Density Measurement, Calibration of a Thermometer and a Pipette Accuracy and Precision in Measurements

Objectives To measure the density of an unknown solid To calibrate your alcohol thermometer To calibrate your volumetric pipette To gain an appreciation for precision and accuracy in temperature, volume and mass measurements in this lab Objectives

Alcohol Thermometers We use alcohol thermometers which are safer and cheaper than Hg ones The liquid used can be pure ethanol, toluene, kerosene or iso-amyl acetate, depending on the manufacturer and the working temperature range. The liquids are all transparent, so a red or blue dye is added yours is suppose to measure to 110oC Less Accurate at high temperatures!

Resistance Temperature Detector A temperature sensor is an RTD These have a thin film of Pt – the thin-film’s resistance depends on temperature. Knowing the resistance as a function of temperature means it can be used as a thermometer Needs software that knows the resistance vs. temperature characteristic for your temperature sensor Pay attention to the type of sensor you use and make sure you use the correct program with it Precision of 0.01oC Accuracy of 0.15oC at 0oC Resistance Temperature Detector

The most accurate way to measure volume in the lab is using either a pipette or a burette Scale marks on burettes read the volume every 0.1mL, so the best we can do is report the volume to a hundredth of a mL The precision of a burette or a pipette is 0.01mL Volume

Part I: Measuring the density of an ‘unknown’ solid This method works only for solids, insoluble in water and more dense than water, or liquids immiscible in water Take an unknown and record its number Weigh a 50.00 mL Volumetric flask m1 = mflask Add your entire unknown to the flask Reweigh the flask and its contents m2 = mflask + munk ½ fill with DI water from your wash bottle ensure no solid pieces stuck to the neck, tap to remove air bubbles Add water up to neck of flask Use a dropper to fill the volumetric up to the meniscus Remove any water adhered to neck of the flask above the meniscus Reweigh the flask m3 = mflask + munk + mwater

Calculation of dunk description label 𝑑 𝑢𝑛𝑘 = 𝑚 𝑢𝑛𝑘 𝑉 𝑢𝑛𝑘 We need Vunk Mass empty flask M0 Mass flask and unknown M1 Mass flask, unknown and water M2 Mass unknown = M1 – M0 Munk Mass water = M2-M1 Mw Density water = 0.998203 g/mL dw 𝑑 𝑢𝑛𝑘 = 𝑚 𝑢𝑛𝑘 𝑉 𝑢𝑛𝑘 We need Vunk 50 mL = Vw + Vunk Vw = mw/dw Vunk = 50.00 mL – Vw Calculation of dunk

Part II: Thermometer Calibration With a partner obtain a Vernier GoDirect Temperature probe and a LabQuest 2 interface Construct the set-up on the left with 125 mL of ice and 75 mL tap water in the 250 mL beaker. (see instructions) Gently stir and when the temperature on the LabQuest stabilizes at or near 0oC read the alcohol thermometer to the nearest 0.1oC this is TR and record the sensor temperature TS Empty beaker at 200 mL of tap water and record the temperature according to the sensor Ts and the temperature according to the thermometer TR at roughly 20oC, 40oC, 60oC, 80oC, and (gentle) boiling, each time turn off Bunsen at the desire temperature and allow the temperature to stabilize before reading Read P on the barometer and look at the tabulated boiling point of water at this pressure Calculate the correction Tc and use it to find the actual temperature TA Graph TR (x-axis) vs TA (y-axis) in Excel. Add a Trendline and record the trendline equation Lab Quest 2 interface

Part II: Thermometer Calibration The sensor will have a small systematic error over the range of the experiment. To assess it we need to know some temperatures exactly then we can assess this error. Here we use the freezing point of water (0.00oC) and the boiling point of water. We then calculate the average systematic error TC for these 2 points 𝑇 𝐶 = 0.000 𝑜 𝐶− 𝑇 𝑆 𝑖𝑐𝑒 + 𝑇 𝑎𝑐𝑡𝑢𝑎𝑙 𝑏𝑜𝑖𝑙𝑖𝑛𝑔 − 𝑇 𝑆 𝑏𝑜𝑖𝑙𝑖𝑛𝑔 2 𝑇 𝐴 = 𝑇 𝑆 + 𝑇 𝐶 Graph TR (x-axis) vs TA (y-axis) in Excel. Add a Trendline and record the trendline equation

Part III: Volumetric Pipette Calibration

Part III: Using the Calibrated Thermometer to correct the density Now we are going to correct our answer from part A for the density duncorr The calculation assumed that we were at 20.0oC for which the density of water = 0.998203 g/mL Use your measured temperature Tr of the water from part A to obtain the true temperature (using your calibration curve) Use the CRC Handbook to find the true value for the density of water at room temperature and recalculate the density of your unknown let’s call the dcorr Calculate the error