 # Chemistry

#### Chapter 1 Review Section 1 Homework

# Reviewing Fundamental Concepts of Measurement and Calculation

## Review Section 1-2 Homework

Textbook assignment: Read Kotz and Triechel, Chemistry and Chemical Reactivity Review Section: The tools of quantitative chemistry (pp. 22-31).

##### Study Notes

This section throws you into the nitty gritty of the material for this text. There are lots of examples. Slow down and work your way through each one. Do not skip over them. The basics you master now will really pay off during the rest of the course.

• 1: Units of Measurement Chemistry is concerned with physical quantity. We don't just deal with numbers but with measurements of a particular kind, or the counting of a particular type of thing. Of the seven base measurements in the Systeme Internationale, chemists deal constantly with three: temperature (in Kelvin but some times more conveniently in Celcius/Centigrade), length (in meters but sometimes in centimeters, millimeters, or nanometers, depending on what we are measuring), and mass (in kilograms and grams). Volume is simply an extension of length into three dimensions, but we spend a lot of time consolidating those measurements into a single "volume" unit -- the liter. Chemists are also concerned with time measurements (seconds) in studying rates of reactions and units of energy (joules and calories) in determining energy exchanges during reactions.

When we use the SI system, we mean the combination of Meters/Kilograms, and seconds (for time). Sometimes we use the CGS (centimeter/gram/second) system when it is more convenient.

Which unit system we use is a choice for our convenience in doing a particular type of analysis. There is no one "right" system for every application. The cardinal rule is that you identify which system and which units you will use, and remain consistent throughout a particular analytic exercise.

For a moment, ignore the actual digits and keep in mind the type of quantity you need to answer the question. The units have to match the answer. "How long?" cannot be answered with " 20 kilograms".

If units for the same type of quantity are given in more than one form (kilometers and meters, or meters and miles, for example), you must first use a standard conversion factor to convert one of the numbers into the form of the other before you can do the final numerical calculation. Sometimes (as in the case of temperature), you must also take into account the fact that the scales you are using have different "0" points. The zero point for Kelvin is absolute zero, where motion stops. The zero point for Celsius is the freezing point of water. When converting from one to the other, we have to both convert the units and then shift the scale.

 Y numbers of UNIT A * conversion factor = X numbers of UNIT B 6.35 kilograms * 1000 grams/kilogram = 6350 grams 15 milliliters * 1 milliliter/1 cubic centimer = 15 cubic centimeters 30 °C * 1Kelvin/1°C + 273.15 = 303.15K
• 2: Making Measurements: Precision, Accuracy, Experimental Error, and Standard Deviation

Precision is a measure of how well you measure a given quantity: three measurements of the same characteristic will be precise if they are nearly the same. Accuracy is a measure of how close the measurements are to the expected value: three measurements of 10.1 meters/second2, 10.05meters/second2, and 10.0meters/second2 may be precise measurements of acceleration due to gravity in a falling body experiment, but they are not very accurate (the expected amount is 9.8 meters/second2).

In order to help us understand results, we look at the difference between our data and the accepted value: this is the experimental error, which may be due to inappropriate methods of measuring, mis-calibration of our measuring devices, or the influence of some hitherto unsuspected physical phenomena. Calculations using statistical techniques, in particular, standard deviation (which we will be discussing in conjunction with writing up your lab reports) help us see whether our data is accurate enough to be useful.

#### Important Formulae

ConceptRelationshipFormulaSymbolsTypical Units
Temperature conversion Kelvin to Celsius T (K): Temperature in Kelvin
T *C: Temperature in Celsius
K or C
Temperature conversion Kelvin to Celsius T (K): Temperature in Kelvin
T *C: Temperature in Celsius
K or C
Energy conversions Calories to Joules 1 cal = 4.184 J cal: calorie (note small c!)
J: Joules
cal: heat required to raise 1 gm water 1 °C, or J
Error Percent error as proportion of measurement % error: error as fraction of measurement
error in measurement: difference between individual measurement and expected value or average
accepted value: standard value or average
NO UNITS!
Standard deviation Statistical error range σ: Standard deviation
n: number of measurements
xi: ith measurement
xavg: average of measurements
NO UNITS!

#### Web Lecture

Read the following weblecture before chat: Scientific Method: the Uses of Experiment

Take notes on any questions you have, and be prepared to discuss the lecture in chat.

#### Videos for Chapter 1: Review

• Scientific Measurement

#### Chat Preparation Activities

• Essay question: The Moodle forum for the session will assign a specific study question for you to prepare for chat. You need to read this question and post your answer before chat starts for this session.
• Mastery Exercise: The Moodle Mastery exercise for the chapter will contain sections related to our chat topic. Try to complete these before the chat starts, so that you can ask questions.

#### Chapter Quiz

• There is no chapter quiz for the review section. You do, however, have to complete the Mastery exercises.

#### Lab Work

Please read Illustrated Guide to Home Chemistry Experiments: All Lab, No Lecture, Chapter 2 - Lab Safety, and survey pages 52-67. Make sure that you understand how to determine the safe storage and disposal methods for a particular chemical.