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CHEM 1411 General Course Guide - LAB

The Basics

Chemistry - The study of the composition of matter and the changes that matter undergoes.

Five different branches of chemistry:

  • Organic: the study of the structure, properties, composition, reactions, and preparation of carbon-containing compounds  (e.g., hydrocarbons, hydrogen, nitrogen, oxygen, halogens, phosphorus, silicon, and sulfur).
  • Inorganic: the study of the remaining subset of compounds other than organic compounds (e.g., metals, minerals, and organometallic compounds).
  • Analytical: the art and science of determining what matter is and how much of it exists.
  • Physical: the study of how matter behaves on a molecular and atomic level and how chemical reactions occur.
  • Biochemistry: the study of the structure, composition, and chemical reactions of substances in living systems.

The following video by Bozeman Science (Mr. Anderson) gives a brief history of the scientific method and discusses it in step-by-step detail.

Steps for the Scientific Method
Steps Explanation
Problem What is wrong?
Hypothesis "Educated Guess" - Predict what will happen based on prior knowledge
Experiment Test your hypothesis
     a) Control Group - This group will stay the same and is used for comparison
     b) Variable Group - This is what is being manipulated and changes
                    1. Independent Variable - This is what the experimenter changes (what I change)
                    2. Dependent Variable - This is what is being observed and measured (the dependent variable DEPENDS on the                                                independent variable)
                    
3. Control Variables - Things that are kept the same during each experiment

Remember, a good experiment only has two variables that change (independent and dependent). All the rest of the variables must be the same.

Data Collection

Observe the data that has been collected and apply statistical analysis.
When graphing, think of DRY MIX:
     D = dependent               R = responding               Y = y-axis
     M = manipulated            I = independent              X = x-axis

Conclusion Based on the observation and might support the hypothesis

Observation - The act of gathering information (actually seeing it).
Inference - An opinion based on the observations.

Examples:
The sky is cloudy today = observation
It is going to rain = inference

Qualitative - Describes physical characteristics (e.g., color, odor, shape, sound, taste, texture)
Quantitative Numerical information (e.g., How much?, How fast? How many? How tall?)

Scientific Law - Summarizing statement of many experiments (has been proven, usually stated in mathematical formula.

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  •  Law of Conservation of Mass - No atoms are created or destroyed in a chemical reaction. Instead, they just join together in a different way than they were before the reaction, and form products.
    • The total mass stays the same during a chemical reaction. This is the law of conservation of mass.
  • Law of Conservation of Energy - This is the first law of thermodynamics. Energy cannot be created or destroyed - it can only be transferred from one type to another.

Scientific Theory - Thoroughly tested explanation of why experiments give certain results (helps predict natural systems; cannot be proven).

  • Dalton's Atomic Theory: All matter is made of atoms; all atoms of a given element are identical in mass and properties; compounds are combinations of two or more different types of atoms; a chemical reaction is a rearrangement of atoms

Matter: Takes up space and has mass. It is the material of the universe.

Three most common states of matter:

  • Solid: Rigid; fixed volume and shape (e.g., ice)
  • Liquid: Definite volume, but no specific shape; assumes shape of container (e.g., water)
  • Gas: No fixed volume or shape; takes on shape and volume of container (e.g., steam)

Most matter around us consists of mixtures of pure substances (e.g., wood, gasoline, wine, soil, and air).

Pure substances: A substance with constant composition. Pure water is composed solely of H2O molecules, but water found in nature is a mixture.

Two classifications of mixtures:

  • ‚ÄčHomogeneous (also called solution): Having a uniform composition and appears visually the same throughout
    • Air is a solution; wine is a complex liquid solution; brass is a solid solution of copper and zinc
  • Heterogeneous: Having visibly distinguishable parts; a mixture with a composition that varies from point to point
    • Chocolate chip cookies; Iced tea with ice cubes

Heterogeneous mixtures usually can be separated into two or more homogeneous mixtures or pure substances (e.g., the ice cubes can be separated from the tea).

Example:

  • Seawater: Contains larger amounts of dissolved minerals.
    • Boiling seawater produces steam, which can be condensed to pure water, leaving the minerals behind as solids.
    • The dissolved minerals in seawater can also separated out by freezing the mixture, since pure water freezes out.
       
  • Physical change: The processes of boiling and freezing (a change in the form of a substance, not in its chemical composition).
    Example: When water freezes or boils, it changes its state but it still remains as water. It is still composed of H2O molecules.
  • Physical property: a characteristic of matter that is not associated with a change in its chemical composition.
    Examples: Density, color, hardness, melting and boiling points, and electrical conductivity.

Methods for separating the components from a mixture:

  • Distillation: a process that depends on differences in the volatility (how readily substances become gases) of the components.
  • Filtration: Used when a mixture consists of a solid and a liquid - the mixture is poured onto a mesh (e.g., filter paper) which passes the liquid and leaves the solid behind.
  • Chromatography: Uses a system with two phases (states) of matter - a mobile phase (liquid or a gas) and a stationary phase (a solid). The separation process occurs because the components of the mixture have different affinities for the two phases and move through the system at different rates.

Example:
High affinity (mobile phase) = moves quickly through the chromatographic system
High affinity (solid phase) = moves more slowly

  • Paper chromatography: Uses a strip of porous paper (filter paper) for the stationary phase. A drop of the mixture is placed on the paper, dipped into a liquid (the mobile phase) that travels up the paper.

Note: When a mixture is separated, the absolute purity of the separated components is an ideal.

Pure substances are either compounds (combinations of elements) or free elements.
Compounds: A substance with constant composition that can be broken down into elements by chemical processes (e.g., electrolysis of water - an electric current is passed through water to break it down into the free elements hydrogen and oxygen).

  • Chemical change: A given substance becomes a new substance or substances with different properties and different composition.
    Example: The formation of rust is a chemical change because rust is a different kind of matter than the iron, oxygen, and water present before the rust formed.
  • Chemical property: The change of one type of matter into another type (or the inability to change).
    Example: Iron combines with oxygen in the presence of water to form rust.

Elements: Substances that cannot be decomposed into simpler substances by chemical or physical means.

Math in Chemistry?

A quantitative observation (measurement) always consists of two parts: a number and a scale (unit).

There are two major systems of measurements: the English system used in the United States and the metric system used by most of the rest of the industrialized world.

For scientists, an international agreement set up a system of units called the International System (le Système International in French), or the SI system. This system is based on the metric system and units derived from the metric system.

Fundamental SI Units

Physical Quantity Name of Unit Abbreviation
Mass kilogram kg
Length meter m
Time second s
Temperature kelvin K
Electric current ampere A
Amount of substance mole mol
Luminous intensity candela cd

Prefixes Used in the SI System (Most common are shown in green).

Prefix Symbol Meaning Scientific/Exponential Notation
exa E 1,000,000,000,000,000,000

1018

peta P 1,000,000,000,000,000

1015

tera T 1,000,000,000,000

1012

giga G 1,000,000,000

109

mega M 1,000,000

106

kilo k 1,000

103

hecto h 100

102

deka da 10

101

-- -- 1

100

deci d 0.1

10-1

centi c 0.01

10-2

milli m 0.001

10-3

micro µ 0.000001

10-6

nano n 0.000000001

10-9

pico p 0.000000000001

10-12

femto f 0.000000000000001

10-15

atto a 0.000000000000000001

10-18

 

 

 

 

 

 

 

 

 

 

An important point concerning mass and weight:

Mass is a measure of the resistance of an object to a change in its state of motion. Mass is measured by the force necessary to give an object a certain acceleration.

On Earth, we use the force that gravity exerts on an object to measure its mass. We call this force the object's weight. Since weight is the response of mass to gravity, it varies with the strength of the gravitational field.
 

Fun fact:

Your body mass is the same on Earth and on the moon

Your weight would be much less on the moon than on Earth because of the moon's smaller gravitational field.

 

 



 


Imagine that five different people took a measurement reading from the same buret. They were measuring volume by reading the meniscus (bottom of the liquid curve). The results are as follows:
                                               20.15 mL, 20.14 mL, 20.16 mL, 20.17 mL, 20.16 mL
‚ÄčThese results show that the first three numbers are the all same (20.1) even though different people took the readings. Since that is the case, this is called certain digits. The digit to the right of 1 is estimated and that is why it is different in almost every answer. That last number is called an uncertain digit.

Measurements always have some degree of uncertainty. The uncertainty of a measurement depends on the precision of the measuring device.

Accuracy: The agreement of a particular value with the true value.

Precision: The degree of agreement among several measurements of the same quantity.

Precision is more on the reproducibility of the measurement while accuracy requires the measurement to be close with the actual answer. The illustration to the left is the results of several darts thrown and shows the differences between accuracy and precision.

a) Neither accurate nor precise
b) Precise but not accurate
c) Accurate AND precise

The numbers of scientific measurements are often very large or very small; thus making it convenient to express them using powers of 10. Scientific/Exponential notation expresses a number as N x 10M.

Examples:

The number 1,300,000 can be expressed as 1.3 x 106 - An easy way to remember is that the exponent (6) is the number the decimal moved to its wanted position.

1,300,000 is a whole number which means the decimal will be found at the end.  1,300,000.

From there, the decimal wants to be behind the first actual number; in this case it is the one. So, the decimal will move to be in the wanted position.

1,300,000.

1,300,00.0 = 1,300,00.0 x 101     Notice here that we moved the decimal once and changed the exponent to a number one. Remember, the decimal WANTS to be behind the first actual number (which is the one) so it needs to keep moving.

1,300,0.00 = 1,300,0.00 x 102     The exponent changes each time you move the decimal.

1,300.000 = 1,300.000 x 103       Again, we moved the decimal three times already, so the exponent is the number three.

1,30.0000 = 1,30.0000 x 104      

1,3.00000 = 1,3.00000 x 105

1.300000 = 1.300000 x 10or 1.3 x 106

Let's do another example! This time we are going to do the number 0.0034. Now, the question you may be having is, "Well, it's already behind a number, should we just leave it alone?" The answer is no. In this case, zero does not count as an actual number so the decimal wants to move behind the three. Whenever you are making your answers to scientific notation and wondering where to move the decimal, the actual numbers to consider are 1 - 9. 

0.0034     We already have a decimal there. Now, we just need to move it behind the three.

00.034 = 00.034 x 10-1     Notice the exponent. This time when we moved the decimal one place, the exponent is now a negative one.

Helpful Tip:

If the start number is more than one (1,300,000), the exponent will be positive.

If the start number is less than one (0.0034), the exponent will be negative.

000.34 = 000.34 x 10-2

0003.4 = 0003.4 x 10-3 or 3.4 x 10-3

What if the teacher wants all your answers in scientific notation, but the answer is 5.6? 

5.6     We need to move the decimal behind the first actual number. Well, it already is so we don't need to move it. This means your answer should be this:

5.6 x 100     Since you move the decimal zero times, your exponent will be zero.

More Scientific Notation Example Problems
Number Answers: Highlight the text inside the box to reveal answer
1985 1.985 x 103
9.9 9.9 x 100
0.011 1.1 x 10-2
0.00000000519 5.19 x 10-9

For more practice problems, visit the University of Missouri-Rolla's self test page.

Significant Figures: the certain digit and the first uncertain digit of measurement.

Rules for Counting Significant Figures
Rule # Rules Examples
1 Nonzero integers. Nonzero integers always count as significant figures. Numbers 1 - 9
2 Leading zeros are zeros that come before all the nonzero digits and DO NOT count as significant figures. 0.0034 - There are 2 significant figures (the 3 & 4). The zeros before do not count.
3 Zero sandwich. Zeros between nonzero digits count as significant figures.

303 - There are 3 significant figures. The zero in between the three's count.
40009 - 5 significant figures

4 Trailing zeros are zeros at the right end of the number and they only count if the number contains a decimal point.

100 - There is 1 significant figure. There is no decimal point so the trailing zeros do not count.
100. - The decimal point is that the end of the 100 and now all of the trailing zeros count. So, you have 3 significant figures.
1.00 x 102 - Scientific notation always has a decimal and the trailing zeros count. So, again you have 3 significant figures.

5 Exact numbers. Sometimes calculations involve numbers that were determined by counting and not using measuring devices (10 experiments, 3 apples, 8 molecules). These are exact numbers and can have an infinite number of significant figures. If a number is exact, it DOES NOT affect the accuracy of a calculation nor the precision of the expression. 1 foot = 12 inches - Depending on how you use it in the equation it can either have 1 significant figure or 2. So, neither of those numbers can limit the number of significant figures used in the calculation.

For a video tutorial on the rules regarding zero, view Significant Figures and Zero by Tyler DeWitt.

Example Problems for Significant Figures

Example Problems Answers: Highlight the text inside the box to reveal answer
3.0800 5 - The first zero is sandwiched between the 3 & 8 making it significant and the trailing zeros are significant because of the decimal.
0.004180 4 - The leading zeros do not count and the trailing zero behind the 8 counts because of the decimal.
7.09 x 107 3 - The zero is sandwiched and there is a decimal. It counts.
30,800 3 - The zero between the 3 & 8 is significant because it is sandwiched, but the trailing zeros do not count because there is no decimal point.

Rules for Significant Figures in Mathematical Operations:

Multiplication or division: The number of significant figures in the result is the same as the number in the least precise measurement used in the calculation.

4.56 X 1.4 = 6.38  -   Because the 1.4 has the least number of significant figures in the equation, your answer should also have 2.

Corrected answer: 6.4  -  Now, the answer has 2 significant figures. The same amount as the 1.4 which is the least number.

Addition or subtraction: The result has the same number of decimal places as the least precise measurement used in the calculation.

12.11 + 18.0 + 1.013 = 31.123  -  18.0 only has one number after the decimal point and that will be considered the limiting term. So, your answer should only have one number after the decimal point.

Corrected answer: 31.1

Helpful Tip:

Multiplication and division - ALL the significant figures are counted.

Addition and subtraction - Only the numbers AFTER the decimal are counted.

Example Problems for Significant Figures in Mathematical Operations
Example Problems Answers: Highlight the text inside the box to reveal answer
(1.05 x 10-3) ÷ 6.135 1.71149 x 10-4 - Corrected to 1.71 x 10-4 because the term with the least amount of significant figures is (1.05 x 10-3) which has 3.
21 - 13.8 7.2 - Corrected to 7 because 21 does not have any numbers after the decimal which makes the number zero the least number of decimal places.
3.461728 + 14.91 + 0.980001 + 5.2631 24.614829 - Corrected to 24.61 because 14.91 has the least number of significant figures after the decimal, which is 2.

For more practice problems, visit the University of Missouri-Rolla's self test page.

In most calculations, you will need to round numbers to obtain the correct number of significant figures.

Rules for Rounding:

In a series of calculations, carry the extra digits through to the final resultthen round.

If the digit to be removed

  1. is less than 5, the digit before stays the same (e.g., 1.33 rounds to 1.3).
  2. is equal to or greater than 5, the preceding digit is increased by 1 (e.g., 1.36 rounds to 1.4).
Rounding Example Problems
Instructions for Examples Examples Answers: Highlight the text inside the box to reveal answer
Round the example to three significant figures. 22.528 22.5 - The 2 after the 5 is less than 5 and must be removed.
Round the example to one significant figure 103,007 100,000 - The 0 after the 1 is less than 5 and the other numbers will be "removed". This number still has only 1 significant figure because the trailing zeros do not count since there is no decimal.
Round the example to two significant figures. 0.000847 8.5 x 10-4 - The leading zeros aren't significant. The 7 after the 4 is higher than 5, so the 4 rounds up to become 5.