Chesapeake Campus Chemistry 111 Laboratory

Chesapeake Campus Chemistry 111 Laboratory Objectives Calculate the density of a sugar solution. Evaluate lab sources of error and their effect on an experiment. Introduction The density of an object is defined as the ratio of its mass to its volume. We write this mathematically by using the equations: Density = Mass Volume Equation 1 For an example of density, consider the following: Imagine a brick that is made of Styrofoam. Imagine a second brick that is made of lead. Note that even though the bricks take up the same amount of space - that is, they have the same volume - there is a major difference in their mass. We would say that the lead is denser, that is it has more mass in the same volume. It is important to note that water has a density of 1.0 g/ml. Objects that have a density less than water, that is, less than 1.0 g/ml, will float on the surface of the water. Those that have a density greater than 1.0 g/ml will sink. Consider our two bricks again. The brick of Styrofoam will float if we toss it into water. The lead will quickly sink. Modern ship manufacturers make use of density when designing the ships they build. They use materials that are denser than water but shape the materials so that they take up enough space to float. Although the ships weigh several thousand tons, that mass takes up a lot of space. Overall, the ship has a density less than water and therefore floats. Two factors have an effect on the density of water: 1) Temperature will have a small effect on the density. For water, density increases as temperature decreases. See Table 1 for the density of water at different temperatures. 2) If more dense materials are dissolved in the water, the solution density will increase. We will see this effect in today's lab when we measure the effect of dissolving sucrose on the density of water. Modified from Lumen Learning 2014 CC-BY 4.0 https://lumen.instructure.com/courses/150410/modules Page 1

Temp ( C) d H 2O (g/ml) Temp ( C) d H2O (g/ml) 18.0 0.99860 22.0 0.99777 18.5 0.99850 22.5 0.99765 19.0 0.99841 23.0 0.99754 19.5 0.99830 23.5 0.99742 20.0 0.99820 24.0 0.99730 20.5 0.99809 24.5 0.99716 21.0 0.99799 25.0 0.99704 21.5 0.99788 25.5 0.99690 Table 1. Density of water at different temperatures In this experiment you will test your laboratory technique by calibrating a 10 ml graduated cylinder, making up an aqueous sucrose solution of a particular mass percent in solute, and measuring the density of the solution with the calibrated graduated cylinder. The density result will be evaluated by students for accuracy and precision. Since the correct density will depend on a correctly prepared sugar solution, careful sample preparation will be critical. There are many ways of describing the concentration of a solution. The mass percent of solute in a solution is given by the symbol (w/w)% and the equation: Mass % = Mass of Solute Total Mass of Solution x 100 Equation 2 The advantage of this type of concentration unit is that it depends only on the mass, which is accurately measured with an analytical balance. It is not dependent on the temperature. Note: Volumes are dependent on temperature. For example, a 10.000 ml volume of water will increase by 0.016 ml when the temperature is raised from 18 o C to 25 o C. Table 1 gives the density of water at different temperatures. Another useful property is using the percent error to determine the amount a measurement is off from the theoretical value. The equation for finding percent error is: Percent Error = (Experimental Value Theoretical Value) Theoretical Value x 100 Equation 3 Modified from Lumen Learning 2014 CC-BY 4.0 https://lumen.instructure.com/courses/150410/modules Page 2

This allows us a more reasonable comparison of numbers than looking at the difference only because the magnitude of the theoretical value is considered. A table of the theoretical values of density for sucrose solutions of various (w/w)% is included in Table 2 below. Mass % Density (g/ml) Mass % Density (g/ml) 0.00 1.000 12.50 1.051 2.50 1.011 15.00 1.062 5.00 1.021 17.50 1.073 7.50 1.030 20.00 1.084 10.00 1.042 22.50 1.102 Table 2: Theoretical Density Values of Sucrose Solutions with Known Mass Percent Graphing Data It is imperative students learn to properly organize and graph data. Students may wish to review graphing data and calculating the slope prior to coming to lab this week if it has been a few years since you have had a math course. A brief review is included here but may not be sufficient for some students. Manual graphs should always: Be drawn on graph paper (included within the lab handout). Include data points (and possibly the labels as well). Have labels for the graph itself (named Y vs. X), the axes (with both name and units), and (if applicable) the legend. Be drawn large enough to visually see all components. Include axis scales that are appropriate (they may not start at 0, depending on the data). Contain a line of best-fit. Graphs done in Microsoft Excel should always Include all of the components of manual graphs. Be in the scatter chart type unless otherwise specified. Include the equation for the line of best fit. Modified from Lumen Learning 2014 CC-BY 4.0 https://lumen.instructure.com/courses/150410/modules Page 3

Materials Student tray containing the following: o 1-50ml beaker o 1-100ml beaker o 1-150ml beaker o 1-10ml graduated cylinder o 1 stir rod o 1 spatula o 1 Plastic pipet o 1 container of sucrose o 1 DI water bottle o Thermometer Student balance Safety and Notes Goggles for this and ALL labs must be worn whenever any lab group is using chemicals The water used in the lab should be D.I. water not tap water. MSDS for sucrose is to be reviewed before class. Use all proper protective gear to include goggles, aprons, and gloves. All waste may be disposed of in the back hood. Use the container labeled CHM 111 Waste. Label ALL glassware with your name, date, chemicals and concentration according to TCC policy. Return all materials to trays and drawers in a clean orderly condition for the next class. Experimental Procedure A. Preparation of the Sucrose Solution 1. The instructor will assign a sucrose concentration of between 5 and 15 % to each lab group. Record the assigned concentration in your data sheet. 2. In the designated space of the data section, calculate the approximate mass of sucrose that is needed to make a 100 g sample of the assigned concentration. 3. Obtain approval of the instructor before continuing. 4. Tare (zero) the scale to be used. Place a 150 ml beaker on the scale and record the mass in your data section. 5. Add the approximate mass of the sucrose calculated to the beaker and record the mass in the data section. 6. Add water to the beaker until you have reached ~100 g of solution (The mass will read the beaker s original mass + 100 grams). Record the final mass of the beaker, sucrose and water in your data section. 7. You may need to gently swirl or stir to dissolve the sucrose. B. Calibration of a 10.0 Graduated Cylinder 1. Record the temperature of the room. Modified from Lumen Learning 2014 CC-BY 4.0 https://lumen.instructure.com/courses/150410/modules Page 4

2. Tare a 50 ml beaker on an analytical balance. (The balance should read zero with the beaker on the measuring pan so that the beaker s mass does not need to be subtracted each time.) 3. Measure exactly 10.0 ml of DI (deionized) water using the graduated cylinder. 4. Transfer the water to the tared beaker and record the mass of the water added. 5. Pour the water down the drain. 6. Repeat this trial five times. Remember to tare the beaker each time it is weighed since it might still contain residual water. 7. Use Table 1 from the Introduction and the temperature of the lab to calculate the volume of DI water actually delivered by your graduated cylinder. C. Mathematically Calculating the Density of Prepared Sucrose Solution 1. Tare a 50 ml beaker on an analytical balance. 2. Measure exactly 10.0 ml of the sucrose solution measured in Part A. using the graduated cylinder. 3. Transfer the solution to the tared beaker and record the mass. 4. Pour the solution into a separate beaker. Do not dispose of it yet as you may need it later. 5. Repeat this trial five times. Remember to tare the beaker each time it is weighed since it might still contain residual solution. 6. Use the recorded masses and the volume of the graduated cylinder (calibrated volume from Part B) to calculate the density of the sucrose solution. D. Graphically Calculating the Density of Prepared Sucrose Solution 1. For this section students will be gathering information to graph. The graph will then be used to determine the density of the sucrose solution. Unlike Part C, the beaker should not be tared or emptied before adding more solution. 2. Obtain the mass of a clean, dry 100 ml beaker. For this section, do not tare the beaker. 3. Measure exactly 10.0 ml of the sucrose solution measured in Part A. using the graduated cylinder. 4. Transfer the solution to the beaker and record the mass. 5. Do not pour the solution out. 6. Measure another 10.0 ml of the sucrose solution and transfer it to the beaker. Record the mass. 7. Repeat this trial five times. 8. Graph the recorded masses and calibrated volumes in your data Table D. 9. The y axis should be the mass in grams. The y-intercept should be the mass of your beaker. You should scale your axis so that the data takes up most of the space. (Do not start at 0). 10. The x axis should have units of volume in ml. 11. The slope of the data will have units g/ml (and therefore be the density of the sucrose solution. Modified from Lumen Learning 2014 CC-BY 4.0 https://lumen.instructure.com/courses/150410/modules Page 5

Pre-lab Assignment/Questions N o t e this pre-lab must be finished before you come to lab. Remember to show your work and use the correct number of significant figures for full credit. 1. A student measured the volume and mass of three samples of methanol and obtained the results given below. Calculate the density for each sample, and then the mean (average) of the three measurements. [Average = Sum of all densities / number of samples] Mass (g) Volume (ml) Density (g/ml) 4.029 5.10 3.704 4.71 3.952 5.03 Average Density = 2. A student is asked to make up approximately 40.0 g of a 29 % (w/w) sucrose solution. Use Equation 2 from the introduction to calculate the g sucrose and g water needed to make this solution. 3. The student made the solution with the following data. Calculate percent error of the solution if it should have been 29% w/w sucrose. Mass of empty flask: 83.148 g Mass of flask with sucrose: 114.290g Mass of sucrose Mass of flask with water & sucrose: 183.816 Mass of solution Mass % of solution Percent Error Modified from Lumen Learning 2014 CC-BY 4.0 https://lumen.instructure.com/courses/150410/modules Page 6

Pre-lab Assignment/Questions Continued 4. While a 25 ml pipet should theoretically measure out exactly 25.00 ml, this is not always the case. A student determined the actual volume of a 25 ml pipet by measuring the mass of the water delivered by the pipet and then calculating the volume from the known density of water. The density of water is 0.99820 g/ml at 20 o C. This measurement was done three times, and the following results were obtained: Calculate the average volume delivered by the pipette and the percent error of the pipet. Trial Mass (g) Volume (ml) 1 24.926 2 24.915 3 24.941 Average Volume = Percent Error = 5. The above problem is an example of how an instrument (in this case, a pipet) can be calibrated. We expected the volume to be 25.00 ml, but the actual volume was significantly less. If you had measured the density of an unknown with this pipet and assumed that V = 25.00 ml, your result would not be correct. Would it be too high or too low? Explain your answer. Modified from Lumen Learning 2014 CC-BY 4.0 https://lumen.instructure.com/courses/150410/modules Page 7

Experimental Data and Results A. Preparation of the Sucrose Solution Before you begin, calculate the approximate mass of sucrose and water needed. Show all work and use the correct number of significant figures to receive full credit. Assigned Sucrose Solution (%w/w) Approximate mass of sucrose to be used Approximate mass of D. I. water to be used Record all data from the scale and determine the %w/w of the solution made. Mass of beaker Mass of sucrose + beaker Mass of sucrose + beaker + water Mass of sucrose Mass of solution Wt % of solution B. Calibration of a 10.0 Graduated Cylinder Record the temperature of the lab. Temperature of Lab = Density of water at this Temperature according to Table 1 = Modified from Lumen Learning 2014 CC-BY 4.0 https://lumen.instructure.com/courses/150410/modules Page 8

Determine the volume of water using the mass of water delivered and the density of water from Table 1. Trial # 1 Mass of Water (g) Volume of Water (ml) 2 3 4 5 Average Volume Delivered = **This is the calibrated volume used in Part C! Be sure to show an example of the calculations necessary to determine the volume of water delivered. C. Mathematically Calculating the Density of Prepared Sucrose Solution Trial # Mass of Solution (g) Density of Solution (g/ml) 1 2 3 4 5 Average Density = Show an example of the calculations necessary to determine the volume of water delivered. Modified from Lumen Learning 2014 CC-BY 4.0 https://lumen.instructure.com/courses/150410/modules Page 9

D. Graphically Calculating the Density of Prepared Sucrose Solution In this section you will collect data to graph. You will graph the mass of the solution + beaker vs. the volume of the solution. Mass of Empty Beaker Trial # 1 (1 x calibrated volume) Volume of Solution (ml) Mass of Solution + Beaker (g) 2 (2 x calibrated volume) 3 (3 x calibrated volume) 4 (4 x calibrated volume) 5 (5 x calibrated volume) Modified from Lumen Learning 2014 CC-BY 4.0 https://lumen.instructure.com/courses/150410/modules Page 10

In the space below: plot a graph of mass on the y-axis versus volume on the x-axis. All graphs must have each axis clearly labeled with numbers and units. The graph must have a title and legend. Draw a best fit straight line through as many of the points as possible. You should use a ruler to draw the line. This graph should not simply be a connect the dots line. 1. Find the slope of this line by using two of the most widely spaced data points you have measured which come closest to the best fit straight line that you have drawn with your ruler. Remember, slope= rise = Δ y / Δ x = (y 2 y 1) / (x 2 x 1) run Slope of the line (density) g/ml 2. Calculate the % Error of the Density of the solution you made by comparing it to the theoretical density in Table 2 from the introduction. 3. (Next, observe where the line intersects the y-axis. This number should be close to the mass of the empty beaker containing 0 ml of the solution. y-intercept of best fit line g 4. Calculate the % error comparing the y intercept value compared to the mass of the empty beaker. Modified from Lumen Learning 2014 CC-BY 4.0 https://lumen.instructure.com/courses/150410/modules Page 11

Post Lab Questions 1. Which method that you used for the calculation of density of the sucrose solution is the most accurate, Part C or D? Explain your answer. 2. When performing today s lab, you measure the temperature of the lab during the calibration of the graduated cylinder. You record the temperature as 2 degrees higher than it actually is. How will this error affect the calculated density (will the resulting calculated density be too high, too low or unaffected)? 3. When making the sucrose solution in Part A you measure the correct amount of sucrose on the scale. However some of the sucrose was spilled on the balance and not into the beaker. How will this error affect the calculated density (will the resulting calculated density be too high, too low or unaffected)? 4. When trying to measure the density of your solution, you do not quantitatively transfer the solution (there is still some in the graduated cylinder when you obtain the volume in Part D). How will this error affect the calculated density (will the resulting calculated density be too high, too low or unaffected)? Modified from Lumen Learning 2014 CC-BY 4.0 https://lumen.instructure.com/courses/150410/modules Page 12