Experiment #1 Mass, Volume, and Density Background Mass (weight) and volume have already been discussed in lecture in terms of solving dimensional analysis problems. Our lab is equipped with four balances that weigh to two places past the decimal; hence these balances are called centigram balances. Also there are two milligram balances (reserved for other chemistry students). Your chemistry draw should contain a 10 ml and 50 ml graduated cylinder; these are used to measure the volumes of liquid. Mass and volume are two of many physical properties of matter, be it gas, liquid, solid, or a mixture. Further, mass and volume are termed “extensive” properties because they change depending on the quantity of the sample. For example, weighing increasingly larger amounts of sodium chloride on the centigram balance would obviously result in increasingly larger weights being displayed by the balance. The same goes for volume; the more liquid transferred to a graduated cylinder, the larger the volume read on the cylinder. Measuring an object’s mass and/or volume is not of much use in trying to identify that object. Once you know that object’s mass and volume, another physical property, called density, can then be calculated from the formula mass (g) Density = ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ volume (ml) Density, unlike mass and volume, is termed an “intensive” property because the density of an object does not depend on the quantity of that object. For example, the density of a piece of metal is the same, regardless of the size of that piece of metal. Furthermore, density may serve as a means for confirmation/identification, as you’ll be doing in today’s experiment. Determining the density of a substance is important for a number of reasons, not just for the (possible) identification of a substance. For example, the charge that car batteries have can be determined by measuring (not calculating) the density of the sulfuric acid in the battery with a special device called a hydrometer. Wine makers follow the progress of fermentation of grapes by measuring the density of the grape juice; the density of the grape juice decreases as fermentation converts grape sugar to alcohol.
More importantly, to allied health professionals, density serves as the basis for “specific gravity” measurements of urine. It turns out that “normal” urine has a specific gravity between 1.005 and 1.030 (specific gravity is unitless). Urine with a specific gravity outside this range may indicate a possible disease process of one type of another. Finally, density is useful in that it can serve as a conversion factor to “move” between mass (weight) and volume, as will be covered soon in lecture if not already covered. Objectives To experimentally confirm the density of water To determine the density of a liquid other than water To determine the density of a solid To determine the density of a gas (exhaled breath) Directions Use of Pipets and Pipetting Graduated cylinders are fairly accurate in measuring the volume of a liquid. A device that is even more accurate in measuring (and delivering) volumes of liquids is the pipet. Pipets are long narrow pieces of glassware made to precisely measure the volumes of liquids. Because they are more accurate than graduated cylinders, you’ll use them in today’s experiment. Obtain a 10 ml pipet and observe the graduation marks. Your pipet may be a volumetric pipet in which there will be only one etch mark, or it could be a Mohr pipet in which you will see many graduation marks, Practice filling and dispensing distilled water from your pipet as demonstrated by the instructor. Mastering this technique of filling and dispensing a liquid from a pipet is essential to this experiment if accurate results are to be achieved.
Density of Water Part 1 Half fill a 125 ml beaker with distilled water. Take this beaker as well as an empty 50 ml beaker and your pipet to the centigram balance. Determine the weight of the empty 50 ml beaker and record on the data sheet. Carefully pipet 10.0 ml of distilled water from the 100 ml beaker to the 50 ml beaker while the beaker is on the balance pan. Be sure to touch the tip of the pipet to the side side of the beaker once all of the water has been delivered. Record the weight of the 50 ml beaker plus the water on the data sheet. Calculate the density of the water and record on the data sheet. Part 2 Determine your percent error according to the following formula and record on the data sheet: ⏐ T‐ E⏐ Percent Error = ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ x 100 T where T = theoretical density of water and E = your experimentally determined density of water. Density of a Liquid Other Than Water Part 3 Obtain either hexane or dichloromethane from the hood an record which liquid you are using on the data sheet. These are organic liquids which are not water soluble, so when you are completely through with using them, put them in the “Waste Organic” beaker located in the hood for proper disposal by the instructor. Rinse out your 10 ml graduated cylinder with acetone (found in the hood, another organic liquid which is soluble in water). Next, rinse out the same pipet with 3‐4 ml of either hexane or dichloromethane as demonstrated by the instructor. Your pipet is now “wet” with your liquid. Be sure to discard the liquid in the “Waste Organic” beaker. Half fill a 50 ml beaker with your liquid. Take this beaker as well as another (empty) 50 ml beaker and your pipet to the centigram balance. Determine the weight of the empty 50 ml beaker and record on the data sheet. Carefully pipet 10.0 ml of your liquid from the 50 ml beaker to the other 50 ml beaker while the beaker is on the balance pan. Be
sure to touch the tip of the pipet to the side side of the beaker once all of the liquid has been delivered. Record the weight of the 50 ml beaker plus the liquid on the data sheet. Calculate the density of the liquid and record on the data sheet. Look up the actual density of your liquid from the Handbook of Physics and Chemistry and record on the data sheet. Determine your percent error using the same formula and record on the data sheet. Density of a Solid Part 4 Obtain a small object found in a cardboard box in the hood and record what it is on the data sheet. Determine the object’s weight on record on the data sheet. Half‐fill a 50 ml graduated cylinder with tap water and record this (initial) volume on the data sheet. Carefully transfer your solid object to the graduated cylinder and roll the object around in the water to remove any trapped air bubbles that may be adhering to the object. Measure the new (final) volume of water in the graduated cylinder and record on the data sheet. Calculate the density of the object and record on the data sheet. Density of Gas (Exhaled Breath) Part 5 Obtain a new 5‐in balloon and record its weight (empty!) on the data sheet. Blow the balloon up up it becomes approximately as big as a cantaloupe. Tie the balloon, reweigh, and record the weight on the data sheet. Measure the circumference of the inflated balloon as demonstrated by the instructor. Using the following formulas, calculate the volune of the inflated balloon and record on the data sheet. Calculate the density of the exhaled breath and record on the data sheet. Volume of a Sphere = V= 4 ⁄ 3 r3, where the circumference is given as C= 2 r
Experiment #1 Name_________________________ Data Sheet Density of Water Part 1 Weight of empty 50 ml beaker: __________________ Weight of 50 ml beaker plus 10.0 ml water: __________________ Weight of 10.0 ml water: __________________ Density of Water: __________________ (Show calculations) Part 2 Percent Error: __________________ (Show calculations) Density of a Liquid Other Than Water (Choose any of the liquids that are marked A, B, C or D) Part 3 Liquid Used: __________________ Weight of empty 50 ml beaker: __________________ Weight of 50 ml beaker plus 10.0 ml liquid: __________________ Weight of 10.0 ml liquid: __________________ Density of Liquid: __________________ (Show calculations)
Density of known liquids from CRC handbook of Physics and Chemistry or from Internet. Dichloromethane __________________ methylene chloride. Ethanol ___________________ (ethyl alcohol) Hexane ___________________ Ethyl acetate ___________________ acetic acid ethyl ester or ethyl ethanoate or acetic ether
Identity of your Unknown liquid: Percent error in density of your Unknown liquid (Show calculations) Density of a Solid Part 4 Identity of solid: Weight of solid: Initial Volume of Water in Graduated Cylinder: Final Volume of Water in Graduated Cylinder: Density of solid: (Show calculations)
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Density of a Gas (Exhaled Breath) Part 5 Weight of empty 5‐in balloon: __________________ Weight of inflated 5‐in balloon: __________________ Measured circumference of inflated balloon: __________________ Volume of balloon: __________________ (Show calculations) Density of exhaled breath: __________________ (Show calculations) Have the instructor check your calculations ___________________ To see if you’re correct, go to “http:www.ask.com” and type “volume of sphere?” Click on “Go” and then click on the link “ABE Volume calculations, Sphere” and then follow the direction. Volume of Sphere (your balloon) from this link: ____________________ Density of exhaled breath ____________________ (Show calculations) After you have completed this section please either discard your balloon or pop it and throw it the trash, do not leave on the desk. Thanks.
Conclusion How might the differences in densities of a gas, liquid, and solid be explained? Give this some thought...
