Burhan Riaz 09/06/2011 Measuring the Accuracy of Glassware Beginning Ideas: When performing most chemistry labs, accuracy is necessary to prevent miscellaneous results. The question then arises, which instrument is most correct in measuring? An Erlenmeyer flask seems too big to accurately measure a liquid because of its lack of readability when compared to other instruments. If someone misreads the measuring line in the Erlenmeyer flask, it could be a big error. The volumetric flask only has only one measuring line and probably isn't the best instrument to measure liquids. The volumetric pipet and graduated cylinder, however, are very slender which only allows them to contain a small amount of liquid and an abundance of measuring lines. We will be testing the accuracy of each measuring device by placing water into them and determining whether they contain the right amount of liquid by calculating the volume of water by using its corresponding density at a measured temperature. Tests: First, a 100ml beaker was filled to the brim with deionized H2O. This beaker was not used as a measuring tool but rather a container where the water was used by each instrument. A thermometer was placed in the beaker for approximately 5 minutes. The instruments where then weighed without any liquids on a digital centigram scale. After the temperature was recorded, all five measuring instruments were administered 10mL of water from the large beaker. Lab participants used their eyes to stop administrating water after hitting the 10mL mark in each instrument. In instruments where 10mL marks did not exist, participants used their good
judgment. The measuring instruments where then weighed again, this time with the water. These trials were done twice to each instrument. Using the temperature from the thermometer and the calculated weight of water in each instrument, the theoretical density was used to a calculate the volume. Please note that the volumetric pipet could not easily be weighed so the 10mL from the pipet were transferred to the graduated cylinder to be weighed. The measuring instruments should be of the smallest scale to accurately measure 10mL. Observations: Data Table Weight w/o H2O (g)
Trial 1 (g)
Trial 2 (g)
Graduated Cylinder
38.89
48.97
48.81
Beaker
23.99
31.51
23.99
Erlenmeyer Flask
40.11
46.51
46.03
Volumetric Pipet
*
48.89
49
Volumetric Flask
23.26
NA
NA
* The weight of the volumetric pipet was not used due its difficulty to place it on the scale. Instead a graduated cylinder was used. Also, due to time constraints, the volumetric flask was not tested upon. Temperature of water was recorded at 19C. When each trial was complete some droplets of H2O still remained in the instruments even after attempting to dry them. Evidence:
Calculated Data Average weight (g)
Weight of H2O (g)
Volume of H2O (mL)
Graduated Cylinder
48.89
10.00
10.01
Beaker
27.75
3.76
3.79
Erlenmeyer Flask
46.27
6.16
6.17
Volumetric Pipet
48.95
10.07
9.30
Volumetric Flask
NA
NA
10.09
The average of both trials for each instrument were calculated by the equation (Trial 1 +Trial 2)/2 The weight of water was calculated by subtracting the weight of the instrument before the trials from the average weight. Ex. Graduated cylinder's weight of water was 48.89 - 38.89 = 10.00g. The volume of water was calculated by dividing the weight of the water by the density at 19C. Density = mass/volume or in our case .998405g/mL1. This would give us a figure in mL and cancel out the grams. Percent error of the closest value to mL (graduated cylinder) was calculated by (actual valuetheoretical)/ theoretical value x 100. (10.01mL - 10mL)/ 10mL x 100 = 1% error Claim: According to the data, the graduated cylinder and the volumetric pipet came quite close to the intended 10mL volume initially measured. The graduated cylinder came the closest and yielded
only a 1% error. Instruments such as the beaker or Erlenmeyer flask did not even come close and are not considered by the evidence to be good measuring tools. Reading: There is a difference between accuracy and precision. Precision is how close your measurements are to each other over multiple trials while accuracy is how close the measurements are to the intended theoretical value2. For example, our trials for the graduated cylinders were precise and accurate. They were precise because both trials produced similar results and were also correctly measuring about 10mL. The Erlenmeyer flask was somewhat precise in that it produced similar results but it was inaccurate because it significantly deviated from the intended theoretical value of 10mL. When recording measurements it is important to include significant figures. This included all of the known numbers and the last number which may deviate on an electric scale3. The instruments used formed a visible meniscus when water was poured in. When a meniscus is concave, it should be read from the bottom. Density was an important constant in this experiment. It was famously discovered by Archimedes through his observations with water and how much it rises when different objects are placed inside of it3. The equation of density is d = m/v. Through this equation we can manipulate the numbers to give us mass or volume. In this case we had the density and mass and used it to find the volume. Reflection: Looking back at our evidence and the original question of what instrument is correct, I have changed my idea on what our experiment actually found out. I think because we measured 10mL of water into each instrument, our question became 'what instrument best measures 10mL.' If
we were to repeat this procedure with 50mL instead we may have gotten different results. For example there were no 10mL ticks in the Erlenmeyer flask and beaker. We simply used our best judgment to guess how much we should pour in. That being said our conclusion that the graduated cylinder is the best tool to measure 10mL of water still holds true because the beaker and Erlenmeyer flask are not good tools to measure small amounts of liquids anyways. The reverse is true if 100mL of water needs to measure. You would need to use the volumetric pipet for example 10 times just to do it and this is an inconvenient way of measuring. Therefore there is not one best instrument. It depends on how much is being measured. Also, because a meniscus formed in the volumetric pipet and in the graduated cylinder, our results may change if the procedure is replicated. Reading the meniscus is prone to human error. Despite these fallacies, the graduated cylinder and the pipet had good accuracy and precision when measuring 10mL of water while the Erlenmeyer flask and beaker had neither. By using the density to calculate volume, the graduated cylinder came closest to the 10mL mark and had the smallest percent error. News Brief: Have you ever noticed that when you get into a bath tub, the water rises? What causes such an action? Everything has a mass or how heavy it is without gravity. When you take something with mass and place it into the bathtub it will rise depending on the mass. This phenomena is called density and everything has it. If you pour cooking oil into a jar of water you will notice that it floats above the water. This is because the density of oil is less than water. The property of density can be very important when trying to calculate mass or volume. Archimedes, a Greek mathematician, used this property to test the validity of king's crown. Since gold and silver have different densities, they should have different results when placing them in water. Archimedes
was able to find out that the king's crown had bits of silver because silver takes up more space in water than gold does. Citations: 1
Chemistry 2046L Laboratory Manual for Chemistry Fundamentals II
2
'Precision and Accuracy.' Web. 06 Sept. 2011.

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