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AP Chemistry: Precision Vs. Accuracy, Density
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Accuracy How close measurements are to the true/accepted value.
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Precision How close measurements are to each other. These measurements are not necessarily accurate.
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Percent Error How much error you have in your lab measurements. Comparison between your experimental value/measurement to the true/accepted value. % Error =(True – Experimental/True)x 100
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Percent Error Problems 1) True value = 21.2g, Measured value = 17.7g 2) True value = 4.15ml, Measured value = 4.26 ml.
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Density
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Physical property of matter/substances Used for substance identification Provides information on how solids/liquids interact Ratio of a substance’s mass and volume (density = m/V) Units = SI Unit (kg/m 3 ), we will use g/cm 3 or g/ml
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Example 1: A student records V1= 2.7ml and V2= 3.4 ml after placing an object in a graduated cylinder. The mass of an empty beaker is 1.13g and the mass of both the beaker and substance is 4.13g. What is the object’s density?
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Example 2: It has been estimated that there are 4 x 10 -6 mg of gold per liter of sea water. At a price of $22.30 per gram of gold, what would be the value of gold in 1.00 cubic kilometers of the ocean?
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Example 3: An empty container weighs 121.3g. Filled with carbon tetrachloride (density = 1.53 g/cm 3 ) the container weighs 283.2g. What is the volume of the container?
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Example 4: A 55.0 gallon drum weighs 75.0 lbs. when empty. What will the total mass be when filled with ethanol based on the following information. Density = 0.789 g/cm 3 1 gal = 3.78 L 1 lb = 454g
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Specific Gravity Removes gravitational influence on a weighed object, density should not vary with weight ! Ratio of density of substance/density of reference = D substance /D reference Liquids/solids, D water = 1.000 g/ml at 3.98°C (decreases with increase/decrease in temperature) Gases, D air = 1.29 g/L At STP (1 atm, 0°C), 1 mole of gas = 22.4 L SO density can be calculated with molecular mass of gas.
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Ex. 1 A solid object has a mass of 3.04 kilograms and a volume of 3.0 x 10 4 cm 3. What is the density and specific gravity of the solid object?
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Ex. 2 Phosphorus trifluoride is present at STP conditions. What is its density and specific gravity?
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Homework p. 28 #31 p. 30 #59-64, 68, 71 What is the density and specific gravity of nitrogen dioxide gas at STP conditions?
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