CHEM 1211K - Determining Density Lab PDF

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Determining Density Introduction Density is the ratio of mass over volume for an object or material (. Since density = mass / volume, the typical units used by chemists are either g/cm3 or g/mL (recall that 1 cm3 = 1 mL by definition). Density is also an example of a derived unit (mass/volume). Density is an intensive property, i.e. a property that is the same regardless of amount of material. For example, 15 mL of water has the same density as 125 L of water. Mass and volume are examples of extensive properties, i.e. properties that depend upon the amount of material present. Intensive properties are useful because they can be used to characterize an unknown material. You will determine the density of an unknown metal and use that to identify the metal. Determination of density requires at least two separate measurements: one for mass, and one for volume. Although mass is typically measured using a balance, there are several ways to determine the volume of an object. For liquids, the volume can be measured directly using a graduated cylinder or other glassware. For solids, however, this will not work. One common method of measuring the volume of solids is via displacement. Typically, this involves taking a known amount of water, plunging an object in the water and observing the differential volume of the object (i.e. the displaced volume). Objectives 1. Determine the density of both a solid and liquid sample 2. Use density to identify an irregularly shaped material 3. Distinguish between intensive and extensive properties STEM Skills Practiced in this Experiment 1. Technical skills – utilize a balance and glassware 2. Data Analysis – calculate densities from experimental data 3. Graphing – generate graphs from your data and use this to find density Procedure COVID-19 Safety Protocol:  Wear goggles, a mask, and gloves at all times while completing this lab. If you touch your face or leave the lab, please change your gloves.  Only use reagents and supplies on your lab bench. Do not remove supplies from lab drawers. If you are missing something, please ask your instructor for assistance. You will only have 1 set of reagents per lab group, but each student should have their own glassware and other supplies.  Dispose of your waste in the waste beaker at your lab bench. At the end of the lab period, your instructor will direct you to dispose of your waste beaker in the designated lab waste container.  Minimize moving around the lab as much as possible. Only use the balance on your lab bench, and only one person should be at the balance at a time.  At the end of the lab please wash all supplies with hot water and soap and return them to your lab bench. Be sure only 1 person is at a sink at a time. Wipe down and sanitize your lab bench, wash your hands with soap and water for at least 20 seconds, and wait for your instructor to dismiss you from lab.  During this experiment students working in groups of 2 should alternate making measurements and being the observer. For example, one person might do the measurements for the first and third trials of Part 1 while the other person does the measurement for trial two of Part 1. Then roles might reverse for Part 2. When not taking measurements, you should be observing and recording data. Part I: Mass and Volume Measurements for Water 1

Densities of liquids can be easily determined through mass and volume measurements. 1. Prepare a table similar to the one shown below in your lab notebook: Table 1: Mass, Volume, and Density of Water Volume of water (mL) Mass (g)

Density (g/mL)

2. Select three approximate volumes of water between 20 and 100 mL. 3. Using a 100-mL graduated cylinder, record the actual volume for these three volumes to the correct number of significant figures in Table 1. You do not need to strive to get the “exact” amount of water you chose; just record the actual amount in Table 1. 4. Using a 150-mL or 250-mL beaker and the balance, measure the mass of each water volume you measured out in Step 3 and record this in Table 1. Be sure to tare the balance so you are only measuring the mass of the water. Part II: Mass and Volume Measurements for an Unknown Metal Densities of solid objects can also be determined. However, in this case, the volume must be measured via displacement. 1. Collect approximately 30 pieces of metal. 2. Prepare a table similar to the one shown below in your lab notebook: Table 2: Mass, Volume, and Density of Unknown Metal Pieces of metal

Mass (g)

Initial Water Volume (mL)

Final Water Volume (mL)

Metal Shot Volume (mL)

Density (g/mL)

10 pieces 20 pieces 30 pieces Make sure to leave enough room to record all the necessary measurements. 3. Measure and record the mass of all 30 pieces of metal in Table 2. 4. Using water displacement, measure the total volume of the 30 pieces of metal. Follow these basic instructions: (a) Pour ~5 mL of water into a 10-mL graduated cylinder. Record this initial volume in your table to the correct number of significant figures. 2

(b) (c) (d) (e) (f)

Gently slide the metal pieces into the graduated cylinder (do NOT drop them in; they will splash and cause error in your measurements). Be sure to shake or tap the graduated cylinder to get rid of any air bubbles. Record the Final Water volume. Determine the Metal Shot Volume by difference. Be sure to dry all objects when finished.

5. Repeat steps 3 and 4 for 20 pieces of metal, and then 10 pieces of metal. Record all measurements in Table 2. 6. Dry all of the metal shot using a paper towel before returning it to the appropriate container. Calculations Part I: Density of Water 1. Calculate the density of each sample of water using the density formula and record these in Table 1. 2. Calculate the average density of water using the three measurements and record this value (including units) in your laboratory notebook. 3. Another way to determine the density from multiple measurements is graphically. This can be useful when there is a large number of measurements and it is inconvenient to calculate the density of each measurement. It is also useful because it makes it easier to see any outliers in the data (i.e. any points that are very far off from the rest of the data). Using Excel, construct an XY scatter plot with mass on the Y-axis and volume on X-axis. Add a linear trendline and R-squared value to your chart along with an appropriate label for each axis and a descriptive title. Draw the graph in your lab notebook. Be sure to include a numbered title (“Figure 1: Graph of…”), axis labels (with units), the equation, and the R-squared value. If you need additional assistance take a look at this YouTube tutorial: https://www.youtube.com/watch?v=Q6itzVJaoVw. Based on the graph, determine the density of water. Record this value in your notebook. Be sure to include units! Part II: Density of Unknown Metal 4. Calculate the density of each sample of metal (10 pieces, 20 pieces, and 30 pieces) using the density formula and record these in Table 2. 5. Calculate the average density of the metal and record this value (including units) in your laboratory notebook. 6. Graph your mass and volume measurements in the same way as you did for the Part I data. Sketch the graph in your laboratory notebook. Based on the graph, determine the density of the metal (with units). Analysis and Discussion 1. Density is an example of an intensive property. a. Do the results from Part I and II demonstrate this? Why or why not? Keep in mind that random experimental error will always prevent them from being exactly the same, but you can check if the values are consistent. b. If the answer to part (a) was “no”, what errors might have caused this? “Human error” is not an acceptable error; be specific about the type(s) of error that may have impacted the results.

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2. Compare the densities determined using the density formula to the densities determined graphically. Are they similar? Which method do you believe is better (or do you believe they are equally useful)? Why? 3. Based on the determined density in Part II and Table 3 below, suggest the identity of the unknown metal. (Recall that 1 mL = 1 cm3.) Table 3: Densities of various metals. Metal Density (g/cm3) Magnesium 1.74 Aluminum 2.70 Yttrium 4.47 Titanium 4.54 Zinc 7.13

Metal Tin Cadmium Nickel Copper Lead

Density (g/cm3) 7.31 8.65 8.90 8.96 11.35

4. Assuming the metal was correctly identified, calculate the percent error in the determined density. Use the density determined graphically for this calculation.

5. Water has a known density of 0.997 g/mL at room temperature. Calculate the percent error in the determined density. Use the density determined graphically for this calculation. Conclusion Summarize the lab by answering the following questions in your lab notebook. Check with your instructor to see whether you should just respond to the questions or if you should place your responses in a single paragraph. 1. What was the main topic/concept investigated today? 2. How was this concept investigated? 3. What were the main results found in this experiment? Include both quantitative results (i.e. important calculated values, including percent error) and qualitative results (i.e. important properties that were investigated). 4. Summarize the main errors encountered in this experiment and how they may have affected the results.

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