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DENSITY OF WATER AT ROOM TEMPERATURE Bassam Attili, PhD Collin College Chemistry Department Introduction: All substances have properties that we can use to identify them. There are two basic types of properties that we can associate with matter; physical and chemical properties. Physical properties are the properties that do not change the chemical identity or nature of matter. Chemical properties are properties or changes that do change the chemical nature of matter. The more properties we can identify for a substance, the better we know the nature of that substance. These properties can then help us model the substance and thus understand how the substance will behave under different conditions. There are two types of physical properties. An intensive property; which is also called a bulk property, is a physical property of a system that does not depend on the system size or the amount of material in the system: it is scale invariant. By contrast, an extensive property of a system depends on the system size or the amount of material in the system. Density is one of the most important physical properties. It relates the mass of an object to its volume. It is defined as the mass in grams per unit volume. Density is an intensive quantity (it does not depend on the quantity), while mass and volume are extensive quantities. Note that the ratio of two extensive quantities that scale in the same way is scale-invariant, and hence an intensive quantity. Density Mass( g ) Volume(ml ) Density can be calculated from measurements of mass and volume or as a slope of the mass versus volume graph. It is typically measured in g/ml for liquids and solids and g/l for gases. Density decreases with an increase in temperature. The known density of water at different temperature can be obtained from a reference book. Lab techniques: Graduated Cylinder and Pipette usage: For accurate volume measurement, a graduated cylinder (Figure 1) and a pipette (Figure 2) are used. Good lab technique requires that one uses clean glassware in all steps of an experiment. Also, a pipette pump or pipettor, Figures 2 and 3, are used to fill pipettes. Figure (1) Figure (2) Figure (3) Water, when in a glass container, normally forms a meniscus (Figure 4), so it is important to be at eye level with the meniscus and read the bottom of the meniscus for an accurate volume. Figure (4) Figure (5) Analytical balance usage: Chemistry lab is always equipped with an analytical balance (Figure 5) that can measure masses to the nearest 0.01, 0.001 g. The following are guidelines in using the balance. - - Do not weigh objects directly on the balance pan. Always use weighing paper or weighing boat. Keep balance ON all the times. Use the same balance for repetitive measurements. Do not shake or move balance from its place. Balance must be leveled all the time. Weigh for a stable reading. Samples that are weighed must be at or very close to room temperature to avoid air currents in the balance chamber. Weighing samples that are at elevated temperatures will result in inaccurate measurements. Remove containers and clean spills immediately. - - When taring your container, wait until balance shows stable zero before placing anything in it. The mass at that point will be the mass of what is in the container not including the mass of the container itself. Clean and dry the balance immediately. Experiment Objectives: Verify that the mass of a substance is directly proportional to its volume Determine the density of water at room temperature using two different methods Perform statistical analysis to determine the density from the graph and to calculate the mean and standard deviation of the average values calculated Suggested Procedures: You will be provided with a beaker, a 25 and 50 ml graduated cylinders and a pipette. Use only the glassware assigned for each method and do not use the same the glassware for both. Make sure to measure the water temperature and to use different quantities (volumes) for the three trials for each method. Method I Method II 100 ml beaker 25 ml graduated cylinder Electronic balance (0.001g) 50 ml graduated cylinder 10 ml pipette Electronic balance (0.001g) Organize the collected data and do statistical analysis for the three trials for each method separately. Draw the graph of mass versus volume and measure the slope for each. Statistical Analysis: Statistics is used when no absolute or accepted value exists. One must rely on the precision of the data collected to determine the validity of the conclusions. This can be done conveniently by calculating the following: Mean or average value (AVG), AVE (Mean) = Sum of individual trials / Total number of trials Deviation from the average (DEV), DEV = AVE – trial (one for each trial) Average deviation (AD) AD = Sum of the DEV / Total number of trials Relative average deviation (RAD) from your data; computed usually as a percent relative average deviation. RAD = (AD / AVG ) * 100% This establishes how significant the average deviation is in proportion to the measured value for the percent salt. This type of calculation can easily be automated using a computer spreadsheet. Critical Data/Discussion to Include in your Lab Report: Note; Follow instructions given by your lab professor in organizing your report. Graph mass versus volume using Microsoft Excel on the lab computer Determine the density of water at room temperature from the slope of the graph Compute the mean value, deviation, average deviation and relative average deviation for each procedure Discuss your interpretation of result and your conclusion. Calculate density percent error Include a data table and plot with your report.