KEYExam1 - Key to Exam 1 Chapters 1-5 CE 3400 Introduction...

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Key to Exam 1 – Chapters 1-5 CE 3400 – Introduction to Environmental Engineering 17 September 2010 Question 1. Worth 10 points. Pipets (photo below) are a basic type of laboratory equipment designed to accurately deliver a desired volume of pure liquid or liquid solution. For this reason, knowledge of the precision and the error (measured as accuracy) associated with specific pipets is important. In my lab, our most commonly used pipet is one that measures 1000 μ L. The manufacturer lists the error (=accuracy) on this pipet as + 0.6%. From time to time, we check the pipets to be sure they are still accurate. We do this by weighing successive aliquots of deionized water and converting them to volumes. Since 1000 μ L of water weighs 1000 mg (=1 g) at room temperature, we are able to quickly and accurately assess the precision and accuracy of any given pipet. The following is data I collected recently on one of my lab’s 1000 μ L pipets. To weigh the aliquots of water, I used an analytical balance (weights are in grams) that has a limit of 0.001 g. Measurement 1: 1.000 g Measurement 2: 1.003 g Measurement 3: 1.004 g Measurement 4: 0.999 g Measurement 5: 1.003 g Based on these measurements, what is your assessment of the condition of my pipet? In other words, shall I continue to use it in my lab or send it in for recalibration? Please justify your answer in a quantitative way using any/all of the information I have provided! Since the manufacturer’s error is listed as + 0.6%, this means that the range of accuracy for this 1000 mL pipet is between 994 mL and 1006 mL which corresponds with the weight measurements of 0.994-1.006 g that were obtained. None of the measured values exceeds this range of values, so I would conclude that the accuracy of the pipet is still intact, and it can be continued to be used in the laboratory with confidence that it’s accuracy is meeting expectations. Question 2. Worth 10 points. Explain the main difference between a zero-order chemical reaction and a first-order chemical reaction? A very brief couple of sentences should do. I don’t need to see any equations or reactions in your response. The rate of a zero-order reaction is independent of the concentration of reactants (r = -k). In contrast, a first-order reaction is dependent on the concentration of reactants (r = -k[C]). Rubric for grading: Many of you provided lots of information about the characteristics of these two types of reactions, but if you did not mention the relationship of respective reaction rates to the concentration of reactants, you received no more than 5/10 points. Question 3. Worth 10 points. The effluent from a wastewater treatment plant is discharged into a nearby river at a flowrate of 50 mgd and has a solids concentration of 0.05%? What is the solids loading rate into the river in terms of lb/day?
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This note was uploaded on 12/08/2010 for the course CE 2400 taught by Professor Colberg during the Spring '10 term at Wyoming.

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KEYExam1 - Key to Exam 1 Chapters 1-5 CE 3400 Introduction...

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