Essay on chemistry 109 freezing point

Submitted By namoore14
Words: 2272
Pages: 10

INTRODUCTION EMBED Equation.3 Equation 1 In this investigation masses of an unknown solute and known solvent will be measured and combined to form a solution. The freezing point depression of the solvent will be measured and the molality of the solution will be calculated. With molaltity and mass of solvent known, the moles of solute and ultimately the molar mass of the unknown can be calculated. Colligative properties of solutions thus offer an effective means for determining molar mass. Along with freezing point depression other colligative properties include boiling point elevation and osmotic pressure. EMBED Equation.3 Equation 2 The freezing point depression will be determined graphically from temperature vs. time plots (Figure 1) for freezing pure solvent and solution. The pure liquids freezing point occurs at the plateau temperature (based on a best-fit line to the plateau data). The solutions freezing point will be the temperature at the intersection of two best-fit lines, one for the initial rapid temperature decrease, and the other for the more gradual temperature decrease that follows. Often a solutions cooling curve has a much less defined plateau region which requires determination of the points that are most equally distributed about a best-fit line. Solidification of a Pure Solvent Compared to Its Solution with a Nonvolatile Solute Figure 1 PROCEDURE SAFETY PRECAUTIONS Cyclohexane is flammable. No flames. Work in the hood Each pair of students will be assigned a hood and will work in the same hood all quarter. ALL WASTE MUST BE DISPOSED IN A DESIGNATED WASTE JAR GENERAL NOTES The freezing point apparatus must be clean and dry. Get unknown and cyclohexane from the stockroom. Cyclohexane will be individually dispensed to prevent contamination of the stock. Part A Determination of the Freezing Point of Cyclohexane Check out the freezing point apparatus. The freezing point apparatus will include a Nessler tube (a flat bottomed tube) and a Lab Quest with two temperature probes. Nest a 250 mL beaker in a 400 mL beaker to use for the ice bath. Record the mass of the empty Nessler tube to the nearest tenth of a milligram. Measure about 12 mL of cyclohexane (d 0.779 g/mL) with a clean, dry graduated cylinder and transfer to the Nessler tube, re-weigh and record the mass. Cover the Nessler tube with Parafilm( while setting up the LabQuest. Plug the temperature probes in channels (CH) 1 and 2. Turn on the Labquest. Tap File, then New. On the right window, make sure that Mode is set to Time Based. Tap on Rate and set the following Rate 4 Interval 0.25 Length 20 then the tap down arrow and tap min Tap OK Tap on the Graph Icon, then Graph, Graph Option. Tap Autoscale Type Time on the X-Axis Column Enter Left 0 Right 20 Graph 1 Y-Axis, enter Top 25 Bottom 0 In the Run 1 box, check Temperature 1 and Temperature 2 Check Point Protectors and Connect Points Tap OK. Make an ice bath with a least a 31 ratio of ice to water. Place the temperature probe connected to the CH2 in the ice bath. Remove the Parafilm from the Nessler tube and place the CH1 probe into the cyclohexane. Place the Nessler tube into the ice bath and press the start icon,. Carefully stir the cyclohexane with the probe through the freezing point determination. Monitor the temperature of the ice bath to make sure that its temperature is always lower than that for the cyclohexane. Add more ice and remove the water with a dropper to keep the ice water ratio 3 to 1. After the determination, insert your thumb drive. Tap on File, click Export. Type your file name, tap on the thumb drive icon, and press OK. Cover with Parafilm( until the next trial. Melt the cyclohexane by holding the Nessler tube with your hand. Part B Freezing Point of Cyclohexane with Unknown Solute Reset the LabQuest. Tap on File, then New. This will return the settings to default. Re-enter the settings as shown above with the