chem-201-chapter-2 - 9/29/11 Analytical Tools Analytical...

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9/29/11 1 Analytical Tools Analytical Balances An modern analytical balance uses a magnetic force to balance the pan and determine the mass of the sample (Figure 2-3) Analytical balances can measure to a precision of 0.0001 g (0.1 mg) Precaution to use when weighing Do not touch sample or container with fingers —the balance is precise enough to weigh finger prints Analytical Balances Precaution to use when weighing Close the doors on the balance to prevent air drafts from affecting results Correction for buoyancy—upward force by air under pan of balance = s a w a d d 1 d d 1 ' m m m = true mass of object m = mass reading from balance d a = density of air d w = density of weights (8.0 g/mL) d s = density of sample Analytical Balances Buoyancy Correction Determine true mass of a CuSO 4 5H 2 O sample with a mass reading of 0.5672 g d s = 2.284 g/mL d w = 8.0 g/mL d a = (P MW)/(RT) = (1 atm)(29 g/mol)/(.08205 Latm/molK)(298K) = 1.186 g/L
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chem-201-chapter-2 - 9/29/11 Analytical Tools Analytical...

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