Chemistry Chapter 5—Measurements and Calculations
I.
II.
III.
IV.
V.
5.1 Scientific Notation
A. An easy way to deal with exceedingly large or exceedingly
small numbers
B. The exponent tells us how many decimal places we have moved
1. positive (+) exponents tell us to move the decimal to
the right
2. negative (-) exponents tell us to move the decimal point
to the left
C. You can only add or subtract numbers in scientific notation if
they have the exact same exponenet
D. You can multiply numbers in S.N. by multiplying the integers
and adding the exponents (and adjusting your answer)
E. You can divide numbers in S.N. by dividing the integers and
subtracting the exponents (and adjusting your answer)
F. You can also use your scientific calculator
Section 5.2 Units
A. We are most familiar with English units (miles, pounds, ounces
etc.)
B. This is unfortunate because science uses the metric system—
asystem of measurement based on the number 10
C. The “International System” of measurement used in Chemistry
uses the metric system
Section 5.3 Length Volume and Mass
A. We use metric units to measure length, mass and volume
B. The meniscus in the graduated cylinders
Section 5.4 Uncertainty in measurement
A. There is always some uncertainty in measurement as there is
no such thing as a perfect measuring tool or a perfect
measurer
Section 5.5 Significant figures
A. In general, you answers to scientific calculations should
contain as many decimal places as the least number of decimal
places in the problem
B. Rounding Off
1. if the number to be removed is less than 5 the
preceeding digit remains the same
VI.
2. if the number to be removed is more than 5 the
preceeding digit should be increased by 1
3. ONLY ROUND OFF ANSWERS! Do not round off
numbers before making your calculations.
Section 5.6 Problem Solving and Dimensional Awareness
A. Often in chemistry and other sciences we must convert
measurements from one unit to another. We do this all the
time, like hours to minutes, days to weeks etc. In chemistry
we use a process called dimensional awareness to convert
units.
B. Dimensional awareness consists of a series of calculations
based on the idea that whenever you multiply something by 1
you end up with what you started with.
C. Example: If we want to find out how many minutes are in 8
days we would carry out the following calculations:
8 days
x
24 hours x 60 minutes = 11,520 minutes
1 day
1 hour
Notice that each of these fractions
equals 1 because the numerator and
the denominator are equal—just the
units are different.
VII. Section 5.7 Temperature conversions
A. Here are the formulae for temperature conversions
1. Farenheit to Celsius
o
C = oF – 32
1.80
2. Celsius to Farenheit
o
F = (1.8)(oC) + 32
3. Celsius to Kelvin/Kelvin to Celsius
o
C + 273 = K and conversely K – 273 = oC
Kelvins are not measured in degrees!
VIII. Section 5.8 Density
A. Density is the very useful ratio of an object’s mass divided by
its volume. Example: 125 g of Au = 19.32g/cm3
6.47 cm3
(this is the density of gold)