Honors Chemistry Chapter 3. Objectives—Chapter 3 Understand and be able to explain the nature of measurement Qualitative, quantitative Accuracy, Precision,

Honors Chemistry

Chapter 3

Objectives—Chapter 3

Understand and be able to explain the nature of measurement Qualitative, quantitative Accuracy, Precision, Error

Demonstrate a knowledge of significant figures through problem solving.

Demonstrate an ability to solve problems with scientific notation

To use proper scientific (metric) units. Be able to calculate density of various objects Convert temperature to a different scale.

Vocab—give example of formula when appropriate!

Scientific Notation Precision Accuracy Error Kelvin Density Metric Units

Joke of the day

what does a chemist say when he finds two helium molecule?

HeHe

What is wrong with this picture?

A group of Civil Engineers were at a conference being held in Central Australia. As part of the conference entertainment, they were taken on a tour of the famous rock, Uluru. "This rock", announced the guide, "is 50 000 004 years old."

The engineers - always impressed by precision in measurement - were astounded.

"How do you know the age of the rock so precisely?" asked one of the group.

"Easy!", came the reply. "When I first came here, they told me it was 50 million years old. I've been working here for four years now."

Rules for Scientific Notation

Example: 3427g to scientific notation.

1. Move the decimal point until the ”root” number is between 1 and 10

3427 3.427

2. Count the number of places the decimal moved.

3. Move to the left. Multiply the “root” number times 10 raised to the

positive power that the decimal moved 3.427 x 103

Rules for Scientific Notation---Part B

Example: 0.003427g to scientific notation.

1. Move the decimal point until the ”root” number is between 1 and 10 .003427 3.427

2. Count the number of places the decimal moved.

3. If move to the right. Multiply the “root” number times 10 raised to a negative

power = number of places moved.

0.003427 3.427 x 10-3

Recall--Exponents

am x an = a (m+n)

am = a (m-n)

an

(am)n = amn

1/an= a-n

Scientific Notation

• Addition and subtraction

– Exponents must be the same.– Rewrite values with the same

exponent.– Add or subtract coefficients.

Uncertainty in Measurement

So what uncertainty would you have in a qualitative (observation—color, smell…) measurement?

What about quantitative (data taken with an instrument—thermometer, scale…) measurement, what would give rise to uncertainty?

Types of Error

Random error is the error that a measurement has an equal probability of being high or low.

Systematic error in the same direction each time, either high or low.

Uncertainty in Measurement

Certain v. uncertain in measurement. Always measure one beyond the scale of the

instrument The last digit is uncertain in a measurement In class demo. Let’s read this graduated

cylinder and see what everyone gets.

Precision and Accuracy

Accuracy is the agreement between a data measurement and the true measurement.

Precision refers to the degree of agreement between several measurements.

Precision v. Accuracy

Evaluating Error

How do you measure discrepancy between the true and experimental value?

Percent error—measures the magnitude of error. Percent Error= |true-experimental| x100%

true

Significant Figures

Error and Significant Figures

The convention is to read a measurement with certain numbers and one uncertain number.

This allows a scientist to indicate precision within their data.

Only as good as your least precise instrument!!!

Copyright©2000 by Houghton Mifflin Company. All rights reserved.

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Rules for Counting Significant Figures

Nonzero integers always count as significant figures.

3456 has 4 sig figs.


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Rules for Zeros-#1

Leading zeros do not count as significant figures.

0.0486 has 3 sig figs.


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Rules for Zeros-#2

Captive zeros always count as significant figures.

16.07 has 4 sig figs.

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Rules for Zeros-#3

Trailing zeros are significant onlyif the number contains a decimal

point. 9.300 has

4 sig figs. 9,300 has 2 sig figs

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Exact Numbers

Exact numbers count, they have an infinite number of significant figures.

12 apples, 1 inch = 2.54 cm, exactly


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Rules for Counting Significant Figures - Overview

1. Nonzero integers (always count)

2. Zeros leading zeros (don’t count) captive zeros (always count) trailing zeros (sometimes count)

3. Exact numbers (always count)


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Rules for Significant Figures in Multiplication and Division

Multiplication and Division: # sig figs in the result equals the number in that has the least sig figs.

7.7meters 5.4meters =41.58 square meters 42 (2 sig

figs)


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Rules for Significant Figures in Addition and Subtraction

Addition and Subtraction: # sig figs in the result equals the number of decimal places in the least precise measurement.

6.8 + 11.934 =18.734 18.7 (3 sig figs)

Exponential Notation and Scientific Figures

The Advantage of writing 1.00 x 102 in this type of notation: Easy to identify the sig figs. Convenient—a lot less zeros to write.


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Nature of Measurement

Measurement - quantitative observation consisting of 2 parts

Part 1 – number

Part 2 - scale (unit)

Examples: 20 grams

6.63 Joule seconds

SI System (King Henry Drinks Much Dark Chocolate Milk)

Units of measurement

SI System

King Henry Drinks Much Dark Chocolate Milk

Allows for easy comparison of data across the world.

Based on 10 which makes conversions simple and consistent

Most important units to remember in chemistry

Volume is in mL or Liters (1000ml = 1L)

Mass is in grams (1kg = 1000g or 1g =1000mg)

Temperature Kelvin or Celsius (273 + C = K)

Celsius and Kelvin Scale

K = C + 273 C = K - 273

Density

The mass of a substance that occupies a given volume.

D= Mass

Volume The SI standard unit of volume is the cubic

meter (m3) but it is more practical to use g/cm3 for liquids and g/dm3 for gases.

Note cm3 = mL

Dimensional Analysis

What is it?

Using Conversion Factors to Solve Problems!

Using Conversion Factors or proportionality to allow you to change the unit without changing the amount.

Example) 1000g or 1kg 1kg 1000g

How do I make a conversion factor?

Using 2 units that equal the same amount or quantity you make a fraction or "factor" Example: 1 dollar = 4 quarters = 10 dimes = 20 Nickels = 100 pennys 1 dollar or 4 quarters 4quarters 1dollar

Kilo = 1000 base units Hecta = 100 base units Deka = 10 base units Base Unit: meter, Liter, Grams 1 base unit = 10 deci 1 base unit = 100 centi 1 base unit = 1000 milli

How do I use a conversion factor?

How many quarters are in $ 3. 75? $1 = 4 quarters 1 dollar or 4 quarters 4quarters 1 dollar Start with given amount that is not a conversion.

.

How do I use a conversion factor cont..

$3.75 4 quarters =15 quarters 1$

How do I decipher a problem? • 1st: Identify Knowns

• 2nd: Identify Unknowns • 3rd: Do I need to make any • conversions? Ifso what • conversion factors do I need?

Use the factor that allows you to cancel out the unit given and get the desired unit.

How do I start a problem?

If it is an equation, and the units match, just plug it in and solve for unknown.

Make Sure Units Match! If they don't match, use conversion factors to convert units.

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Honors Chemistry Chapter 3. Objectives—Chapter 3 Understand and be able to explain the nature of measurement Qualitative, quantitative Accuracy, Precision,