Mass, Volume and Density

Introduction

In this exercise you will determine the densities of a liquid, an irregular solid and a regular solid.  In the course of performing the experiment you should learn how to correctly use a quad-beam balance, an electronic balance, a graduated cylinder and Vernier calipers.  You should also gain a fuller understanding of the concepts of mass, volume and density.

Chemistry is the study of matter and matter is generally defined as "that which has mass and occupies space".  Mass is then, and intrinsic property of matter.  It is important to distinguish at this time between the concepts of mass and weight as they are not the same.  Weight is the effect of the acceleration of gravity on mass.  (An astronaut in orbit about the earth may be effectively weightless, but not mass less.)  In the English system of weights and measures the equivalent mass of one pound weight is called a slug.  Although it is important to understand the differ­ence between the concepts of weight and mass, the common interchangeable usage of the terms is of little practical concern as we generally are "weighing" objects right here on the surface of the earth where (in metric units) a gram mass exerts a force of a gram weight.

To state that matter occupies space is equivalent to stating that matter has vol­ume.  Volume, like mass, is an extensive property of matter.  An extensive prop­erty is one, the value of which depends on the amount of matter.  Simply put, the more matter you have the greater is its mass and the more space it occupies.  Units of volume may be unique to volume as in a pint, a gallon, or a bushel or may be expressed as cubic length as in a cubic foot or a cubic meter.

Density is an intensive property of matter.  Intensive properties are independent of the amount of matter involved.  Density is defined as the "mass per unit vol­ume" or the quotient of mass and volume of a sample of matter.  D = M/V.  The density of a substance is one means by which we can identify it.  Pure water, for instance has a density at room temperature of 1.00 grams per cubic centimeter.  The densities of normal solids range in values from a few tenths of a gram per cubic centimeter to a bit more than 20 grams per cubic centimeter.  Liquid water with a density of 1.0 g/cm3 will float on liquid mercury with a density of 13.6 g/ cm3  ( is mercury the densest substance on Earth?) while vegetable oil at 0.8 g/cm3 will float on water.  Likewise a piece of wood, less dense than water will float in water, while a stone, more dense than water will sink.

Measurement

Chemistry is an experimental science and central to experiment is precise and accurate measurement.  The determination of density requires measurement of both mass and volume.  Mass is determined using a balance.  Your instructor will demonstrate the proper use of the quad-beam balance (used in parts one and two) and the more precise electronic balance (used in part three).  Some procedural rules apply to all balances.


1.   Always keep the balance pans clean and dry.  Only clean dry surfaces of glass metal or weighing paper must ever touch the balance pan.  Under no circum­stances weigh reagents directly on the balance pan.


2.   Always check the zero adjustment before making a mass determination.  With the quad-beam balances this means double-checking to see that movable weights are set to the zero position.

3.   Objects for mass determination must always be near room temperature.  Hot objects cause convection currents that interfere with mass determination and on sensitive balances can produce incorrect readings due to bouncy effects.

4.   Record the mass of the object to the limit of the precision of the balance.  Mass determination on the quad-beam balance must always be to the nearest 0.01 grams, and 0.001 g  or 0.0001 g (three or four decimal places) on the electronic balances.

5.   When done return movable masses to their zero positions.

The volume of a liquid sample can be determined using a graduated cylinder.  The curved surface of the water in the cylinder is called the meniscus.  Volume is measured using the very bottom of the meniscus.  In the laboratory every mea­surement is made to the limit of the precision of the instrument used.  In the case of a graduated cylinder, you must interpolate the position of the bottom of the meniscus to the nearest one tenth (0.1) mL in every measurement.  Your instructor will demonstrate how to use the graduation marks on the cylinder to avoid paral­lax error in reading the meniscus.

In the third part of the experiment, you will determine the volume of a regular object (a cylinder) by calculation from linear measurements made with Vernier calipers.  Your instructor will demonstrate how to read the Vernier calipers. Make sure to record all measurements made with the calipers to a precision of 0.01 cm.

Procedure

Part I: Determination of the density of an unknown liquid

Obtain a liquid sample with your name on it from you instructor.  The sample is a salt solution with a density between 1.00 g/mL (the density of pure water) and 1.50 g/mL.  Pour between 45 and 50 mL of the solution into a graduated cylinder and record the volume of the sample to the nearest 0.1 mL.  Determine the mass of a clean, dry 100 or 150 mL beaker to the nearest 0.01 g on a quad-beam balance.  Pour the sample from the cylinder into the tared beaker and reweigh the beaker.

From the beaker pour between 40 and 45 mL of the solution back into the gradu­ated cylinder.  Record the new volume of the cylinder (again to the nearest 0.1 mL).  Dry the beaker and transfer the solution from the cylinder into the beaker.  Record the new mass of the beaker and solution.  From the two determinations, calculate the density of the solution and the average density from the two deter­minations.  (watch your significant figures!)  Obtain from your instructor the actual density for your unknown and calculate your percent error in the density according to the formula  actual - theoretical  x 100.

                                            (theoretical)

Part II: Determination of the density of an irregular solid

Determine which of the five solids you have been assigned as an unknown.  Weigh a clean, dry 50 mL beaker to the nearest 0.01 g on the quad-beam balance.  Observe the examples set out for you on the reagent shelf and obtain an equiva­lent amount of you assigned substance in the beaker.  Weigh the beaker and sam­ple on the quad-beam balance and record the mass on you data sheet.  Add about 25 ml of water to a graduated cylinder and record the initial volume to the nearest 0.1 mL.  Carefully transfer all of the solid into the cylinder containing the water.  Avoid splashing.  Record the new volume in the cylinder to the nearest 0.1 mL.  Decant the water from the cylinder into the sink (use a wire screen to avoid dropping metal into the sink) and pour the wet sample onto a paper towel.  Dry the sample as best you can with the paper towel and deposit it in the proper container found on the reagent bench. Do not place the wet metal back into the reagent jar!

Repeat the entire process with a fresh, dry sample of your assigned substance.  It is essential that the change in volume of the cylinder readings (volume of the sam­ple) be greater than 10.0 mL.  If your change in volume is less than 10 mL repeat the experiment until you have two determinations with volume changes of a least 10 mL.

Calculate the density of your sample from each of the two determinations.  Look up the actual density of your sample in the CRC Handbook of Chemistry and Physics.  Calculate your percent error for each determination.  (If you are assigned calcium carbonate for your unknown, calculate the percent error from the density listed on the bottle.)

Part III: Determination of the density of a cylinder

Record the number of your assigned unknown cylinder.  Determine the mass of the cylinder to the nearest milligram (0.001g ) or 0.0001g using the electronic, top-loader bal­ance.  Using Vernier calipers, determine the values of the length and the diameter of your assigned cylinder to the nearest 0.01 cm.  Calculate the volume of the cyl­inder from the formula.  From the mass and the volume, calculate the density of the cylinder (remember significant figures!).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Name                                          

Date                                             

Section                                        

 

Prelaboratory Exercise

Experiment #1

Mass, Volume, and Density

 

1.    A 50 ml graduated cylinder should be read to the nearest ___________ ml.

 

2.    (True/False)  You will get the same weight for an object regardless of whether you weigh it hot or cold.

 

3.    100 mg is what part of a gram?

 

4.    (True/False)  Even though the metric system is extremely useful, it is more difficult to add different sized quantities in the metric than in the British system.

 

5.    With how many different volumes should you determine the density of the liquid in this experiment?

6.    The curved surface at the top of a liquid in a graduated cylinder is called a       ___ ___ ___ ___ ___ ___ ___ ___ .

 

7.    (True/False)  The densest substance in the world is mercury.

 

 

8.    The surface of a liquid in a graduated cylinder should be read at eye level to prevent a ___ ___ ___ ___ ___ ___ ___ ___error.

 

9.    You can get the true value of the density of the liquid you use in this experiment from the ___ ___ ___ ___ ___ ___ ___ ___ ___ ___.

 

 

10.A student forgot to zero the balance before weighing his beaker (in Part 1) and got a value that was actually too high.  When he weighed the beaker with the liquid in it, however, he remembered to zero and got an accurate value for this weighing.  Will the value he gets for the weight of the liquid be high, low or unchanged?

 

11.If the same student then subsequently measures his liquid volume accurately, will his density result be high or low or unchanged?

 

 

12.If a student determines the density of a substance to be 1.80 g/ml, but the actual density is known to be 2.000 g/ml, what is his percent error?

 

13.A graduated cylinder originally contained 17.2 ml of water. An object weighing 14.1 g was then submerged in the water and the new reading was 21.1 ml.  The balance can be read to 0.1 g and the cylinder to 0.2 ml.  Which measurement is the less precise?

 

 

14.What is the density of the object in question 13?


Name                                    

Date                                     

 

Density determinations Data Sheet

Part I: Density of unknown liquid

 


TABLE  

 

 

Trial 1

Trial 2

Trial 3

Mass of beaker and liquid

 

 

 

Mass of beaker

 

 

 

Mass of liquid

 

 

 

Volume of liquid

 

 

 

Density of liquid

 

 

 

Actual Value

 

 

 

Percent error

 

 

 

 

Sample Calculations

 

 

 

 

 

 

 

 

Part II: Density of unknown solid

Unknown Metal ____________________

 

TABLE  

 

 

Trial 1

Trial 2

Trial 3

Mass of beaker and solid

 

 

 

Mass of beaker

 

 

 

Mass of solid

 

 

 

Volume of liquid and solid

 

 

 

Volume of liquid

 

 

 

Volume of solid

 

 

 

Density of solid

 

 

 

Literature density of solid

 

 

 

Percent error

 

 

 

Sample Calculations

 


 Part III: Density of a cylinder

Cylinder Number                    

TABLE  

 

 

Trial 1

Trial 2

Mass of cylinder

 

 

Length of cylinder

 

 

Diameter of cylinder

 

 

Volume of cylinder

 

 

Density of cylinder

 

 

 

Sample calculations

 

 

 

 

 

 

 

 

 

 


Questions


1.   Why could you not determine the density of salt in the same way you deter­mined  the density of the unknown (water displacement) in part 2?

 

 

 

 


2.   How could the experiment be modified to determine the density of salt?

 

 

 

 

2.   In part 1, could you increase the number of justifiable significant figures in you answer by using the electronic top-loader balance in place of the quad-beam balance?  Explain.

 

 

 

 

 

 

2.   If a graduated cylinder initially contains 28.8 mL of water and addition of 70.42 grams of metal shot causes the water level to rise to 37.3 mL, what is the density of the metal shot?