M. It is expressed in units such a g/cc,

Transcription

1 Experiment 2 Density Part I: Density of a solid Density is defined as mass per unit volume, D = V M. It is expressed in units such a g/cc, g/ml, lb/ft 3, etc. Determination of mass is done by weighing an object on a balance. Determination of the volume of a solid can be done several ways. If the object has a regular geometrical shape like a cube, the appropriate dimensions call be measured and the volume calculated by an appropriate formula. If the solid is irregular its volume can be determined by displacement of a liquid by a weighed amount of the solid. Graduated cylinder filled with water. (Note the level of the water is read at The bottom of its curved upper surface, which is called the meniscus.) Rubber stoppers are added to the cylinder. The water level rises to 45 ml. The difference in volume, 45 ml - 30 ml or 15 ml, is the volume of the rubber stoppers. RECORD ALL INFORMATION ON WORKSHEET PROVIDED Part A Obtain either a block or a cylinder of metal, and measure its various dimensions (in cm) and its mass (in g). Then, calculate its volume (in cm 3 ) and finally, its density (in g/cm 3 ). Repeat the measurements for confidence. (How do you calculate the volume?) Part B Use the same piece of metal and determine its density (in g/ml) by the method of volume displacement in water. (This method has some limitations. What are some?) Repeat the determination for precision.

2 Part C. Density of a prepared solution Each student or group prepares a sodium chloride solution with a given (by instructor) concentration. For example if the concentration is 5.00 % by mass, then the student needs to weigh exactly 5.00 g of NaCl and add g of water to it. According to the formula you prepared a 5.00% NaCl solution. masssolute % mass 100% masssolution Procedure 1. You will prepare the solution in a 250ml Erlenmeyer flaks. Based on your concentration calculate how much salt and how much water you need to add to the beaker to prepare a 100 g solution. 2. Use a small beaker to weigh the correct amount of salt. Put the small beaker onto the balance and tare the balance. Add slowly with a spatula the correct amount of salt into the beaker. Once you are close to the desired mass, write the amount of salt measured and add the salt to a 250ml Erlenmeyer. 3. For measuring out the water use a small beaker (100 ml). Tare the balance with the beaker on it and add the exact amount of water to it (make sure you do not spill any water onto the balances!). Add the water to the Erlenmeyer and stir until all the salt is dissolved. 4. Once your salt is completely dissolved we can proceed and measure the volume and mass of the solution. Take a small, empty and dry beaker and measure its mass. Take the beaker back to your place. 5. Use a 10ml volumetric pipet, draw up 10.00ml of solution (make sure to use pipet pumps) and transfer the solution to the dry beaker. 6. Reweigh the mass of the beaker with the 10.00ml solution and record your new mass. 7. Repeat step 4 6 a second time in order to have two trials. 8. Calculate the mass of the solution and calculate the density of the solution for each trial; calculate the average density of the solution. 9. Make sure to rinse your volumetric pipet a few times with RO water. Graphing exercise with class data: After obtaining your average density for you solution, write your concentration and density on the white board. Once all groups have finished their work use the data to prepare a graph. Plot the density of the different solutions as a function of concentration on a graphing paper. Make sure you label the axis properly and draw a best fit line through your data.

3 Part D. Density of floating golf ball Procedure Obtain a golf ball from the instructor. Carefully place it in a 400 ml beaker and add enough water to cover it (about 300 ml). Add solid sugar, a scoop at a time, with stirring to dissolve the solid from each scoop until your golf ball just floats. Make sure the ball floats in the solution and does not break the surface of the solution. In case you added too much sugar you can correct this by adding extra water. Determine the density of the sugar solution (which should be the same as the density of the ball) by transferring 10.00ml of the solution with a volumetric pipet to a pre-weighed small beaker. Measure the mass of the beaker with the solution after the transfer and calculate the net mass of the 10.00ml solution. Finally calculate the density of the sugar solution and therefore the density of the ball. Make sure to rinse all your glassware with plenty of RO water in order to wash out all the sugar.

4 DATA Experiment 2 Densities Name Part A. Description of metal Mass Length Diameter Height Width Volume Density Average density Part B Use the same piece of metal and determine its density (in g/ml) by the method of volume displacement in water. (This method has some limitations. What are some?) Repeat the determination for precision. Initial volume reading Final volume reading V H2O = V metal Mass (from above) Density Average density Some Final Thoughts: Find out from your instructor what the metal is. Which method gave you a more accurate density result? How could you improve the accuracy?

5 Part C Density of prepared solution DATA Assigned concentration of solution: % Prepare solution: Mass of solute (NaCl) Mass of solvent (water) Density: Trial 1 Trial 2 Trial 3 (if needed) Mass of empty beaker Volume of solution Mass of cylinder plus solution Calculations: Net mass of solution Density of solution (for ea. Trial) Average density: Graph: Plot all the class date on a graphing paper. Make sure you label the axis properly. X-axis is concentration and y-axis is density. Answer the following question: Is the data linear? Are all the points close to the trend line? What does it mean if some points are somewhat away from the trend line? Explain.

6 Part D Determine Density of floating golf ball. Mass of empty beaker Volume of transferred solution Mass of beaker plus solution Net mass of solution Density of ball Density Problems: Fill in the following table: Density (g/ml) Mass (grams) Volume (ml) 15.7 g 1.90 ml 3.44 g/ml 148 ml 7.7 g/ml 21.0 g Answer the following questions: 1. Iron (Fe) has a density of 7.87 g/cm 3. If a block of Fe has a length of 15.0 cm and width of 13.0 cm and a height of 10.0 cm what is the mass of the block in grams? grams 2. The following is a table of densities of metals Metal density (g/ml) Nickel 8.91 Titanium 4.50 Lead Zinc 7.14 Tin 7.23 a. If a block of pure metal has a mass of 74.9 and a volume of 6.60 ml what is the identity of the metal? b. If the mass of a piece of metal is g and it is 2.20 cm long, 2.00 cm wide and 1.09 cm thick, what is the identity of the element?