AP Chemistry: Precision Vs. Accuracy, Density. Accuracy How close measurements are to the true/accepted value.

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AP Chemistry: Precision Vs. Accuracy, Density

Accuracy How close measurements are to the true/accepted value.

Precision How close measurements are to each other. These measurements are not necessarily accurate.

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

Percent Error Problems 1) True value = 21.2g, Measured value = 17.7g 2) True value = 4.15ml, Measured value = 4.26 ml.

Density

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

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?

Example 2: It has been estimated that there are 4 x 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?

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?

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 = g/cm 3 1 gal = 3.78 L 1 lb = 454g

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 = 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.

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?

Ex. 2 Phosphorus trifluoride is present at STP conditions. What is its density and specific gravity?

Homework p. 28 #31 p. 30 #59-64, 68, 71 What is the density and specific gravity of nitrogen dioxide gas at STP conditions?