Chemistry Notes Unit 2 Measurement Metrics Scientific Notation Uncertainty

The Metric System Created by the French in 1795 Two type of units  Fundamental – deals with single measurements that describes a single phenomenon  Ex: time, distance, mass  Derived – deals with two or more measurements that describe a single phenomenon  Ex: m/s, Kg*m/s 2, m/s 2, N*m

The Metric System  The metric system is much easier than our standard system of measurement because units with in the metric system Are powers of ten.  Ex: convert 1200 g to kg  Answer  1.20 kg (just move the decimal)  Ex: convert 40 oz.  lbs  Answer  2 ½ lbs (have to do some math)

The Metric System  Some basic units  see table 2.1 on page 29  Length  meters  Time  seconds  Mass  kilogram  Metric Prefixes  see table 2.2 on page 29  Micro m  Mill m  Centi m  Deci -0.1 m  Kilo-1000 m

The Metric System Definitions of fundamental measurement  Mass = the amount of matter in an object  Measured in kilograms (kg)  Not to be confused weight. Weight is more comparable to force,  W = mg  A balance is an instrument used to determine mass

The Metric System  Mass v. Weight

The Metric System Definitions of fundamental measurement  Length = the distance covered by a straight line segment between two points  Measured in meters (m)  Time – interval between two occurrences  Measured in seconds (s)

The Metric System Definitions of fundamental measurement  Temperature = the average kinetic energy of the particles that make of matter.  Measured in Kelvin (K)  Kelvin is a scale based on molecular motion.  We also use Celsius ( o C)  To convert from ( o C) to K:  K = ( o C)  ( o C) = K –  Example problem: convert 25 o C to Kelvin  Example problem: convert 400 Kelvin to 25 o C

Scientific Notation  Used in science because scientist often work with very large or very small numbers  Ex: Astronomers, Bacteriologists, Chemists

Scientific Notation  To add and subtract numbers that are in scientific notation you have to:  Be sure the numbers are to the same power  Be sure numbers are in the same units  NOT one in grams and one in kilograms  Examples:  2.30 X X = __________  1.14 X 10 7 – 3.11 X 10 9 = __________

Scientific Notation  To multiply or divide numbers that are in scientific notation you have to:  Add exponents if you are multiplying  Subtract exponents if you are dividing  Examples  (6.87 X ) (8.24 X ) = _________  (4.87 X ) / (8.55 X ) = ________

Uncertainty  Recall: in chemistry (or any science) we deal with a lot of uncertainty. Scientists have to be skeptical of their measurements and explanations – but how skeptical?  Question: how do we handle uncertainty?  Answer: Calculations like percent error and significant digits allow scientist to see how far off they may be.

Uncertainty Percent Error: is calculated as seen below Percent Error = experimental value – theoretical value X 100 theoretical value  Because absolute value is used % error will always be a positive number.  Ex: what is the % error of an experiment where a student produces 43 g of sodium sulfite from the reaction including sodium, sulfur and oxygen? The theoretical mass of sodium sulfite was 49.2 g.  Ex: what is the % error of an experiment where a student observed that the time a certain reaction lasted was s? The theoretical time that was calculated was s.

Uncertainty Significant Figures  Question: Which of the following measurements is the most precise?  m  12 m

Uncertainty  Answer: m is more precise because what is being said is that this measurement was to the closest m opposed to being measured to the closest meter.  These numbers are not saying the same thing.  is saying the measurement is between m and m  12 is saying the measurement is between 11 m and 13 m  contains 5 significant digits whereas 12 only contains 2  All numbers but the last ones are certain.

Uncertainty Determining Significant digits:  All non-zero numbers are significant   4 sig figs  Zeros after decimal points are significant   5 sig figs  Zeros between non-zero numbers are significant   10 sig figs  Zeros that are place holders are not significant   2 sig figs   4 sig figs

Summary  What questions do you still have?  What was unclear?