S2i Sunday, September 8, 2012

Transcription

1 Agenda: 1. How to study for this course 2. Review of Significant Figures 3. Practice Problems S2i Sunday, September 8, 2012 How to Study for CHEM 212 DO THE HOMEWORK o Start it early in case you are having problems o Go back and re-work homework problems before the test Do the practice problems in the book- the odd problems have answers in the back Use tutoring services Use practice tests, but don t overuse If you aren t getting a concept after going to class or s2i, read the book- it helps to have it explained 3 different ways Make sure to have key items memorized Significant Figures Correct the following rules: Zeros always count Zeros count when they are surrounded by integers or are after a decimal Zeros at the front of the number count Zeros at the front of the number do NOT count Numbers that are not zero only count after a decimal point. Numbers that are not zero always count

2 Zeros at the end of the number never count Zeros at the end of the number count if the number contains a decimal point. Exact numbers have 0 significant figures Exact numbers have infinite significant figures. When multiplying, dividing, adding, or subtracting, the answer should only have the same amount of significant figures as the most precise measurement Practice Problems: 1.62 What type of quantity (for example length, volume, density) do the following units indicate? a) ml volume b) cm 2 area c) mm 3 volume d) mg/l density e) ps time f) nm length g) K temperature

3 Conversion Factors: 1 inch = 2.54 centimeters 1 foot = 12 inches 1 gallon = L 1 pound = g 1 ounce = 28.3 g 1 m = 1.09 yd 1 cm 3 = 1 ml Calculate the density in g/ml of the following: a) 15.2 mg in 300 L solution 15.2 mg 1 g 1 L 300 L 1000mg 1000 ml = 5.07 x 10-8 g/ml b) 3.4 oz in 20 cm oz 28.3 g 1 cm 3 20 cm 3 1 oz 1 ml = 4.8 g/ml c) 5 lb in 5 liters 5 lb g 1 L 5 L 1 lb 1000 ml = 0.5 g/ml A cube has a volume of 200 cm 3 and weighs 50 lbs. What is its density in g/ml? 50 lbs g 1 cm cm 3 1 lb 1 ml = 100 g/m

4 A sheet of paper has a density of 0.2 g/cm 3, a surface area of 93.5 in 2, and a weight of 2.0 mg. What is the thickness of the sheet? V= 93.5 in 2 x thickness 2.0 mg 1 g 1 cm mg 0.2 g = 0.01 cm in cm 2.54 cm 1 in 1 in = cm 2.01cm 3 / cm 2 = 2 x 10-5 cm 1.71 A g sample of a solid is placed in a flask. Toluene, in which the solid is insoluble, is added to the flask so that the total volume of a solid and liquid together is ml. The solid and toluene together weigh g. The density of toluene at the temperature of the experiment is g/ml. What is the density of the solid? g total g sample= g toluene 32.12g toluene * 1 ml/0.864g = ml toluene ml total ml toluene= ml sample g sample/ ml = 1.91 g/ ml

5 1.72 A thief plans to steal a gold sphere with a radius of 31.1 cm from a museum. If the gold has a density of 19.3 g/cm 3 what is the mass of the sphere is pounds? (V =!! πr! ) V= 4/3 * 3.14 * (31.1 cm) 3 = 1.26 x 10 5 cm g 1.26 x 10 5 cm 3 1 lb 1 cm g = 5630 lbs 1.74 A 50-lb container of peat moss measures 15 x 12 x 11.2 in. A 50-lb container of topsoil has a volume of 3.2 gal. a) Calculate the average densities of peat moss and topsoil in units of g/cm 3. b) How many bags of peat moss are need to cover an area measuring 15.0 x 20.0 ft to a depth of 4.0 in? Peat Moss V peat moss = 15 * 12 * 11.2 in= 2016 in 3 50 lb 1 in g 2016 in 3 (2.54) 3 cm 3 1 lb = 0.69 g/cm 3 Topsoil 50 lb g 1 gal 1 L 1 ml 3.2 gal 1 lb g 1000 ml 1 cm 3 = 1.87 g/cm 3 b) 15*20= 300 ft ft 2 (12) 2 in 2 = 4320 in 2 * 4 in= in 3 1 ft in 3 1 bag peat moss 2016 in 3 = 86 bags of peat moss

6 1.75 A package of aluminum foil contains 70 ft 2 of foil, which weights approximately 10.2 oz. Aluminum has a density of 2.70 g/ cm 3. What is the thickness of the foil in millimeters? 10.2 oz 28.3 g 1 oz = g = g 1 cm 3 (10) 3 mm g 1 cm 3 = 1.07 x 10 5 mm 3 70 ft 2 (12) 2 in 2 (2.54) 2 cm 2 (10) 2 mm 2 1 ft 2 1 in 2 1 cm 2 = mm x 10 5 mm 3 / mm 2 = 0.02 mm