MEASUREMENT Using Measurements

A. Units Scientists need a standard set of units when communicating information. In 1960, an international committee of scientists met to update the existing metric system. This revised system was called the Systeme Internationale d’Unites (SI)

B. Base Units There are seven base units in SI and they are as follows. Quantity Base Unit Time Second (s) Length Meter (m) Mass Kilogram (kg) Temperature Kelvin (K) Amount of Substance Mole (mol) Electric Current Ampere (amp) Luminous intensity Candela (cd)

B. Base Units

C. Temperature When describing if something is hot or cold, it is useful to use temperature. Fahrenheit is the temp scale used in the US Water freezes at 32oF and boils at 212oF Celsius is used throughout much of the rest of the world, especially in science. Water freezes at 0oC and boils at 100oC

C. Temperature Conversions The following equations are used for converting between Celsius and Fahrenheit. oF = 1.8(oC) + 32 oC = (oF – 32)/1.8 Example. If the temperature is 41oC, what is it in Fahrenheit?

C. Temperature Conversions  

C. Temperature Conversions To convert between Celsius and Kelvin, use the following. K = oC + 273 or oC = K - 273 Example. What is 451 K in Celsius? Example. What is 21oC in Kelvin?

D. Derived Units Not all quantities can be measured, some are calculated. When a combination of SI units are used, this is called a derived unit. Volume is the space occupied by an object. Vol = L x W x H (units cm3) Density is amount per unit volume Density = mass/volume Volume units g/mL or g/cm3

D. Density Calculations Example. An object has a mass of 25.6 g and a volume of 18.3 mL. What is the density of the object?

D. Density Calculations Example. A rectangular prism shown at right has a mass of 137 grams. What is its the volume and density? 8 cm 5 cm 3 cm

D. Density Calculations If the object is irregularly shaped volume can be found by water displacement. Example. The plastic fish shown at right has a mass of 11 grams. What is its volume? _____________________ What is its density? _____________

D. Density Calculations Example. An object has a density of 3.14 g/cm3 and occupies 5.1 cm3. What is the mass of the object? Example. Ethyl alcohol has a density of 0.789 g/mL. What volume will 25.0 g occupy?

E. Accuracy vs. Precision Accuracy - how close a measurement is to the accepted value Precision - how close a series of measurements are to each other ACCURATE = CORRECT PRECISE = CONSISTENT

F. Significant Figures Indicate precision of a measurement. Recording Sig Figs Sig figs in a measurement include the known digits plus a final estimated digit C. Johannesson

F. Significant Figures

F. Significant Figures Counting Sig Figs Count all numbers EXCEPT: Leading zeros -- 0.0025 Trailing zeros without a decimal point -- 2,500

Counting Sig Fig Examples F. Significant Figures Counting Sig Fig Examples 1. 23.50 1. 23.50 4 sig figs 2. 402 2. 402 3 sig figs 3. 5,280 3. 5,280 3 sig figs 4. 0.080 4. 0.080 2 sig figs

F. Significant Figures Calculating with Sig Figs Multiply/Divide - The # with the fewest sig figs determines the # of sig figs in the answer.

F. Significant Figures Calculating with Sig Figs (con’t) Add/Subtract - The # with the least number of decimal places determines the number of decimal places in the answer.

F. Significant Figures Calculating with Sig Figs (con’t) Exact Numbers do not limit the # of sig figs in the answer. Counting numbers: 12 students Exact conversions: 1 m = 100 cm “1” in any conversion: 1 in = 2.54 cm C. Johannesson

F. Significant Figures Practice Problems 18.9 g - 0.84 g (15.30 g) ÷ (6.4 mL) 18.9 g - 0.84 g

G. Scientific Notation 65,000 kg  6.5 × 104 kg Converting decimal into Sci. Notation: Move decimal until there’s 1 digit to its left. Places moved = exponent. Large # (>1)  positive exponent Small # (<1)  negative exponent Only include sig figs. C. Johannesson

G. Scientific Notation 6.5 × 104 kg  65,000 kg Converting Sci. Notation into decimal: For positive exponent, move decimal to the right. For negative exponent, move decimal to the left. C. Johannesson

G. Scientific Notation Practice Problems 2,400,000 g 0.00256 kg 7  10-5 km 6.2  104 mm

G. Scientific Notation Calculating with Sci. Notation (5.44 × 107 g) ÷ (8.1 × 104 mol) =