Lab Exam 1

Lab Exam 1 - Gaurav Gharti-Chhetri TA Answer Sheet LAB 1...

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Unformatted text preview: Gaurav Gharti-Chhetri TA: Answer Sheet 10/04/08 LAB 1: Prelab assignment Answers: 1) (2pts) 4.00% (by weight) of 60.0 grams solution is: 4.00%/100.% × 60.0g = 2.40 g sugar 2.40 g of sugar is required to prepare 60.0 g of solution with 4.00 % (by weight ) of sugar. Other results: To prepare 60.0 g of solution with concentration of sugar (by weight): 8.00 % requires 4.80 g of sugar 12.0% requires 7.20 g of sugar 16.0% requires 9.60 g of sugar. 2) (2pts) weight % of solute, x = (y – 0.987)/4.81 × 10-3 = (1.045 – 0.987) / 4.81 × 10-3 = 12.06 %. Solution weight is 100. g. The amount of solute in 100. g of solution is 12.06 %/100.% × 100. g = 12.06 g = 12 g. Note that the final result is rounded to 2 sig.fig.. Explanation: (1.045 – 0.987) = 0.058 (2 sig.fig). In this case you may deduct only a tiny fraction of points (e.g. 0.2) if the result is given as 12.1 g (3 sig.fig). But explain why the correct answer should be given to 2 sig.fig. and warn your students that next time a larger fraction of points might be deducted. Take larger fraction of points off if the final result is left with 4 or more sig. figures. 3) (2pts) The density of the solution is : 1.036 g/mL = 1.036 g/mL × × × = 1036 kg/m 3 . To get maximum number of points conversions between corresponding units must shown with the correct result . Post-Lab Questions: Q1: Assess random and systematic errors in your density measurement for water. Is your measurement more precise or more accurate? List several possible sources of errors in your measurement and state how these errors affected precision and accuracy of your measurement. Students compare REA and RSD for density of water. If RSD is larger than REA then random errors in their measurement are larger than systematic errors and that means that the measurement is more accurate than precise. In that case students should list sources of random errors: 1 pipetting errors – diferent pipeting in each measurement, e.g., some measurement had last “partial” drop not delivered, in some remaining solution in the pipet was forced from the pipet , dirty balance (non consistent mass measurement, etc) If REA > RSD the measurement is more precise than accurate and sources of systematic errors should be listed: calibration of 10 mL pipet, calibration of balances, systematic error in a procedure – consistently forcing remaining drop out of pipet ( only if the measured density is higher than the true value), last “partial” drop not delivered (only if the measured density is lower than the true value )If REA > RSD the measurement is more precise than accurate and sources of systematic errors should be listed: calibration of 10 mL pipet, calibration of balances, systematic error in a procedure – consistently...
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Lab Exam 1 - Gaurav Gharti-Chhetri TA Answer Sheet LAB 1...

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