BICOL UNIVERSITY College of Science

Department of Chemistry

GENERAL CHEMISTRY LECTURE HANDOUT 1

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INTERNATIONAL SYSTEM OF UNITS (SI)

The International System of Units, abbreviated as SI (from the French name Le Système International d’Unités), was established in 1960 by the 11th General Conference on Weights and Measures (CGPM) as the modern metric system of measurement.

The SI base units are defined as follows:

meter: The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

kilogram: The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.

second: The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.

ampere: The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2∙10–7 newton per meter of length.

kelvin: The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

mole: The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.

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SI Prefixes The following prefixes have been approved by the CGPM for use with SI units. Only one prefix may be used before a unit. Common (black) and uncommon (grey) SI unit prefixes

Prefix Meaning Abbreviation Exponential

Notation

Yotta- septillion Y 1024

Zetta- hexillion Z 1021

Exa- quintillion E 1018

Peta- quadrillion P 1015

Tera- trillion T 1012

Giga- billion G 109

Mega- million M 106

kilo- thousand k 103

hecto- hundred h 102

deka- ten da 101

deci- tenths of d 10-1

centi- hundredths of c 10-2

milli- thousandths of m 10-3

micro- millionths of µ 10-6

nano- billionths of n 10-9

pico- trillionths of p 10-12

femto- quadrillionths of f 10-15

atto- quintillionths of a 10-18

zepto- hexillionths of z 10-21

yocto- septillionths of y 10-24

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Some Non-SI units encountered in chemistry

Non SI unit Unit type SI conversion Notes

liter (L) volume 1 L = 1000 cm3 1 quart = 0.946 L

Angstrom (Å) length 1 Å = 10-10 m typical radius of an atom

atomic mass unit (u)

mass 1 u = 1.66054×10-27

kg

about the mass of a proton or neutron; also known as a 'dalton'

or 'amu'

Volume – SI derived unit for volume is cubic meter (m3)

Example: 1 cm3 = (1 x 10-2 m)3 = 1 x 10-6 m3 1 dm3 = (1 x 10-1 m)3 = 1 x 10-3 m3 1 L = 1000 mL = 1000 cm3 = 1 dm3 1 mL = 1 cm3

Density – SI derived unit for density is kg/m3

𝑑 =𝑚𝑎𝑠𝑠

𝑣𝑜𝑙𝑢𝑚𝑒

1 g/cm3 = 1 g/mL = 1000 kg/m3

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Example: A piece of platinum metal with a density of 21.5 g/cm3 has a volume of 4.49 cm3. What is its mass? m = d x V = 21.5 g/cm3 x 4.49 cm3 = 96.5 g

Temperature

K = °C + 273.15

℃ =𝟓

𝟗(℉ − 𝟑𝟐)𝒐𝒓 ℉ =

𝟗

𝟓(℃) + 𝟑𝟐

Example:

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The temperature of interstellar space is 2.73 K. What is this temperature on the

Celsius and Fahrenheit scales?

K = °C + 273.15 or °C = K - 273.15

℉ =9

5(℃) + 32

Arithmetic with units

addition and subtraction: units don't change 2 kg + 3 kg = 5 kg 412 m - 12 m = 400 m

o consequence: units must be the same before adding or subtracting! 3.001 kg + 112 g = 3.001 kg + 0.112 kg = 3.113 kg 4.314 Gm - 2 Mm = 4.314 Gm - 0.002 Gm = 4.312 Gm

multiplication and division: units multiply & divide too 3 m × 3 m = 9 m2 10 kg × 9.8 m/s2 = 98 kg m/s2

o consequence: units may cancel 5 g / 10 g = 0.5 (no units!) 10.00 m/s × 39.37 in/m = 393.7 in/s

Dimensional Analysis Method of Solving Problems

1. Determine which unit conversion factor(s) are needed 2. Carry units through calculation 3. If all units cancel except for the desired unit(s), then the problem was solved

correctly.

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How many mL are in 1.63 L?

Conversion Unit 1 L = 1000 mL

The speed of sound in air is about 343 m/s. What is this speed in miles per

hour?

1 mi = 1609 m

1 min = 60 s

1 hour = 60 min

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Uncertainty in Measurement A digit that must be estimated is called uncertain. A measurement always has some degree of uncertainty. Accuracy – how close a measurement is to the true value Precision – how close a set of measurements are to each other

good precision & good accuracy

good accuracy but poor

precision

poor accuracy but good precision

poor precision & poor accuracy

Types of Error

Random Error (Indeterminate Error) - measurement has an equal probability of being high or low. Systematic Error (Determinate Error) - Occurs in the same direction each time (high or low), often resulting from poor technique or incorrect calibration.

SIGNIFICANT FIGURES

• Any digit that is not zero is significant (Nonzero integers)

1.234 kg 4 significant figures

• Zeros between nonzero digits are significant (Captive Zeros) 606 m 3 significant figures

• Zeros to the left of the first nonzero digit are not significant (Leading Zeros) 0.08 L 1 significant figure

• If a number is greater than 1, then all zeros to the right of the decimal point are significant (Trailing Zeros) 2.0 mg 2 significant figures

• If a number is less than 1, then only the zeros that are at the end and in the middle of the number are significant 0.00420 g 3 significant figures

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Rounding off

If the next digit to the right is greater than 5, we round up; if it is less than 5, we round down.

For cases in which the next digit is exactly 5, a good rule to follow is to round to the nearest even number, and there are no additional insignificant digits or else all subsequent insignificant digits are O.

For example, rounding off 45.45 or 45.450 to 3 significant figures makes it 45.4; however, we would round off 45.55 or 45.550 to 45.6. Rounding 45.450001 to 3 significant figures makes it 45.5.

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Scientific Notation A method of representing very large or very small numbers in the form:

M x 10n

M is a number between 1 and 10 n is an integer

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Book References:

CRC Handbook of Chemistry and Physics 89th Ed 2009

Chemistry. 9th edn. Raymond Chang. 2007.

Internet Resources:

http://antoine.frostburg.edu/chem/senese/101/measurement/

http://physics.nist.gov/cuu/Units/index.html

http://www.sciencegeek.net/Chemistry/index.shtml