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Enviado por: miangasu 15 octubre 2013
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Practice 4. DENSITY AND A CALIBRATION CURVE
Density is a characteristic physical property of a pure substance (element or compound). It is an intensive property derived from a ratio two extensive properties, mass and volume. Therefore, a sample of larger mass and volume has the same density as a sample of the same substance with a smaller mass and volume. Only changes in pressure or temperature alter the density of an element or compound. Density is determined experimentally by measuring the mass of an accurately known volume of sample (eqn 1):
Example: A student finds the mass of a clean dry 25 mL beaker, 2.348 g. After using a volumetric pipet to deliver 2.00 mL of mercury to the beaker, she finds the combined mass of the mercury and beaker, 29.548 g. She calculates the density of mercury as follows:
Density = Mass / Volume = (29.548 g – 2.348 g) / 2.00 mL = 13.6 g /mL
In this experiment, solutions will be created from the combination of two pure substances: water and sugar. Density measurements of solutions and pure substances have many similar characteristics with one notable exception: Solution density changes with solution concentration. This effect of sugar concentration on solution density will be analyzed. The sugar concentration will be quantified as mass percent (also known as weight percent, %w/w). Mass percent is determined by dividing the mass of the solute by the mass of the solution and then multiplying by 100 (eqn 2):
Density will be plotted as a function of mass percent to create a calibration curve. A calibration curve results from the graphing of some dependent variable versus standard solution concentration (independent variable). Ideally the calibration curve is not a curve at all, but a straight line. Once the dependent variable (density in this experiment) is found experimentally for a solution of unknown concentration, the “linear” calibration curve can be used in two ways to determine the solution’s concentra
tion (the independent variable). The first way is visual: a horizontal line is drawn from the value of the experimentally found dependent variable on the y axis to the calibration curve; a vertical line is then drawn to the x-axis to determine the value of the independent variable (the unknown solution concentration). The second way is mathematical, utilizing the line equation for the calibration curve. This equation is easily generated by a graphing program (for example Excel).
Safety goggles and aprons must be worn in lab at all times. Do not ingest any of the solutions used or prepared in the laboratory, as they may be contaminated. Clean up any spills immediately.
Plug in the experimentally found value for y and solve for x to find the solution concentration.
Students will use volumetric pipets and digital scales to prepare sugar solutions of known composition. The density of these solutions will be determined and a calibration curve of den ...
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