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Determination of the identity of an unknown liquid Essay

Purpose
The purpose of this lab is to determine the identity of an unknown liquid by measuring its density and its boiling point and try to match it with those solutions given in Table 2 of experiment 2.

Procedure
In Part A, The main purpose was to find the determination of the density of the unknown (j41) and by doing that we had to determine volumes of the unknown liquid (j41) using three different volumetric devices which are graduated cylinder, pipette and burette. Then we had to perform three measurement trials on each device.

Graduated cylinder: I dried a clean 50.0 ml graduated cylinder and also measured with a balance and I got 66.39g after measuring the empty cylinder I added 15.0ml of the unknown liquid (j41) and measured it and I got 77.65g and I continued adding 15 ml until I had the total liquid in the cylinder to be at 45.0ml and I got 89.30g and 100.72g for my trials at 30.0 ml and 45.0 ml.

Pipette and burette: I weighed a clean dry 150 ml beaker and I saw it was 64.21g and I used a pipette to measure out 10ml of the unknown liquid (j41) and poured it into the beaker and reweighed the beaker which was now 83.22g and i added more 10ml to my 2nd trial and 20ml for my 3rd trial and I got 102.41g and 121.35g respectively. (I accidently used a 25ml pipette which I noticed at the end of the lab so instead of adding 10 ml each into each trial I was adding 25ml).

I filled a burette with the unknown liquid (j41), having an initial burette reading of 0.00ml and by using the burette I was pouring in 13ml of the liquid into the beaker and measuring the mass of the beaker for the 1st trial with 13ml I found its mass to be 74.02g and the 2nd trial with 26ml I found its mass to be 83.95g and on the 3rd trial I found its mass to be 93.68g.

Part B In the part I clean and dried a 75 ml test tube and added 15 drops of the unknown solution (j41) into the test tube. Given a capillary tube I kept it into the test tube with its open end in the solution I kept the test tube after I attached it with a thermometer in a water bath and started boiling it. I did this experiment three times changing the capillary tube each time and got 99.0OC, 98.0OC and 100.0OC for the three trials respectively.

Total Average density= (0.759 + 0.762 + 0.757)/ 3 = 0.759g/ml Using the table to find out my unknown solution (j41) I noticed that my density was closer to 2-propanol (isopropyl alcohol) than any other liquid on the table but my boiling point was closer to 1propanol than any other liquid on the table. But to go with one liquid I will say my unknown was 2-propanol (isopropyl alcohol) because it was closer to the density I calculated for my unknown and I know my density experiment went very smoothly I trust my density more than my boiling point.

Guided Discussion
1) The datas that were key in identifying my unknown was the density and boiling point because the data table given to help identify the unknown element contained only the density and boiling point of the liquids given. They we others key values to find like the mass of the liquid and some other datas but they were only key in finding the density and boiling point but not the unknown liquid (j41). I don’t think there was any data that was useless in this experiment because each made a participation in finding the density or boiling point of the unknown solution.

2) When measuring the density of the unknown liquid I used the graduated cylinder, volumetric pipette and burette. The graduated cylinder was accurate to +-0.5 while the volumetric pipette was accurate to +-0.01 and when measuring with the pipette I noticed that I measured 10ml with the pipette but when I poured the liquid into a beaker it was measuring up to 40ml that was when I concluded that the pipette was very accurate and very much more accurate than the beaker.

The burette was accurate to +- 0.04, the burette is so accurate but not as accurate as the pipette. The pipette is like a calculator with the full value of the measurement given while the burette is also like a calculator but with estimating values and the graduating cylinder is like a human trying to get the measurement of liquid with just his/her brain. If I was to repeat this experiment I would use the volumetric pipette since it is the most accurate out of the three.

3) The nature of the devices obviously limited the precision of my measurements and there was a significant error when measuring the density of the unknown liquid using the pipette I accidently used a 25ml pipette instead of 10ml which would have made my average value for my density get higher because when I was

calculating the density of each trial I was dividing by 10ml not aware of my mistake and I was getting values like 1.901,1.910 and 1.913 which were outside the range of 0.75 – 0.76 I was got when I calculated the density for the other devices but when my T.A. notified me about it which was after I was done with lab I just divided by 10ml and got within the range.

4) There isn’t any much of an advantage between the graphical method and numerical method to me I think both the graphical

and numerical method are easy to do and can be done without any stress and wouldn’t waste any time since we have the
calculator, calculating with masses like 11.26g, 22.9g or 33.33g using the graduated cylinder wasn’t hard at all. It is also the same finding the density graphically it was made very easy with the excel available all my data in their third decimal place and even the scale to use didn’t pose as a problem at all.

5) I didn’t exclude any data point in my analysis. 6) My boiling point would not be so accurate especially my 1st trial becausein my 1st trial I didn’t add boiling chip in my water bath so it very hard to notice the bubbles at the end of the capillary tube until 99.0oC that I saw bubbles faintly at the end of the capillary tube but no liquid seemed to be entering the tube until I noticed that my capillary tube wasn’t in the unknown liquid. My 2nd and 3rd trials went smoothly. And I also noticed that it is very difficult being so accurate while checking for the bubbles and so looking at the thermometer at the same time because we can see the liquid rising and before we look at the thermometer to see the

temperature it happened the thermometer would have risen some degrees and one might not even notice the bubbles at the end of the capillary tube on time because of the bubbles in the water bath so it is very hard to be so accurate in the boiling point. 7) The density of the unknown liquid was more accurate than the boiling point because the in finding the density I had to find the mass and the volume of the unknown liquid using three devices having three trials each and each of their average density were within the range of 0.75 – 0.76 g/ml but for the boiling it is hard to be accurate because it was only a method used and only three

trials available and trying to find the boiling I had to be very observant so that I could see when the unknown liquid was rising in the tube immediately which was made more difficult by the bubbles in the water bath.

8) As I said earlier in finding the unknown liquid there are two main datas needed to find them which are density and boiling point and if these key datas are found accurately then the unknown liquid will be found accurately also. The procedural change I will make is in the measurement aspect and specifically the

measurement of the masses of the devices used like beaker and cylinder I would have measured their masses three times and find out the average mass which I will now use in the calculation.

 


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