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Problem Solving in Chemistry

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Presentation on theme: "Problem Solving in Chemistry"— Presentation transcript:

1 Problem Solving in Chemistry
Dimensional Analysis Used in _______________ problems. *Example: How many seconds are there in 3 weeks? A method of keeping track of the_____________. Conversion Factor A ________ of units that are _________________ to one another. *Examples: 1 min/ ___ sec (or ___ sec/ 1 min) ___ days/ 1 week (or 1 week/ ___ days) 1000 m/ ___ km (or ___ km/ 1000 m) Conversion factors need to be set up so that when multiplied, the unit of the “Given” cancel out and you are left with the “Unknown” unit. In other words, the “Unknown” unit will go on _____ and the “Given” unit will go on the ___________ of the ratio. conversion units ratio equivalent 60 60 7 7 1 1 top bottom

2 If your units did not ________ ______ correctly, you’ve messed up!
How to Use Dimensional Analysis to Solve Conversion Problems Step 1: Identify the “________”. This is typically the only number given in the problem. This is your starting point. Write it down! Then write “x _________”. This will be the first conversion factor ratio. Step 2: Identify the “____________”. This is what are you trying to figure out. Step 3: Identify the ____________ _________. Sometimes you will simply be given them in the problem ahead of time. Step 4: By using these conversion factors, begin planning a solution to convert from the given to the unknown. Step 5: When your conversion factors are set up, __________ all the numbers on top of your ratios, and ____________ by all the numbers on bottom. Given Unknown conversion factors multiply divide If your units did not ________ ______ correctly, you’ve messed up! cancel out

3 How many hours are there in 3.25 days?
Practice Problems: How many hours are there in 3.25 days? (2) How many yards are there in 504 inches? (3) How many days are there in 26,748 seconds? 24 hrs 3.25 days 78 hrs x = 1 day 1 ft 1 yard 504 in. 14 yards x x = 12 in. 3 ft 1 min 1 hr 1 day 26,748 sec days x x x = 60 sec 60 min 24 hrs

4 Scientific Measurement
Qualitative vs. Quantitative Qualitative measurements give results in a descriptive nonnumeric form. (The result of a measurement is an _____________ describing the object.)‏ *Examples: ___________, ___________, long, __________... Quantitative measurements give results in numeric form. (The results of a measurement contain a _____________.)‏ *Examples: 4’6”, __________, 22 meters, __________... Accuracy vs. Precision Accuracy is how close a ___________ measurement is to the ________ __________ of whatever is being measured. Precision is how close ___________ measurements are to _________ ___________. adjective short heavy cold number 600 lbs. 5 ºC single true value several each other

5 Practice Problem: Describe the shots for the targets.
Bad Accuracy & Bad Precision Good Accuracy & Bad Precision Bad Accuracy & Good Precision Good Accuracy & Good Precision

6 Significant Figures Significant figures are used to determine the ______________ of a measurement. (It is a way of indicating how __________ a measurement is.)‏ *Example: A scale may read a person’s weight as 135 lbs. Another scale may read the person’s weight as lbs. The ___________ scale is more precise. It also has ______ significant figures in the measurement. Whenever you are measuring a value, (such as the length of an object with a ruler), it must be recorded with the correct number of sig. figs. Record ______ the numbers of the measurement known for sure. Record one last digit for the measurement that is estimated. (This means that you will be ________________________________ __________ of the device and taking a __________ at what the next number is.)‏ precision precise second more ALL reading in between the marks guess

7 Significant Figures Practice Problems: What is the length recorded to the correct number of significant figures? length = ________cm 11.65 (cm) length = ________cm 58

8 The SI System (The Metric System)‏
Here is a list of common units of measure used in science: Standard Metric Unit Quantity Measured kilogram, (gram) ______________ meter ______________ cubic meter, (liter) ______________ seconds ______________ Kelvin, (˚Celsius) _____________ The following are common approximations used to convert from our English system of units to the metric system: 1 m ≈ _________ kg ≈ _______ L ≈ 1.06 quarts 1.609 km ≈ 1 mile gram ≈ ______________________ 1mL ≈ _____________ volume 1mm ≈ thickness of a _______ mass length volume time temperature 1 yard 2.2 lbs. mass of a small paper clip sugar cube’s dime

9 The SI System (The Metric System)‏


11 kilo- hecto- deka- deci- centi- milli-
Metric Conversions The metric system prefixes are based on factors of _______. Here is a list of the common prefixes used in chemistry: kilo- hecto- deka deci- centi- milli- The box in the middle represents the standard unit of measure such as grams, liters, or meters. Moving from one prefix to another involves a factor of 10. *Example: 1000 millimeters = 100 ____ = 10 _____ = 1 _____ The prefixes are abbreviated as follows: k h da g, L, m d c m *Examples of measurements: 5 km 2 dL 27 dag 3 m 45 mm mass cm dm m grams Liters meters

12 Other Metric Equivalents
Metric Conversions To convert from one prefix to another, simply count how many places you move on the scale above, and that is the same # of places the decimal point will move in the same direction. Practice Problems: 380 km = ______________m mm = _________m 461 mL = ____________dL cg = ____________ dag 0.26 g =_____________ mg ,000 m = _______km Other Metric Equivalents 1 mL = 1 cm L = 1 dm3 For water only: 1 L = 1 dm3 = 1 kg of water or mL = 1 cm3 = 1 g of water (1) How many liters of water are there in 300 dm3 ? ___________ (2) How many kg of water are there in 500 mL? _____________ 380,000 4.61 0.0004 260 230 300 L 0.5 kg

13 Area and Volume Conversions
If you see an exponent in the unit, that means when converting you will move the decimal point that many times more on the metric conversion scale. *Examples: cm2 to m move ___________ as many places m3 to km move _____ times as many places Practice Problems: km2 = _________________m2 4.61 mm3 = _______________cm3 k h da g, L, m d c m twice 3 380,000,000 grams Liters meters

14 Steps for Writing Numbers in Scientific Notation
Scientific notation is a way of representing really large or small numbers using powers of 10. *Examples: 5,203,000,000,000 miles = x 1012 miles mm = 4.2 x 10−8 mm Steps for Writing Numbers in Scientific Notation (1) Write down all the sig. figs. (2) Put the decimal point between the first and second digit. (3) Write “x 10” (4) Count how many places the decimal point has moved from its original location. This will be the exponent...either + or −. (5) If the original # was greater than 1, the exponent is (__), and if the original # was less than 1, the exponent is (__)....(In other words, large numbers have (__) exponents, and small numbers have (_) exponents. + +

15 Scientific Notation Practice Problems: Write the following measurements in scientific notation or back to their expanded form. 477,000,000 miles = _______________miles m = _________________ m 6.30 x 109 miles = ___________________ miles 3.88 x 10−6 kg = __________________ kg 4.77 x 108 9.10 x 10−4 6,300,000,000

16 Evaluating the Accuracy of a Measurement
The “Percent Error ” of a measurement is a way of representing the accuracy of the value. (Remember what accuracy tells us?)‏ % Error = (Accepted Value) − (Experimentally Measured Value) x (Accepted Value)‏ Practice Problem: A student measures the density of a block of aluminum to be approximately 2.96 g/mL. The value found in our textbook tells us that the density was supposed to be 2.70 g/mL. What is the accuracy of the student’s measurement? (Absolute Value)‏ % Error = |2.70−2.96| ÷ 2.70 = …x 100 = 9.63% error

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