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CHEMISTRY 20 TEACHER NOTES
UNIT 1: REVIEW OF SCIENCE 10
TEXT PAGES: 213
NUMERAL RULES
1. If a numeral value is less then 1 a “0” shall precede the decimal point e.g. 0.234 NOT .234.
2. To help reading long numbers the digits are commonly separated by a space into groups of
three counted from either side of the decimal point. e.g. 123 432. 345 67
CONVERTING UNITS
You will often have to convert from one unit to another. There are two basic rules when
converting units using the method I prefer. These rules are being used in answering the question
“How many mm are in 6 m?”
1. Begin by writing down the units you want in the final answer.
e.g. x mm =
2. Then write down the given information and multiply by the appropriate conversion factor to
get the answer:
e.g.
x mm = 6 m x
1000 mm
1 m
= 6000 mm
3. Check to see that all units, which are not required, are canceled and all units required are
arranged properly.
SIGNIFICANT DIGITS
All measured quantities have a degree of error. The significant digits of a measured value
include all the number that can be read directly from an instrument and includes one estimated
digit. The greater the number of significant digits the greater the precision.
The following rules apply for significant digits:
1. For all nonlogarithmic values, any digit from 1 to 9 is significant and 0 may be.
2. Leading zeros are not significant e.g. 0.087 has 2 significant digits, 2.009 has four significant
digits
3. Trailing zeros to the right of the decimal are significant e.g. 168.300 has 6 significant digits
4. For logarithmic values such as pH, any digit to the left of the decimal is not
significant. e.g. pH of 1.23 has 2 significant digits, pH 7 has 0
For manipulating measures the following rules apply:
1. An answer obtained by adding or
subtracting value must have a precision equal to the least
precise value used, or the value taken to the least number of decimal points e.g. 12.6 + 2.07 +
0.142 = 14.812 on calculator but is reported as 14.8.
2. An answer obtained by multiplying or dividing measured values has a certainty equal to the
least certain value or the value with the fewest significant digits e.g. 0.024 89 • 6.94 = 0.172
736 6 on a calculator but is reported as 0.173.
3. When a series of calculations is performed, each interim value should not be rounded before
carrying out the next calculation. The final answer should then be rounded to the same
number
1
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View Full Documentof significant digits as are contained in the quantity in the original data with the fewest
number
of significant digits.
4. When calculations involve exact numbers (counted and defined values) the calculated answer
should be rounded
based upon the precision of the measured values (number of decimal
points) e.g. 5 mol
• 32.06 g/mol = 160.30
SCIENTIFIC NOTATION
Sometimes a number is too large and must be reported in scientific notation using the following
rule:
1. Move the decimal place so that only one nonzero digit appears to its left.
2. Drop all nonsignificant digits.
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 Spring '08
 Goneer
 Chemistry

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