CHAPTER 2 Measurements and Calculations

Chapter 2

Measurement

• Quantitative observation

• Properties that can be measured are called quantitative

properties

– Expressed using a number.

– Must always include the appropriate unit

• Uses numbers and units

– Number tells comparison

– Unit tells scale

Using Scientific Notation

• Placement and direction of a decimal point
– The number of places the decimal point is moved
determines the power of 10
– The direction of the move determines whether the power of
10 is positive or negative

Unit

The scale or standard being used to represent the results
of a measurement

– Scientists require common units to measure quantities such as
mass, length, time, and temperature

Most widely used standard systems of units
– English system (used in the United States (ft, Ga, lb....)
– Metric system (used in most of the world (m, L, Kg,....)
➡Preferred for scientific work

International System or SI

– Comprehensive system of units set up by an international
agreement
– Units based on and derived from the metric system
– General Conference on Weights & Measures (GCWM: 1960)

extensive property

depends on the amount of matter (sample size).

➡ Value of the same extensive properties are additive.
➡ Example: Mass, volume, length,....

intensive property

does not depends on the amount of matter (sample size).

➡ intensive properties are not additive.
➡ Example: temperature, density,....

Scientist use variety of devices (volume, T, mass, ....) to measure
the properties of matter

volumetric flask

graduated cylinder

pipette

burette

density

of a substance is the ratio of mass to volume.

kilogram per cubic meter (kg/m3)
Other common units:
g/cm3 (solids)
g/mL (liquids)
g/L (gases)

Volume

• Measure of the amount of 3-D
space occupied by a
substance
• SI unit = Cubic meter (m3 )
• Commonly measured in cm3
• 1 mL = 1 cm 3
• 1 L = 1 dm 3

Mass

• Measure of the amount of
matter present in an object
• SI unit is Kilogram (kg)
• 1 kg = 2.2046 lbs
• 1 lb = 453.59 g

Uncertainty in Measurement

There are two types of numbers used in chemistry:
• Exact numbers which have defined values
- 1 kg = 1000 g,
- 1 dozen = 12 object,
- any number obtained by counting

• Inexact numbers
- Numbers obtained by any method other than counting
- Measured numbers are inexact because of the instruments and
operators or both

Certain and Uncertain Measurements

• A digit that must be estimated is called uncertain
• A measurement always has some degree of uncertainty
• Certain digits are recorded first followed by uncertain digits or the estimated number

The Reliability of a Measurement-Significant Figures

• An inexact number must be reported so as to indicate its uncertainty.
• This is done using significant figures.
• Significant figures are the meaningful
digits in a reported number.
• The last digit in a measured number is
referred to as the uncertain digit.
•The uncertainty is considered to be ±1 in the
place of the last digit.

Relative Uncertainty and % Uncertainty

Dimension Analysis

• Most chemistry problems you will solve in this course are unit conversion problems.
• Using units as a guide to solving problems is called dimensional analysis.

Three Units for Measuring Temperature

• Fahrenheit
• Celsius
• Kelvin

The Celsius ( o C) degree - Kelvin (K) degree conversion.

The Fahrenheit degree - Celsius degree conversion.

Precision and Accuracy

– precise if they are consistent with one another.
– accurate only if they are close to the actual value