1 UNITS OF MEASUREMENT Essay

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LEARNING OUTCOMES
After the class, students will be able to:
 To identify the SI units
 To identify the significant figures in calculation  To use scientific notation to express very small and very large numbers
 To use Factor label method to solve problems in a particular situation

WHAT IS CHEMISTRY?

WHAT IS MATTER?

Matter = anything that has mass and occupies space
ATOMS – smallest distinctive unit in a sample of matter
MOLECULES – larger units of 2 or more atoms. EOS

PROPERTIES OF MATTER
Physical property: characteristic displayed by a sample of matter without undergoing any change in its composition
e.g., color
Chemical property: characteristics displayed as a result of change in composition
e.g., flammability

EOS

PHYSICAL AND CHEMICAL CHANGES
Physical Change: changes in appearance but not in composition
(same substance before and after)
e.g., melting of ice in the winter ice (solid) → water (liquid)
Chemical Change: changes resulting in altered composition and/or molecular structure (different substance before and after) e.g., electric current pass through water
Water → H2 gas + O2 gas

UNITS OF MEASUREMENT

SI BASE UNITS
Scientists worldwide use common measurement units called the
International System of Units (SI)

DERIVED UNITS
Physical
quantity

Name of units

Symbol of unit

distance x distance

m2

Volume

distance3

m3

Velocity

distance/time

m/s or ms-1

Acceleration

distance/time2

m/s2 or ms-2

mass x acceleration

kg.m.s.-2
(1 Newton)

Area

Force

COMMON SI PREFIXES

Examples:
Gigahertz
(GHz)
Megabytes
(MB)
Terawatts (TW)

COMMON UNITS IN
LAB.
MEASUREMENTS

MEASURING LENGTH
• The SI unit for length, the meter (m), is too large for most laboratory purposes.
• More convenient units are the centimeter
(cm) and the millimeter (mm).
1 m = 100 cm = 1000 mm
1 cm = 10 mm

MEASURING VOLUME
•Volume is a derived unit with dimensions of (length)3.
•SI unit for volume is the cubic meter, m3.
•Volume units typically use the Liters base
•Very convenient for measuring the volume of irregularly shaped containers MEASURING VOLUME - CONT

Regularly shaped objects can use a variant of the volume unit … cubic distance units Some volume equivalents: 1 m3
I dm3

=
=
=
1 cm3 =
=
1 mm3 =

1000 dm3
1000 cm3
1 L = 1000 mL
1000 mm3
1 mL = 1000 µL
1 µL

MEASURING MASS
• The base unit for measuring mass is the kilogram (kg)
• However, gram (g) is a more convenient unit for laboratory measurements
1 kilogram = 1000 gram

MEASURING TEMPERATURE
Temperature is a measure of the intensity of heat.
The Celsius scale is the temperature scale in general scientific use.
The SI unit of temperature is the kelvin
(K)
The conversion of kelvin to degree
Celcius and Celcius to Fahrenheit are given by these mathematical equations

K = °C + 273.15

°F = (1.8 x °C) + 32

MEASURING TEMPERATURE

MEASURING DENSITY
The density of an object is its mass per unit volume,

where d is the density, m is the mass, and V is the volume. • The SI unit of density is kilogram per cubic meter (kg/m3).
• Typically, density is in units of g/L (g/dm3) or g/mL (g/cm3).

SIGNIFICANT FIGURES

PRECISION AND ACCURACY IN
MEASUREMENTS
• Precision refers to how closely individual scientific measurements agree with one another. • Accuracy refers to the closeness of the average of a set of scientific measurements to the “correct” or “most probable” value.
• Sampling errors occur when a group of scientific measurements do not represent the entire population of the variable being studied.

PRECISION AND ACCURACY ILLUSTRATED

SIGNIFICANT FIGURES
• All digits in a number that are known with certainty plus the first uncertain digit.
• The more significant digits obtained, the better the precision of a measurement.
• The concept of significant figures applies only to measurements.

An exact number is a number that arises when you count items or when you define a
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