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## U nit 2 :

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**Unit2:**Intro to Chem & Measurements**In this map, gray areas denote metric territory. English**units are used primarily in red zones.**MEASUREMENTS**• Science is based on measurements • All measurements have: • Magnitude • Uncertainty • Units**NUMBERS**• Math is based on numbers • Exact numbers are obtained by: • Counting • Definition**Also known as the “SI system”**• “SI” stands for “Le Syste’me Internationl d’Unites” • Law in France since 1790. • It is based on the number 10 • ALWAYS used in science**Other Base Units to Know**Quantity Base Unit Symbol Volume liter L Count/Quantity mole mol Temperature* Celsius C *temp can be C or K, depending on what you are doing**Volume of a solid length x height x width m3**Usually derived units are expressed with positive/negative exponents ex. m2**Temperature Conversion you need to know -**K = °C + 273.15**PREFIXES**Prefixes go in front of the unit.**Changing prefix by moving the decimal**103 102 101 100 10-1 10-2 10-3 10-6 Kilo HectoDeka (unit) DeciCentiMilliMicro (k) (h) (da) (d) (c) (m) (µ) King Henry Died Unexpectedly Drinking Chocolate Milk _ _ Moo KHDUDCM__ __ µ**Steps to move decimal**• Start at given prefix • Count # of letters (prefix initials) to get to desired prefix • Note direction • Move decimal same number as letters counted in same direction • May need to add zeros to the left or right of original decimal.**Practice**0.5 km = ________ dm King Henry Died Unexpectedly Drinking Chocolate Milk _ _ Moo KHDUDCM__ __ µ 4 Right Move decimal 4 Right: 0.5000 km = 5000dm (note: needed to add extra zeros to the right of decimal)**Practice**2000 cm = ________ m King Henry Died Unexpectedly Drinking Chocolate Milk _ _ Moo KHDUDCM__ __ µ 2 Left Move decimal 2 Left: 2000 cm = 20m (note: meter is base unit, so moving to U)**A**B C How “good” is your measurement tool?**A**B C 3 cm 3.3 cm 3.31 cm The measurement you record needs to indicate how “good” your measurement tool is!**A**B C 3 cm 3.3 cm 3.31 cm The last number recorded in a measurement is estimated (your best guess)**3 cm**3.3 cm 3.31 cm A B C The more divisions on your measurement tool, the more precise your measurement can be (and the more significant figures it will have).**Significant Figures**Since all measurements are uncertain, we must only use those numbers that are meaningful. In science, that means we record the digits that we are certain of, plus one more that we have to guess. These digits are all referred to as “significant figures”.**1 significant figure: we had to take a guess at that one**digit 2 significant figures: we were certain the object was at least 3 cm, but we guessed at the tenths spot. 3 significant figures: we were certain the object was at least 3.3 cm, but we guessed at the hundredths spot. 3 cm 3.3 cm 3.31 cm**Significant Figure “Rules”**Important when you did not take the measurement directly, and you need to know how many significant figures are in the measurement Non-zero digits: always meaningful, count them as “significant” 22.4 has 3 sig figs Zeros stuck between non-zeros: count them as “significant” 6.022 x 1023 has 4 sig figs Zeros at the beginning of a number less than 1: NEVER count them as “significant” 0.0042 has 2 sig figs Zeros at the end of a number less than 1: ALWAYS count them as “significant” 0.00420 has 3 sig figs Zeros at the end of a number greater than1 DO count them as “significant” if you see a decimal point DON’T count them as significant if decimal point is absent 290. has 3 sig figs 290 has 2 sig figs**Atlantic Pacific Ruleor how to quickly determine the number**of significant figures NO Rules!!!!!!**P A**Decimal Present – shoot arrow left to right stop at first number-count 0.00030 203.47 Decimal Absent – shoot arrow right to left stop at first number-count 789000 4 2SF 5SF 3SF 1SF**How many significant figures in that measurement?**The following numbers have 1 significant figure: 2 0.04 80 700 The following numbers have 2 significant figures: 2.0 0.040 80. 7.0 x 102 The following numbers have 3 significant figures: 2.00 0.0403 80.2 700.**How many significant figures in that measurement?**The following numbers have 1 significant figure: 2 0.04 80 700 The following numbers have 2 significant figures: 2.0 0.040 80. 7.0 x 102 The following numbers have 3 significant figures: 2.00 0.0403 80.2 700.**Rounding**Quick rules for rounding: 1) Find the place value you want (the “rounding digit”) 2) Look at the digit just to the right of it: a) If that digit is less than 5, -do not change the rounding digit -drop all digits to the right of it b) If that digit is greater than or equal to five, -add one to the rounding digit -drop all digits to the right of it Examples: 3.04 rounded to 2 sig figs is 3.0 112.511 rounded to 2 sig figs is 110 0.4203 rounded to 2 sig figs is 0.42 Examples: 4.55 rounded to 2 sig figs is 4.6 0.0865 rounded to 2 sig figs is 0.087 40.523 rounded to 2 sig figs is 40. (note the decimal point!)**Rounding a Calculated Answer:**Addition & Subtraction An answer is no more precise than the least precise measurement used to get the answer. Determine the decimal place of the last significant figure in each of your measured values. Add or subtract in normal fashion. Round the answer to the least precise decimal place seen in the measured values.**Rounding a Calculated Answer:**Multiplication & Division An answer is no more precise than the least precise measurement used to get the answer. Determine the total number of significant figures in each of your measured values. Multiply or divide in normal fashion. Round the answer to the fewest total number of significant figures seen in the measured values.**Applied Problems: Volume**V = L x W x H V = 1.20m x 1.20m x 1.90 m V = _____________ V = _____________ (correct sig figs)**Applied Problems: Volume**V = L x W x H V = 1.20m x 1.20m x 1.90 m V = 2.736 cm3 (unrounded) V =2.74 cm3 (rounded to 3 sig figs)**Applied Problems:**Volume Displacement Initial Volume of water without object: 17.1 mL Final Volume of water with object:19.8 mL Volume of object: 19.8 mL –17.1 mL = ____ mL 19.8 mL – 17.1 mL = ____ mL (SF)**Applied Problems:**Volume Displacement Initial Volume of water without object: 17.1 mL Final Volume of water with object:19.8 mL Volume of object: 19.8 mL –17.1 mL = 2.7 mL 19.8 mL – 17.1 mL = 2.7 mL (SF) 2.7 mL is the unrounded answer, but happens to have the correct number of sig figs!**Applied Problems:**Volume Displacement Final Volume of water with object: 19.8 mL Initial Volume of water without object: 17.1 mL Volume of object: 19.8 mL –17.1 mL = 2.7 mL 2.7 mL is the unrounded answer, but happens to have the correct number of sig figs!**Applied Problems:**Density Density = Mass Volume Density = 51.842 g 4.7 mL Density = 11.03021277 g/mL (unrounded answer) Volume = 4.7 mL Density = 11 g/mL (rounded answer)**Applied Problems:**Kelvin Temperature The temperature measured on this thermometer is 26.5ºC To convert to Kelvin temperature, use the Kelvin conversion equation: K = C + 273 ºC K = 25.5 + 273 K = 298.5 (unrounded answer) K = 299. (rounded answer) Celsius Thermometer**Note on reading graduated lines on lab equipment**Value of incremental lines = Large# - Smaller# #of lines in between Significant Number place value = Place Value of incremental lines + one place value to the right 40-30 =1mL/line 10 36.5mL**Note on reading graduated lines on lab equipment**Value of incremental lines = Large# - Smaller# #of lines in between Significant Number place value = Place Value of incremental lines + one place value to the right 40-30 = 2mL/line 5 38.0mL**Unit Conversion WS**• Finish as HW • Ask for help**Day 2 Precision and Accuracy**Backside of page one of notes**PRECISION**• Reproducibility • Check by repeating measurements • Poor precision results from poor technique**ACCURACY**• Correctness • Check by using a different method • Poor accuracy results from procedural or equipment flaws