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

2. 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

3. 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!

4. 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

5. 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

6. 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 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

7. 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 = g/mL dw
𝑑 𝑢𝑛𝑘 = 𝑚 𝑢𝑛𝑘 𝑉 𝑢𝑛𝑘
We need Vunk
50 mL = Vw + Vunk
Vw = mw/dw
Vunk = mL – Vw
Calculation of dunk

8. 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

9. 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 = 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

10. Part III: Volumetric Pipette Calibration

11. 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 = 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