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**Chemistry Unit 2: Scientific Measurement**

Salisbury High School Spring 2007

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Chemistry February 12, 2007 1) Begin Unit on Measurements 2) Notes on measurements, accuracy, precision 3) HW: WS 2.1

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**Measurements – Opening Questions**

1. What is the purpose of a measurement? 2. Why are measurements important to science?

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Measurements Measurements are fundamental to science Measurements may be: a. Qualitative b. Quantitative

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**Measurements Qualitative Measurement: is a non-numerical measurement**

Example: The solution turned brown when ammonia was added to iron (III) chloride

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**Quantitative Measurements**

Quantitative Measurements: consist of two parts a. A number b. A scale (or unit)

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**Quantitative Measurements**

Scale (unit) Example: The rock has a mass of 9 kg Number

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**Practice: Classify as qualitative or quantitative**

1. Water is a liquid 2. The temperature was 9°C. 3. The book is 12 cm long 4. The mixture contains 5 blue marbles and 12 g of red clay

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**Terms Used for Measurements**

In comparing scientific results, the following terms are applied: a. accuracy b. precision

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Accuracy Accuracy refers to the agreement of one experimental result to the true or accepted value The closer the value is to the true value, the more accurate one is

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Accuracy Examples of accuracy: kicking a soccer goal hitting a three point shot determining the value of to be 3.13

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**Practice: Tell if the accuracy is good or poor**

a. Finding the percent H2O2 to be 2.9% (the bottle says it contains 3%) b. Finding that a 2 Liter bottle only contains 1.5 Liters of soda c. Filling a 1000 mL volumetric flask with 1050 mL of water

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Precision Precision refers to how close several different experimental values are to each other

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Precision Examples: Scoring three goals in a soccer game Shooting an air ball five times Finding the value for to be 3.12, 3.15, 3.13

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**Precision and Accuracy Problems**

Precision problems usually arise from the skill of the person doing the experiment or the division of the measuring instruments

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**Precision and Accuracy Problems**

Accuracy problems usually related to the quality of the equipment used to make measurements Precision relates to how consistent the results are; accuracy relates to how correct the results are

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**Describe the accuracy and precision of the following**

A student makes the following grades: a. 99, 100, 98, 100 b. 45, 43, 44, 42 c. 100, 23, 60, 89

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Scientific Notation Occasionally some measurements are really small or exceptionally large 1, 400, 000 km, the distance to the sun , the Universal Gravitation Constant To help you use these, you may express them in scientific notation

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**Scientific Notation Continued**

In scientific notation, a number is written as the products of two numbers: a coefficient and some power of 10

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**Scientific Notation Continued**

The general form for scientific notation is: M x 10n where M is 1 but < 10 n is an integer and exponent {integers must be a positive or negative whole number}

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**Rules for Scientific Notation**

1. Determine “M” by moving the decimal point in the original number to the left or to the right so that only one non-zero digit remains to the left of the decimal

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**Rules for Scientific Notation**

2. Determine “n” by counting the number of places that the decimal point was moved. If the decimal was moved to the left, n is positive; if the decimal is moved to the right, n is negative

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**Guided Practice Express the following in scientific notation:**

1, 400, 000 km, the distance to the sun , the Universal Gravitational Constant

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**Independent Practice Express the following in scientific notation:**

b c d e f

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**Express the following scientific notations in the long form**

1. 7 x 104 x 104 3. 7 x 10-5 x 1010 x 105

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**Express the following in scientific notation**

B C. 456

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**Rules for Calculations Involving Scientific Notation**

1. Addition/Subtraction: In order to add or subtract numbers in scientific notation, the numbers must be expressed in the same exponent or “n”

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**Rules for Calculations Involving Scientific Notation**

Example: 6.3 x x 105 1) Convert one number to the same exponent 2) Add the number

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Solution 6.3 x104 = 0.63 x 105 0.63 x 105 x 105 2.73 x105

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**Rule for Multiplication**

2. When multiplying, multiply the coefficients and add the exponents together (they do NOT have to be in the same power)

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**Rule for Multiplication**

Example: 4 x 105 * 2 x 102 4 x 105 * 2 x 102 = 8 x 107

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Rule for Division When dividing, divide the coefficients and subtract the exponents (top exponent minus bottom exponent) Example: (4 x 105) / (2 x 107) Ans: 2 x 10-2

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**Complete the following calculations**

1. (2.4 x 10-3) - (1.2 x 10-2) 2. (7.4 x 10-8) / (3.4 x 104) 3. ( 3.45 x 104) * (2.3 x 103)

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Chemistry February 13, 2007

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SI System The SI system is a universal system of measurements based on powers of “10” The US and Burma are the only major countries that do not use the SI system

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**SI System The SI system used prefixes “Kilo-” means 1000; symbol is k**

“Centi-” means 0.01; symbol is c “Milli-” means 0.001; symbol is m “Deci-” means 0.1; symbol is d

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**Review-Chemistry Copy and complete the chart in your notes:**

Name of Prefix Symbol Meaning milli c 1000 deci

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Length Length is the distance between two points The SI unit of length is the meter (m) Devices used to measure length: Rulers, Tape Measurers, and Meter Sticks

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Volume Volume is the amount of space an object occupies Volume is length * width * height The SI unit for volume is m3 The liter (L) is also used for volume of liquids

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**Volume Continued Conversions for volume:**

1 dm3 (decimeter cubed ) = 1 L 1 cm3 = 1 mL (milliliter) Devices Used to Measure Volume: Ruler, Graduated Cylinder

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**Mass Mass is the amount of matter that an object contains**

Mass is measured in kilograms (kg) Grams (g) are also used but are very small

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Mass and Weight Mass does not change Weight does change Weight is the force that the Earth exhibits on a mass Weight is measured in Newtons (N)

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**Mass & Weight Continued**

Question: How much mass does a 60 kg person have on the moon? Answer: 60 kg Question: How much does a 480 N object weigh on the moon? Answer: 80 N

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Time Time is the interval between two events Unit of time is the second

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Temperature Temperature is the measure of the average kinetic energy of particles Temperature measures how hot or cold an object is

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Temperature Temperature is measured in Kelvin in the SI system K = °C Temperature is usually given in °C

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**Convert the following temperatures**

°C to K K to °C K to °C °C to K

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**Density Density is the ratio of mass to volume**

Units for density include g/cm3, g/mL, or kg/m3 Mathematically:

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**Substances and their densities: Water 1.00 g/mL Table Sugar 1.59 g/mL **

Density Continued Substances and their densities: Water g/mL Table Sugar g/mL Gold g/mL Ice g/mL Ethanol g/mL

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Density 1. What is the density of an object that has a mass of 5 g and a volume of 2.5 cm3? 2. What is the mass of an object with a density of 10 g/mL and a volume of 2 mL?

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**Demonstration of Density**

OBJ: to see if different sodas have different densities We will use coke and diet coke to see if the two sodas have different densities Question: Based on what was observed, what can you conclude about the density of coke and diet coke?

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**Review-Copy and Answer in Notes**

1. What is density? 2. How much space does 5 g of a substance occupy if it has a density of 7.6 g / mL?

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Significant Figures Significant Figures (Sig Figs): include all the numbers that are known precisely plus one last digit that is estimated

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Significant Figures Brainstorm: Think about when you make a measurement. Your reading is based on the instrument that you are using. The last digit in a measurement is an estimate

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**In a measurement, the last digit is uncertain (meaning it is estimated and not known exactly)**

Question: What is the value in having the last digit in a measurement estimated? By estimating the last digit, one can obtain additional information about the measurement

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Activity for Sig Figs Three students measure the length of the board using 3 different meter sticks. One of the meter sticks only measures in units of 1. The second meter stick measures in unit of The third meter stick measures in units of 0.01. Question: How precisely can each student measure the board?

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Activity for Sig Figs The following measurements are obtained: Student 1: 3.5 m; Student 2: 3.70; Student 3: m

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Activity for Sig Figs 1. Which student has the most precise measurement? 2. If another student had to repeat the measurements, which ruler should the student use in order to be precise? 3. Why did student 2 report the value as 3.70?

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**Rules for Determining the Amount of Significant Figures**

In order to determine the number of significant figures in a measurement: 1. Every non-zero digit is significant. Example (Ex): has 3 sig figs

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**2. Zeros between non-zero digits are significant**

Ex: has 4 sig figs (the zero is between non zero digits) 3. Zeros appearing in front of all non-zero digits are NOT significant Ex: has only 2 sig figs

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**4. Zeros at the end of a number and to the right of a decimal point are significant**

Example: has 3 sig figs *The zero after a decimal point is significant only if a non-zero digit precedes it

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5. Zeros at the end of a measurement and to the left of the decimal point are not significant unless the zero was measured Example: 300 has only 1 sig fig 300.0 has 4 sig figs

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**Explain why “300 m” has only 1 sig fig but “300.0 m” has 4 sig figs?**

Question Explain why “300 m” has only 1 sig fig but “300.0 m” has 4 sig figs? Consider the precision and what type of ruler may have been used to determine these values

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**Determine the number of sig fig:**

a g b mL c kg d cm e mg f m g g a. 3 b. 3 c. 2 d. 4 e. 5 f. 5 g. 1

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**Determine the number of sig figs in: a. 456 b. 4.00 c. 6.090 d. 4000 **

Review Determine the number of sig figs in: a. 456 b c d e

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Practice – Sig Figs 1) Visit Sig Fig Link (home.carolina.rr.com/bwhitson) 2) Take Quiz at on Sig Figs (individually)

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**Rules for Sig Figs in Caculations**

An answer cannot be more precise than the least precise measurement from which it was calculated Basically, if you have to do a calculation with sig figs, your answer cannot be more precise than your least precise measurement

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**Multiplication and Division**

Multiplication and Division: The number of significant figures in the answer (result) is the same as the number of significant figures in your least precise measurement

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**Report 6.3 * 6.45 * 8.589 with the correct amount of sig figs**

Example Problem Report 6.3 * 6.45 * with the correct amount of sig figs

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Example Problem 1. Determine the amount of sig figs in each number: 6.3 = 2; = 3; = 4 2. Calculate your answer 3. Report your answer to the correct amount of sig figs

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**Your answer should have 2 sig figs 349.01401**

Solution 6.3 * 6.45 * 8.589= Your answer should have 2 sig figs First 2 Sig Figs Ans) 350 or 3.5 x 102

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**Additional Practice a. 2.7 / 5.27 b. 4.5 * 9.56 c. 2 * 5.6**

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**Rules for Addition and Subtraction**

Addition and Subtraction: The answer (result) has the same number of decimal places as the least precise measurement

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Example Problem 1. Determine which measurement is least precise 2. Calculate 3. Place in correct number of sig figs Solution: is the least precise = Ans) 31.1

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**Report the following answers with the correct amount of sig figs:**

Practice Report the following answers with the correct amount of sig figs:

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Percent Error (PE) Percent Error is the comparison of the actual (or true) value to the experimental Essentially, it is the “percent off” that you are from the actual answer

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**Experimental Value is what you obtained **

Percent Error (PE) Experimental Value is what you obtained True (Actual) Value is what the true value is (correct answer you should have gotten)

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Percent Error = PE

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Example Problems 1. You measure the mass of an object to be 5.11 g. The true mass is 5.20 g. What is the percent error? 2. You find the value for to What is your percent error?

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Direct Relationship Adsf dsafd

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**Indirect (Inverse) Relationship**

Adsf dsafd

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