# UNIT: Chemistry and Measurement

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UNIT: Chemistry and Measurement
TOPIC: Measurement Objectives: Lesson 2 of 4 You will learn how to convert extremely large or small numbers into scientific notation You will learn the difference between the terms Accuracy and Precision You will be able to solve problems with Significant Figures

Quickwrite In 1-2 sentences answer one of the questions below:
What are some measurements you might take in a Chemistry lab? Why do you think it is important to make accurate and precise measurements? Finally, in your own words, try to define the term accurate:

Scientific Notation To make large number seem small, scientist use something call scientific notation Lets look at the speed of light Light travels 9,500,000,000,000,000 kilometers in one year 9,500,000,000,000,000 written in scientific notation is x 1015 How did I get 9.5 x 1015? By moving the decimal over 15 places between the 9 and the 5 9,500,000,000,000,000 x 1015

Scientific Notation You have about 50,000,000,000,000, cells in your body! Let’s write this in scientific notation x 1016

Scientific Notation Scientific notation is a useful way to represent numbers that are very large or very small All numbers are written in a form of exponents of ten such as 9.5 x 1015

What is Scientific Notation?
A way to represent numbers that are very large or small Numbers are written in a form of exponents of ten such as 9.5 x 1015

Practice: Try to write 602,300,000,000,000,000,000,000 in scientific notation: 602,300,000,000,000,000,000,000 x 1023

Accuracy and Precision
It is important to make measurements that are both accurate and precise Accuracy is how close a measurement is to the correct or accepted value Precision is how close a measurement agrees with other measurements of similar value Lets apply these terms to a dart board, and our bull's-eye is our “Accepted Value” then we can begin to understand the difference between Accuracy and Precision Precise, Not Accurate Not Precise or Accurate Precise and Accurate

What is the difference between Accuracy and Precision?
Accuracy is how close a measurement is to the correct or accepted value Precision is how close a measurement agrees with other measurements of similar value

Significant Figures So, which on is it? Is it or 1.56 centimeters? It is important to realize that the first two numbers are the same and are therefore certain; however, the third estimate (hundredths place) can vary and is therefore uncertain Measurement always has some level of uncertainty Significant Figures are numbers recorded in a measurement that include all certain numbers plus the first uncertain number Whenever a measurement is made with a device such as a ruler or Graduated Cylinder, a certain degree of estimate is required For example, lets say we want to measure the length of this nail We can see the that the length of the nail is between 1.5 and 1.6 centimeters Because no scale exist between 1.5 and 1.6, we must estimate the nails length Using a visual estimate, we could estimate the nails length as either or 1.56 centimeters certain 1.54 cm 1.56 cm? uncertain

What are Significant Figures?
The numbers recorded in a measurement that include all certain numbers plus the first uncertain number

Significant Figure Rules
Chemistry requires many types of calculations and measurements To help us obtain accurate and precise results, me must use a set rules known as the Significant Figure Rules; these rules are as follows: All Nonzero integers or numbers count as significant For example, in the number below this includes integers 7, 5, and 8 Zeros in front of nonzero numbers or integers are NOT significant For example, in the number below this includes the first three zeros Zeros between nonzero integers or numbers are significant For example, in the number below this includes the zero between 7 and 5 Trailing zeros at the end of a number are only significant if the number contains a decimal For example, in the number below this includes the zero after 8 because a decimal is present significant 5 significant figures TOTAL significant significant NOT significant

Practice: Determine the number of significant figures in each measurement below: A piece of magnesium metal weighs grams Answer: the number contains 3 significant figures A piece of hair from a crime lab weighs grams Answer: The number contains 5 significant figures

Rules for Rounding Off Lets say we want to round the number below to nearest hundredths place Because the last digit, 3 is less than 5, our answer would be Now lets say we want to round the number below to the nearest tenths place Remember, any number greater than or equal to 5, we round up, which gives us an answer of 845.7 It is important to realize that when you are performing calculations on a calculator, only round off until you have arrived at your final answer Sometimes our calculators give us answers with a very large number of digits, therefore it is important we learn how to round off Consider the number below, it is made up certain place names For example, the 8 is located in the thousands place The 4 is located in the hundreds place The 5 is located in the tens place The 6 is located in the tenths place The 5 is located in the hundredths place The 3 is located in the thousandths place hundreds place tens place tenths place hundredths place thousandths place thousands place

Practice: Round each number below to the nearest “tenths” place:
A piece of magnesium metal weighs 1.58 grams Answer: Because 8 is greater than or equal to 5, the answer is 1.6 grams A sample of water has a mass of grams Answer: Because 1 is less than 5, the answer is 150.1

Significant Figure Rules in Calculations
You will often be performing calculations that involve multiplication, division, addition and subtraction When using significant figures in calculations, there are rules we must consider, these rules are as follows: For multiplication or division, the number of significant figures in your answer will be the same as the number with the smallest number of significant figures For example, let’s say you perform the following calculation below: 4.56 x = 6.384, the smallest number 1.4, contains 2 significant figures, so our answer must contain two significant figures Therefore our answer will be 6.4 3 significant figures 2 significant figures For addition and subtraction, the number of significant figures in your answer will be the same as the number with the smallest number of decimal places For example, let’s say you perform the following calculation below: = 18.28, the number with the smallest number of decimal places is 6.2, so our answer must contain only one decimal place Therefore our answer will be 18.3 2 decimal places 1 decimal place

What are the Significant Figures Rules?
All Nonzero numbers count as significant Zero's in front of nonzero numbers are NOT significant Zero's between nonzero numbers are significant Trailing zero's at the end of a number are only significant if the number contains a decimal Calculation Rules: For multiplication/division, the number of significant figures in your answer will be the same as the number with the smallest number of significant figures For addition/subtraction, the number of significant figures in your answer will be the same as the number with the smallest number of decimal places

Practice: Solve each problem below, make sure your answer contains the correct number of significant figures: (remember to round) 5.18 x x 1.1 = ?????? Answer: 12 Because 1.1 has 2 significant figures, therefore our answer must contain only 2 significant figures – 4.6= ?????? Answer: 2.7 Because 4.6 has only one decimal place therefore our answer must contain only one decimal place (don’t forget to round up) (1.33 x 2.8) = ?????? Answer: 12.1 Because x 2.8 = = 3.7, = , notice 3.7 has only one decimal place therefore our answer must contain only one decimal place 12 2.7 12.1

Summary (you can always write your own summary)
In the expression (1.33 x 2.8) = 12.1, describe how you were able to determine the answer using significant figures Imagine you are measuring an object with a ruler, explain the difference between a certain and an uncertain measurement How many significant figures does contain? How many significant figures does 22.1 contain? Summarize the significant figure rules: