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Lecture 1

Department

ChemistryCourse Code

CHEM 130Professor

Charles Mc MoryLecture

1This

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Chem CH R Lecture notes

R-1 units of measure

• A quantitive observation is called a measurement

• A measurement always has two parts: a number and a unit

•unit: a ﬁxed standard of measurement

• ex. Meter (length), kg (mass)

•Preﬁxes: indicate size of unit

• Be familiar with international system and chart/ conversions

R-3: Signiﬁcant Figures and Calculations

•signiﬁcant ﬁgures: digits that carry meaning and contribute to a number’s precision

• Rules for counting signiﬁcant ﬁgures

1) nonzero integers: all nonzero digits are signiﬁcant

• ex. 1486 has 4 signiﬁcant ﬁgures

2) zeros

!a) leading zeros are not signiﬁcant

• 0.0215 has 3 signiﬁcant ﬁgures

! ! b) captive zeros between nonzero digits are signiﬁcant

• 107.77 has 4 signiﬁcant ﬁgures

! ! c) trailing zeros at end of number are signiﬁcant (if they are to the right of the !

!!!decimal point, but not immediately to the right)

• 0.07200 has 4 signiﬁcant ﬁgures

• CLICKER QUESTION: how many sig ﬁgs in 0.00403700 cm?

• answer: 6 signiﬁcant ﬁgure

!3) Exact numbers: is determined by counting (or by deﬁnition) and has inﬁnite sig ﬁgs

• Example: 3 atoms in a water molecule, 100g in kg, 2.45 cm in an inch

• Always measurements have some degree of uncertainty

• Mathematical Operations and Rounding

!1) multiplication and division: your answer should have the same number of sig ﬁgs as the !

! ! measurement with the least number of sig ﬁgs

!2) addition and subtraction: your answer should have the same number of decimal places !

! ! as are in the measurement with the least number of decimal places

!3) rounding

• Carry extra digits through to the ﬁnal result of a problem, then round

• At the end, look at the leftmost digit to be dropped

• If digit is 5 or greater: add 1 to the last digit

• If digit less than 5: last digit stays the same

• CLICKER QUESTION: calculate with correct sig ﬁg and rounding: 8.52 + 4.1586 x

18.73 + 153.2

• answer: 239.6

!4) Exponential Notation

R-5: Dimensional Analysis

• Conversion factors/ unit factors: a factor equal to 1 that converts a quantity expressed in one

unit to a quantity expressed in another unit

• example: 1g/1000mg = 1 = 1000mg/1g!

• Determine the diffraction that would cancel the unwanted units

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