Welcome to AP Chemistry !

Scientific Measurements

• SI Units

• Significant Figures

SI Units

•Measurement system universal to scientists.•Measurement standards (base vs. derived)

•Base unit—•Quantity we can MEASURE

•Derived unit--•Quantity provided by CALCULATION

SI Units (cont.)

Base•Mass = kilogram (kg)• Length = Meter (m)• Time = Second (s) • Temperature = Kelvin (°K)

Derived • Volume = m3, we will use cm3/ml• ml is NOT an SI unit (1ml = 1cm3)

• Density = kg/m3, we will use g/cm3 or g/ml

Scientific Measurements

• SI Units

• Significant Figures

• ALWAYS WRITE YOUR UNITS ON YOUR ANSWER !!!!----units provide meaning to numbers.

Significant Figures

• The number of digits in a scientific measurement known with certainty and one estimated or uncertain digit.

• Method of reporting scientific measurements and calculations.

Significant Figure Basic Rules

• 1. All numbers that are NOT zeroes are significant• Ex. 211, 345 =3

• 2. All zeroes BETWEEN 2 significant figures are significant.• Ex. 2003, 1203 = 4

• 3. All zeros that are FINAL and PAST a decimal are significant.• Ex. 0.00020 =2

• 4. Final zeros are only significant if a DECIMAL POINT is added to the end of the number. • Ex. 100 = 1 • Ex. 100. = 3• Ex. 100.000 = 6

Examples

• 1) 567

• 2) 0.00022

• 3) 10.020

• 4) 200

Significant Figures: Adding/Subtracting

• # of digits to RIGHT of decimal = # of digits RIGHT of decimal from number with the LEAST digits.

• Ex.1 67.14 kg + 8.2 kg = • 75.3 kg

• Ex. 2 101.23 g – 90.614 g = • 10.62 g

Significant Figures: Multiplying/Dividing

• # of significant figures = # of significant figures in number with LEAST significant figures.

• Ex. 1 12.1 / 3.1 = • 3.870967742 ~ 3.9

• Ex. 2 45.67 * 1.23 = • 56.1741 ~ 56.2

Significant Figure Practice---If Necessary

• 1) 900 6) 804.5

• 2) 1020 7) 0.0144030

• 3) 0.00020 8) 1002

• 4) 100034 9) 0.000625000

• 5) 0.00010010 10) 17.982 g / 4.13 cm3 =

• 11) 12.4 + 1.345 = 12) 2.0 * 4.35 =

• 13) 20500. 14) 31400

• 15) 3500

Dimensional Analysis

Dimensional Analysis

• Method of converting one unit to another• Conversion factor—ratio relationship

between 2 units

• Write units, BE SURE THEY CANCEL ! ! !

• “Where are we starting? Where are we trying to go?”

•KNOW

•2.54 cm = 1 inch

Example 1: How fast do you have to run?

• A. Cheetah Speed 70 miles/hr ----- ft/sec. How fast do you have to run?

• B. 1.95x10-3 oz to μg

Example 2:

• Science fiction often uses nautical analogies to describe space travel. If the starship U.S.S. Enterprise is traveling at warp factor 1.71, what is its speed in knots?

• 1.71 Warp = 5.00 times the speed of light• Speed of light = 3.00x108 m/s• 1 knot = 2000 yd/h exactly

Example 3:

• Apothecaries use the following set of measures in the English system: • 20 grains ap = 1 scruple• 3 scruples = 1 dram ap• 8 dram ap = 1 oz. ap• 1 dram ap = 3.888g

• What is the mass of 1 scruple in grams?

Conversion Factors: In Detail

• = 1 value• 12 inches = 1 ft. • 2.54 cm = 1 inch• 5280 ft. = 1 mile

• SO • (12 inches = 1 ft.) 3 = 13 = 1 same conversion factor

In Detail Cont.

• Use same conversion factor but raise the factor to the correct power for conversion.

• Cubic inches to ft3

• (12 inches = 1 ft)3

• When converting between squared and cubic units (m2 to cm2 /m3 to cm3)

Example 4:

• Volume of 2.56 cm3 convert to m3

Example 5:

• BMI is a measurement to determine if a person is obese. A BMI > 25 is considered overweight. Calculate the BMI for a 100 lb person with a height of 52 inches. Is this person overweight?

• Hint: Do conversion factors FIRST, then cancel units

Temperature Conversions

• °F = 1.8 (T°C) + 32

• °K = 273 + T°C

Homework

• Read pp. 20-22

• Problems pp. 29 #37, 39-41, 42(b,c), 47, 49-50, 53