Kimia Dasar Chapter 1

7/29/2019 Kimia Dasar Chapter 1

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U N I V ERSI T Y I N D O N ESI AOF

Introduction:

Matter and Measurement


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Introduction: Matter

and Measurement

The Study of Chemistry

Classification of Matter

Properties of Matter Units of Measurement

Uncertainty in Measurement

Dimensional Analysis


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The Study of

Chemistry

Chemistry is the study of matter and the

changes that matter undergoes.

Matter is made up of almost infinitesimally

small building blocks called atoms.

Atoms can combine together to form

molecules.

Molecules of a few familiar substances are

represented here.
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The Study of

Chemistry


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The Study of

Chemistry


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The Study of

Chemistry


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The Study of

Chemistry


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The Study of

Chemistry


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Matter can exist in one of three states of

matter: a gas, a liquid, or a solid.

A gas is highly compressible and will assume both

the shape and the volume of its container.

A liquid is not compressible and will assume the

shape but not the volume of its container.

A solid also is not compressible, and it has a fixedvolume and shape of its own.
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Matter can also be classified according to its

composition.

Most of the matter that we encounter exists in

mixtures, which are combinations of two or

more substances.
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Mixtures can be homogeneous or

heterogeneous.

Mixtures can be separated into

pure substances, and pure substances can be

either

compounds or

elements.


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A familiar example of a mixture is salt water.

A sample of salt water has the same

composition throughout.

It can be separated into pure substances

water and ordinary table saltby a physical

process, such as distillation.


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Pure water is collected in the flask on the right.

When all of the water has been distilled from the

mixture, pure saltNaClwill remain in the flask

on the left.


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Both water and salt are pure substances.They cannot be further separated into simpler substances

by any physical process.

Each, however, can be decomposed into other substances

by a chemical process, namely electrolysis.


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Electrolysis
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The substances produced by the electrolysis of

water cannot be further separated by any

physical or chemical means.

Oxygen and hydrogen are elements.

When water is separated into its constituent

elements, the relative amounts of those

elements are always the same.


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Water is 11 percent hydrogen and 89 percent

oxygen by mass.

This is an example of the law of constant

composition, also known as the law of

definite proportions.
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Salt can also be separated into its constituent

elements, sodium and chlorine, by

electrolysis.

Sodium chloride also has a constant

composition, as do all pure substances. It is 39

percent sodium and 61 percent chlorine by

mass.
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U N IV ERSIT Y I N D ON ESIAOF

Properties of Matter

Different types of matter have different

distinguishing characteristics that we can use

to tell them apart.

These characteristics are called physical

propertiesand chemical properties.

Physical and chemical properties may be

intensive or extensive.
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Intensive properties such as density, color, and

boiling point do not depend on the size of the

sample of matter and can be used to identify

substances.

Extensive properties such as mass and volume

do depend on the quantity of the sample.


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Physical properties are those that we candetermine without changing the identity of thesubstance we are studying.

For instance, we can observe or measure thephysical properties of sodium metal. It is a soft, lustrous, silver-colored metal with a

relatively low melting point and low density.

Hardness, color, melting point and density are all

physical properties. Figure 7.15 shows a chunk of metallic sodium, which is

soft enough to be cut with a knife.


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Chemical properties describe the way a

substance can change or react to form other

substances.

These properties, then, must be determined

using a process that changes the identity of

the substance of interest.


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One of the chemical properties of alkali metals

such as sodium and potassium is that they

react with water.

To determine this, though, we would have to

combine an alkali metal with water and

observe what happens.


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Section 1.3


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Sodium metal (Na) reacts rather vigorously

with water to produce sodium hydroxide

(NaOH) and hydrogen gas (H2).

After the reaction has occurred, although we

now have evidence of one of sodium metal's

chemical properties, we no longer have

sodium metal.


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Potassium reacts even more vigorously withwater to produce potassium hydroxide (KOH)and hydrogen gas.

As with sodium, once we have determined achemical property of potassium metal, we nolonger have potassium metal.

To determine the chemical properties of asubstance, it is necessary to change thesubstance's chemical identity.


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The changes undergone by sodium and

potassium when they react with water are

chemical changes, also known as chemical

reactions.

Matter can also undergo physical changes in

which the chemical identity of the matter

does not change.


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One example of a physical change is the

melting of a solid.

When ice melts, it changes from a solid state

to a liquid state, but its chemical identity

(H2O) is unchanged.

All changes of state are physical changes.
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Units of Measurement

The scientific community uses SI units for

measurement of such properties as mass,

length, and temperature.

There are seven SI base units from which all

other necessary units are derived.
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Although the meter is the base SI unit used forlength, it may not be convenient to report thelength of an extremely small object or anextremely large object in units of meters.

Decimal prefixes allow us to choose a unit that isappropriate to the quantity being measured.

Thus, a very small object might best be measured

in millimeters (1 millimeter = 0.001 meters),while a large distance might best be measured inkilometers (1 kilometer = 1000 meters).


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The SI unit of temperature is the kelvin,

although the Celsius scale is also commonly

used.

The Kelvin scale is known as the absolute

temperature scale, with 0 K being the lowest

theoretically attainable temperature.

K = oC + 273.15
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Note that there are no units of volume in

Table 1.4.

For measurements of volume, density, and

other properties, we must derive the desired

units from SI base units.

In the case of volume, which has units of

length cubed, (length)3, the basic SI unit for

volume is the cubic meter (m3).



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This is an extremely large volume, though, andmore often you will see volumes reported inliters, L (1 cubic decimeter, or 1 dm3), ormilliliters, mL (which are the same as cubiccentimeters: 1 mL = 1 cm3).

Density has units of mass per unit volume and isoften reported as grams per cubic centimeter,

g/cm3

.



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Uncertainty in

Measurement

Even the most carefully taken measurements

are always inexact.

This can be a consequence of inaccurately

calibrated instruments, human error, or any

number of other factors.



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Uncertainty in

Measurement

Two terms are used to describe the quality of

measurements: precision and accuracy.

Precision is a measure of how closely individual

measurements agree with one another.

Accuracy refers to how closely individually

measured numbers agree with the correct or

"true" value.



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Uncertainty in

Measurement



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Uncertainty in

Measurement

Whatever the source, all measurements

contain error.

Thus, all measured numbers contain

uncertainty.

It is important that these numbers be

reported in such a way as to convey the

magnitude of this uncertainty.



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Uncertainty in

Measurement

Consider a fourth-grade student who, when

asked by his teacher how old the Earth is,

replies "Four billion and three years old."

(The student had been told by a first-gradeteacher three years earlier that the Earth was four

billion years old.)

Obviously, we don't know the age of Earth tothe year, so it is not appropriate to report a

number that suggests we do.



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Uncertainty in

Measurement

In order to convey the appropriate uncertainty

in a reported number, we must report it to the

correct number ofsignificant figures.

The number 83.4 has three digits.

All three digits are significant.

The 8 and the 3 are "certain digits" while the 4 is

the "uncertain digit.



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Uncertainty in

Measurement

As written, this number implies uncertainty of

plus or minus 0.1, or error of 1 part in 834.

Thus, measured quantities are generally

reported in such a way that only the last digit

is uncertain.

All digits, including the uncertain one, are

called significant figures.



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Uncertainty in

Measurement

Guidelines

Nonzero digits are always significant457 cm (3significant figures); 2.5 g (2 significant figures).

Zeros between nonzero digits are alwayssignificant1005 kg (4 significant figures); 1.03 cm(3 significant figures).

Zeros at the beginning of a number are never

significant; they merely indicate the position ofthe decimal point0.02 g (one significant figure);0.0026 cm (2 significant figures).


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Uncertainty in

Measurement

Guidelines

Zeros that fall at the end of a number or after the

decimal point are always significant0.0200 g (3

significant figures); 3.0 cm (2 significant figures). When a number ends in zeros but contains no

decimal point, the zeros may or may not be

significant130 cm (2 or 3 significant figures);

10,300 g (3, 4, or 5 significant figures).


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Uncertainty in

Measurement

To avoid ambiguity with regard to the number ofsignificant figures in a number with tailing zerosbut no decimal point, such as 700, we usescientific (or exponential) notation to express the

number. If we are reporting the number 700 to three

significant figures, we can leave it written as it is,or we can express it as 7.00 102.

There is no ambiguity in the latter regarding thenumber of significant figures, because zeros aftera decimal point are always significant.


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Uncertainty in

Measurement

However, if there really should be only two significantfigures, we can express this number as 7.0 x102.

Likewise, if there should be only one significant figure,we can write 7 x102.

Scientific notation is convenient for expressing theappropriate number of significant figures.

It is also useful to report extremely large and extremelysmall numbers.

It would be most inconvenient for us to have to writeall of the zeros in the number 1.91 10-24(0.00000000000000000000000191).


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Uncertainty in

Measurement

When measured numbers are used in a calculation, thefinal answer cannot have any greater certainty than themeasured numbers that went into the calculation.

In other words, the precision of the result is limited by

the precision of the measurements used to obtain thatresult.

For example: If we measure the length of one side of acube and find it to be 1.35 cm; and we then calculate

the volume of the cube using this measured length, weget an answer of 2.460375 cm3.


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Uncertainty in

Measurement

Our original measurement had three significantfigures.

The implied uncertainty in 1.35 is 1 part in 135.

If we report the volume of the cube to sevensignificant figures, we are implying an uncertaintyof 1 part in over two million!

We can't do that.

In order to report results of calculations so as toimply a realistic degree of uncertainty, we mustfollow the following rules.


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Uncertainty in

Measurement

When multiplying or dividing measurednumbers, the answer must have the samenumber of significant figures as the measured

number with the fewest significant figures. When adding or subtracting, the answer can

have only as many places to the right of thedecimal point as the measured number with

the smallest number of places to the right ofthe decimal point.


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Uncertainty in

Measurement

Using these rules, we would report the

volume of the cube in the example above as

2.46 cm3.


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Uncertainty in

Measurement


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Uncertainty in

Measurement



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Solving problems in chemistry requires carefulmanipulation of numbers and their associatedunits, a method known as dimensional

analysis. For example: What is the volume of a 5.25-

gram sample of a liquid with density 1.23g/mL?

The density of the liquid can be used as aconversion factor.

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For the liquid in the example, 1.23 grams are

equal to 1 milliliter (1 mL).

When the numerator and denominator of a

fraction are equal, the fraction has a value of1, meaning that we can multiply by it for the

purpose of changing units.



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The density conversion factor can be

expressed in either of the following two ways.



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Thank You

Kimia Dasar Chapter 1

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