CHEM 1105 January 10, 2014

Transcription

1 CHEM 1105 January 10, 2014 Today: Measurements and Problem solving (Chapter 1, sections ) SI units and prefixes Converting units Temperature, Volume, Density Significant figures More unit conversion problems Measurements in the lab Basic SI Units Length Mass Time Temperature Amount metre (m) kilogram (kg) second (s) kelvin (K) mole (mol) (chemical amount of a substance) (6.02 x units) 1

2 SI (Metric) prefixes Prefix Symbol Decimal Equivalent Power of 10 mega- M 1,000,000 Base x 10 6 kilo- k 1,000 Base x 10 3 deci- d 0.1 Base x 10 1 centi- c 0.01 Base x 10 2 milli- m Base x 10 3 micro- m or mc Base x 10 6 nano- n Base x 10 9 pico p Base x Temperature Scales: Comparison 2

3 Temperature Conversions 54 C to Kelvin 430 K to 350 F to C Conversions with Google Metric System: Converting units 14.5 g to kg 93 mg to g 54 pg to ng 23.0 mm to cm 55 nm to km 3

4 Conversions between Metric and Imperial units kg to pounds 8.93 miles to km 539 ml to ounces Derived Units (derived from the basic units) Area Volume Density (Length) 2 (Length) 3 Mass/volume (or mass/length 3 ) Energy units (later on) 4

5 Volume Volume: units of (length) 3 m 3 or cm 3 By definition: 1 cm 3 = 1 ml (millilitre) 1 ml = 10-3 L or 1000 ml = 1 L = 1 litre Density Density = mass volume D = m/v Commonly used units: g/ml g/cm 3 g/l 5

6 Density problem Ethylene glycol is the substance used in antifreeze solutions. Ethylene glycol has a density of 1.11 g/cm 3. What is the mass of 95 ml of ethylene glycol in grams? How much mass in grams is there in 1 gallon of ethylene glycol? Significant Figures and Measurements 1. Significant figures in a numerical value dependent on the precision of the MEASUREMENT. 2. Measurements always consist of CERTAIN digits and UNCERTAIN digits. The significant figures are the CERTAIN digits plus the FIRST UNCERTAIN digit. 3. Conversion factors and numbers resulting from counting are treated as exact values. a. We do not count significant figures in exact values b. Examples: 1 in = 2.54 cm 100 pennies = $1 12 pieces = 1 dozen 6

7 Significant Figures and Measurements For instruments marked with a scale, you determine the last digit by estimating between the marks. Mentally divide the gap between marks into tenths, then estimate which tenth the indicator mark would be pointing toward. For digital instruments, the last digit in the reading is uncertain (the wavering digit in the reading). Remember that all the certain digits plus the first uncertain digit are significant digits. Significant Figures All non-zero values are significant. 536 has three significant figures has four significant figures Zeros between non-zero digits are also significant has four significant figures. Zeros to the left of a non-zero digit are NOT significant has two significant figures has three significant figures. 7

8 Significant Figures, continued Zeros to the right of a non-zero digit are significant if the number has a decimal point has three significant figures; all the zeros are significant. If the number does NOT have a decimal point, the number of significant figures is often ambiguous. You need more information about the number. 700 has at least 1 significant figure 4320 has at least 3 significant figures How Many Significant Figures? ,

9 Significant Figures and Scientific Notation 301 in scientific notation is 3.01 x in scientific notation is 3.01 x 10-2 Both of these value indicate THREE significant figures. NOTE: It is often easier to determine the number of significant figures in a number if you first convert it to scientific notation Significant Figures and Mathematical Operations Mathematical operations dictate the reporting of significant figures in an answer. 1. Multiplication and Division The least precise measured value determines the number of significant figures in the reported answers. 2. Addition and Subtraction The value with the smallest decimal measurement determines the answer s significant figure. 9

10 Multiplication and Division Examples 3.56 cm x 4 cm = g / 9.21 cm 3 = x 10-2 x x 10 2 = ml x 5 = (as in, 5 samples, each containing ml) Addition and Subtractions Examples: Examples: in in = cm cm = 4.05 x x 10-1 = 10

11 Unit conversion problem A rectangular region of forest measuring 5.6 mi by 9.2 mi was destroyed by fire. Compute the area of this region and express your final answer in units of hectares. (1 hectare = 10 4 m 2 ) Unit conversion problem In Blaine, Washington on Friday, the cost of gasoline is $4.044/gallon (in US dollars). Convert this to Canadian dollars per litre. Use the exchange rate 1 US dollar = Canadian dollars. (1 gallon = 3.79 L) 11