AP Chemistry: Precision Vs. Accuracy, Density. Accuracy How close measurements are to the...

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AP Chemistry: Precision Vs. Accuracy, Density

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Transcript of AP Chemistry: Precision Vs. Accuracy, Density. Accuracy How close measurements are to the...

Page 1: AP Chemistry: Precision Vs. Accuracy, Density. Accuracy How close measurements are to the true/accepted value.

AP Chemistry: Precision Vs.

Accuracy, Density

Page 2: AP Chemistry: Precision Vs. Accuracy, Density. Accuracy How close measurements are to the true/accepted value.

Accuracy

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

Page 3: 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.

Page 4: AP Chemistry: Precision Vs. Accuracy, Density. Accuracy How close measurements are to the true/accepted value.

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

Page 5: AP Chemistry: Precision Vs. Accuracy, Density. Accuracy How close measurements are to the true/accepted value.

Percent Error Problems

• 1) True value = 21.2g, Measured value = 17.7g

• 2) True value = 4.15ml, Measured value = 4.26 ml.

Page 6: AP Chemistry: Precision Vs. Accuracy, Density. Accuracy How close measurements are to the true/accepted value.

Density

Page 7: AP Chemistry: Precision Vs. Accuracy, Density. Accuracy How close measurements are to the true/accepted value.

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/m3), we will use g/cm3 or g/ml

Page 8: AP Chemistry: Precision Vs. Accuracy, Density. Accuracy How close measurements are to the true/accepted value.

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?

Page 9: AP Chemistry: Precision Vs. Accuracy, Density. Accuracy How close measurements are to the true/accepted value.

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?

Page 10: AP Chemistry: Precision Vs. Accuracy, Density. Accuracy How close measurements are to the true/accepted value.

Example 3:

• An empty container weighs 121.3g. Filled with carbon tetrachloride (density = 1.53 g/cm3) the container weighs 283.2g. What is the volume of the container?

Page 11: AP Chemistry: Precision Vs. Accuracy, Density. Accuracy How close measurements are to the true/accepted value.

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/cm3

• 1 gal = 3.78 L

• 1 lb = 454g

Page 12: AP Chemistry: Precision Vs. Accuracy, Density. Accuracy How close measurements are to the true/accepted value.

Specific Gravity • Removes gravitational influence on a weighed object,

density should not vary with weight !

• Ratio of density of substance/density of reference

• = Dsubstance/Dreference

• Liquids/solids, Dwater = 1.000 g/ml at 3.98°C (decreases with increase/decrease in temperature)

• Gases, Dair = 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.

Page 13: AP Chemistry: Precision Vs. Accuracy, Density. Accuracy How close measurements are to the true/accepted value.

Ex. 1 A solid object has a mass of 3.04 kilograms and a volume of 3.0 x 104 cm3. What is the density and

specific gravity of the solid object?

Page 14: AP Chemistry: Precision Vs. Accuracy, Density. Accuracy How close measurements are to the true/accepted value.

Ex. 2 Phosphorus trifluoride is present at STP

conditions. What is its density and specific

gravity?

Page 15: AP Chemistry: Precision Vs. Accuracy, Density. Accuracy How close measurements are to the true/accepted value.

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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