Lab 1A Results 090822

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University of Northwestern St. Paul *

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1006

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Chemistry

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Jan 9, 2024

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Name: Student ID: Date: 09/6/2023 Lab 1A Results Measurements Record the uncertainties for which blanks are provided. Thereafter, we will assume that the uncertainties are the same for the rest of the values that are measured with that particular device. This will save a lot of tedium. Part 1: Measuring with a Pipet Trial Mass of 25.00 ml of Water, g 1 24.67 ± 0.1 2 25.09 ± 0.1 3 25.15 ± 0.1 4 24.84 ± 0.1 5 24.95 ± 0.1 Average mass of water, g 24.9 ± 0.1 Smallest mass of water, g 24.67 ± 0.1 Largest mass of water, g 25.15 ± 0.1 Part 2: Measuring with a Graduated Cylinder Trial Mass of 100.0 ml of water, g 1 98.88 ± 0.1 2 101.13 ± 0.1 3 100.25 ± 0.1 4 98.67 ± 0.1 5 99.77 ± 0.1 Average mass of water, g 99.68 ± 0.1 Smallest mass of water, g 98.67 ± 0.1 Largest mass of water, g 101.13 ± 0.1 Questions 1. Calculate the density of water using the average mass of the five 25.00 ml samples from Part 1. Show all of your calculations. Show all of your calculations. To indicate multiplication, use the asterisk, which, on most keyboards, shares the same key as the numeral 8. To indicate division, use the forward slash, which, on most keyboards, is on the same key as the question mark. Indicate addition and subtraction with the usual symbols. 24.9 / 25.00 = 0.996g/ml. Percentage Error = ∣ 0.996 − 0.9982 ∣ / 0.9982 x 100% = 0.22% 2. We will assume that the temperature of your distilled water was 20 °C. At this

temperature, the density of water is 0.9982 g/ml. What is your percentage error for the density that you calculated in Question 1 above? You will not be graded on how well your value compares to this given value, since density is affected by temperature and your water may not have been at exactly 20.0 °C. 3. The manufacturing tolerance of the pipet, which is related to its precision and accuracy, is sometimes marked on the pipette by the manufacturer and is probably about ± 0.03 ml for the pipet in the kit. Using your density from Question 1, what would be the mass of 0.03 ml of water? 0.03 / 24.9 = 0.00120482 g/ml 4. Suppose that the scale could only detect differences in mass as small as ±0.05 g instead of ±0.01 g, as with your scale. (In other words, a mass of 24.02 g would display as 24.00 g since the last digit on such as scale can only be a 5 or a 0, and 24.02 is closer to 24.00 than to 24.05.) Based on your answer to Question 3, if your pipetting technique is excellent, such that your volumes are actually identical to the nearest hundredth of a milliliter, what should be true of the five masses that you would obtain in Part 1 and how do your actual values compare to this prediction? (Hint: It might be instructive to list the values in this way: “24.72 g would appear as 24.70, etc.”) 24.7 25.1 25.2 24.8 25.0 5. Since the graduated cylinder is graduated in 1 ml increments, you can probably read the volume to the nearest 0.1 ml, perhaps 0.2 of a ml, or, at the very worst, 0.5 ml, which is half of the smallest graduation, or "least count". Even if your ability to measure with a grad cylinder is pretty good, ± 0.1 ml, what will likely be true of the five masses that you obtained in Part 2? How will the range of values for the graduated cylinder differ from the range with the pipet? For the graduated cylinder: Volume measurement accuracy: ±0.1 ml For the pipette: Volume measurement accuracy: Much more precise than ±0.1 ml, often accurate to the nearest 0.01 ml or better. 6. Let us assume that the uncertainties in making multiple measurements are additive. In other words, if you used the pipet ten times, the precision would be 10 times ± 0.03 ml or ± 0.3 ml. It is possible that the errors cancel each other out, too, but we will assume the worst case scenario. Using only the volumetric equipment that is in the kit (those designed to measure volumes as opposed to the digital scale which measures masses), what would be the best way to measure out 50 ml of water as precisely as possible? What would be the best way to measure out two hundred milliliters of water? Assume that your ability to measure with a graduated cylinder is pretty good, as explained in Question 5

above. I think that the pipette would be better, but the graduated cylinder would be better for larger numbers like two hundred milliliters of water. 7. A calibration weight used to be included in the kit, but now we calibrate the scales before putting them in the kits. The calibration weight is simply a chunk of metal machined to fairly strict tolerances. A one hundred gram calibration weight will generally be marked simply "100 g". This is a bit misleading, as there is no indication as to how precise this value is. Which of the following is a good estimate for the tolerance of the calibration weight? There is no one single correct answer to this question, but circle one of the following seven choices. ±1 g ±0.5 g ±0.2 g ±0.1 g ±0.05 g ±0.01 g ±0.001 g 8. Suppose that each one of the Seven Dwarfs has a tape measure, all of their tape measures are made by the same company, and each dwarf measures the length of the same used car at Joe’s Used Car Lot in order to see if the car will fit into their garage. Dwarf Car Length, inches Bashful 187.0 ± 0.5 Doc 188 ± 1 Dopey 189 ± 1 Grumpy 189 ± 1 Happy 188 ± 1 Sleepy 195 ± 1 Sneezy 188.5 ± 0.5 Which dwarf(s) seems to have the best eyesight or the most confidence in using a tape measure? Sneezy Does Sleepy’s measurement agree with Grumpy’s? no Does Sneezy’s measurement agree with Bashful’s? no What is the best value for the actual length of the car? 188.5 ± 0.5 What caused Sleepy’s measurement to be significantly different from the other six measurements? Give two different possible explanations. An error in the measurement process Sleepy's car could be a different model, make, or condition compared to the other cars Do not complete the next lab exercise until you receive your grade for Lab 1A.

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