CHEM 130 Lecture 1: unit r
2 pages21 viewsWinter 2019
Course CodeCHEM 130
ProfessorCharles Mc Mory
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Wednesday, January 9, 2019
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
!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
• 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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