with tap water. Once all the soap residue is gone, rinse once with distilled water. There is no need to dry the inside of the glassware with a paper towel.