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CHEMISTRY 20 TEACHER NOTES UNIT 1: REVIEW OF SCIENCE 10 TEXT PAGES: 2-13 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 non-logarithmic 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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of 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 non-zero digit appears to its left. 2. Drop all non-significant digits.
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This note was uploaded on 04/09/2010 for the course CHEMISTRY 1AA3 taught by Professor Goneer during the Spring '08 term at McMaster University.

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