Scientific Measurement

Measurement A measurement is a quantity that has both a number & a unit. Examples: 67, 154, & 70 are just numbers. 67 inches, 154 pounds, & 70 miles per hour, are measurements!

Precision vs. Accuracy Precision: the ability to reproduce the same value in a measurement. In other words, how repeatable measurements are. Accuracy: The agreement between a measured value & the true value. In other words, how close a measurement comes to the actual value (the target). Precise does not always mean accurate.

Precision vs. Accuracy cont.

Significant Figures/Digits Significant Figures are all the digits in a number that are known & the last digit that is estimated. They determine where to round off a calculation.

Significant Figures Rules: Non-zero numbers are always significant. Zeros between digits are always significant. Initial zeros are not significant. (Example: 0.0003 has only 1 sf.) Zeros at the end of the number are significant IF there is a decimal point.(24.0 cm has 3 sf)

Scientific Notation Purpose: to express very large or very small values easily. Ex. 1,000 = 1 x 103 0.001 = 1 x 10-3 To convert- Move the decimal point until there is only one digit (number) before the decimal point. The number of moves = the exponent Move to the left = positive exponent Move to the right = negative exponent

Scientific Notation cont. Let your calculator do the work (use the EE (or exp) button) Examples: 5,280 = 3 moves to the left so, 5.28 x103 0.0899 = 2 moves to the right so, 8.99 x 10-2

SI Units There are two types of units: fundamental (or base) units; derived units. There are 7 base units in the SI system. - We only use 4-5

Metric System Quantity Unit Symbol Mass* Gram g Length* Meter m Time* Second s Temperature* Kelvin K Amount* Mole mol

SI Units

Metric Table M k H da d c m u mega kilo hecto deca Base Unit deci centi milli micro M k H da d c m u 1,000,000 1,000 100 10 Gram g Liter L Length m 0.1 0.01 0.001 0.000001 1.0 x106 1.0 x103 1.0 x 102 1.0 x101 1.0x10-1 1.0x10-2 1.0x10-3 1.0x10-6

Derived Units Derived Unit is a combination of other units to reach a measurement. There are 2 common derived units we will use. Volume = cm3 or m3 = formula (L x W x H) Density = g/cm3 = from formula (D = m/v)

Volume The units for volume are given by (units of length)3. SI unit for volume is 1 m3. Common: 1 mL=1 cm3. Other volume units: 1 L = 1 dm3 = 1000 cm3 = 1000mL.

Volume

Density A physical property. Defined as mass divided by volume: Units: g/cm3 or g/mL Originally based on mass (density was defined as the mass/vol. of 1.00 g of pure water).

SI Units Temperature There are three temperature scales: 1. Kelvin Scale Used in science. Same temperature increment as Celsius scale. Lowest temperature possible (absolute zero) is zero Kelvin. Absolute zero: 0 K = -273 oC.

Temperature 2. Celsius Scale 3. Fahrenheit Scale Also used in science. Water freezes at 0 oC and boils at 100 oC. To convert: K = oC + 273. 3. Fahrenheit Scale Not generally used in science. Water freezes at 32 oF and boils at 212 oF. To convert: Look up the formula – then forget it.

Temperature Scales Know MP and BP of water in all three.

What the Heck is a Mole? Atoms & molecules are too tiny & numerous to count. (Imagine counting grains of sand!) It is much easier to weigh atoms & molecules. Chemists use a mole to represent a specified number of particles. One mole of an element =6.02 x 1023 = the atomic mass. (See periodic table) Kind of like a dozen = 12. Except a mole = 6.02 x 1023

Dimensional Analysis Method of calculation utilizing a knowledge of units. Conversion factors are simple ratios (fractions):

Using a Conversion Factor Example: convert a length of 3.25 meters to length in centimeters: Solution: Given x Conv. = Ans. factor 3.25 meters x 100 cm = 325 cm 1 m

Using Two or More Conversion Factors Example to convert length in meters to length in inches: 3.00 meters x 100 cm x 1 inch = 118 in 1 m 2.54 cm This takes practice. Be patient.

Percent Error – used to compare data (observed values) to accepted/true values in a meaningful way │ Obs – Acc│ x 100 = % error Acc