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Chapter 2. Measurements and Calculations. Lab Demo Page 69: Zinc and HCl. Objectives:. Describe the difference between a qualitative and a quantitative measurement. Describe the difference between accuracy and precision. Write a number in scientific notation.

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## Chapter 2

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**Chapter 2**Measurements and Calculations**Lab Demo**Page 69: Zinc and HCl**Objectives:**• Describe the difference between a qualitative and a quantitative measurement. • Describe the difference between accuracy and precision. • Write a number in scientific notation. • State the appropriate units for measuring length, volume, mass, density, temperature and time in the metric system. • Determine the number of significant figures in a measurement or calculation. • Calculate the percent error in a measurement. • Calculate density given the mass and volume, the mass given the density and volume, and the volume given the density and mass.**Chapter 2**Section 1 Scientific Method**Scientific Method is a logical approach to solving problems**by observing and collecting data, formulating hypotheses, testing hypotheses and formulating theories that are supported by data. Observations Hypothesis Experimentation Theory**Observations**• Collecting data • Measuring • Communicating with other scientists**Measurements**Measurements are divided into two sets: Qualitative – a descriptive measurement. Color, hardness, shininess, physical state. (non-numerical) Quantitative – a numerical measurement. Mass in grams, volume in milliliters, length in meters.**Hypothesis**A tentative explanation that is consistent with the observations (educated guess). An experiment is then designed to test the hypothesis. Predict the outcome from the experiments.**Theory**Attempts to explain why something happens. Has experimental evidence to support the theory. Observations, data and facts.**Classwork**• What is the scientific theory? • What is the difference between qualitative and quantitative measurements? • Which of the following are quantitative? • The liquid floats on water? • The metal is malleable? • A liquid has a temperature of 55.6 oC? How do hypothesis and theories differ?**Section 2**Units of Measurement**Measurements represent quantities.**A quantity is something that has magnitude, size or amount. All measurements are a number plus a unit (grams, teaspoon, liters).**Number vs. Quantity**• Quantity = number + unit UNITS MATTER!!**UNITS OF MEASUREMENT**Use SI units — based on the metric system Length Mass Volume Time Temperature Meter, m kilogram, kg Liter, L Seconds, s Celsius degrees, ˚C kelvins, K**UNITS OF MEASUREMENT**Use SI units — based on the metric system Amount Electric current Luminous Intensity mole, mol ampere, A candela, cd**mega-**tera- hecto- deka- giga- kilo- M G k h T da 1012 106 102 101 109 103 deci- d 10-1 centi- c 10-2 milli- m 10-3 micro- 10-6 BASE UNIT --- 100 pico- nano- p n 10-9 10-12 SI Prefix Conversions Prefix Symbol Factor move left move right**Learning Check**1. 1000 m = 1 ___ a) mm b) km c) dm 2. 0.001 g = 1 ___ a) mg b) kg c) dg 3. 0.1 L = 1 ___ a) mL b) cL c) dL 4. 0.01 m = 1 ___ a) mm b) cm c) dm**SI Prefix Conversions**• 20 cm = ______________ m 2) 0.032 L = ______________ mL 3) 45 m = ______________ m**Derived SI Units**Many SI units are combinations of the quantities shown earlier. Combinations of SI units form derived units. Derived units are produced by multiplying or dividing standard units.**Volume**Volume (m3) is the amount of space occupied by an object. length x width x height Also expressed as cubic centimeter (cm3). When measuring volumes in the laboratory a chemist typically uses milliliters (mL). 1 mL = 1 cm3**Density**Density – the ratio of mass to volume, or mass divided by volume. Density = D = Density is often expressed in grams/milliliter or g/mL mass volume m v**Density**Density is a characteristic physical property of a substance. It does not depend on the size of the sample. As the sample’s mass increases, its volume increases proportionally. The ratio of mass to volume is constant.**Density**Calculating density is pretty straightforward. You measure the mass of an object by using a balance and then determine the volume. For a liquid the volume is easily measured using for example a graduated cylinder.**Density**For a solid the volume can be a little more difficult. If the object is a regular solid, like a cube, you can measure its three dimensions and calculate the volume. Volume = length x width x height**Density**If the object is an irregular solid, like a rock, determining the volume is more difficult. Archimedes’ Principle – states that the volume of a solid is equal to the volume of water it displaces.**Density**Put some water in a graduated cylinder and read the volume. Next, put the object in the graduated cylinder and read the volume again. The difference in volume of the graduated cylinder is the volume of the object.**Volume Displacement**A solid displaces a matching volume of water when the solid is placed in water. 33 mL 25 mL**Learning Check**What is the density (g/cm3) of 48 g of a metal if the metal raises the level of water in a graduated cylinder from 25 mL to 33 mL? 1) 0.2 g/cm3 2) 6 g/cm3 3) 252 g/cm3 33 mL 25 mL**PROBLEM: Mercury (Hg) has a density of 13.6 g/cm3. What is**the mass of 95 mL of Hg in grams?**mass**( g ) = Density volume ( ml ) PROBLEM: Mercury (Hg) has a density of 13.6 g/cm3. What is the mass of 95 mL of Hg? First, note that1 cm3 = 1 mL Strategy Use density to calc. mass (g) from volume.**)**( g mass mass g ( ) = Density = 13.6 g/mL ( volume ( ) ml 95 ) ml PROBLEM: Mercury (Hg) has a density of 13.6 g/cm3. What is the mass of 95 mL of Hg? Mass = 1,292 grams**Learning Check**Osmium is a very dense metal. What is its density in g/cm3 if 50.00 g of the metal occupies a volume of 2.22cm3? 1) 2.25 g/cm3 2) 22.5 g/cm3 3) 111 g/cm3**Solution**Placing the mass and volume of the osmium metal into the density setup, we obtain D = mass = 50.00 g = volume 2.22 cm3 = 22.522522 g/cm3 =22.5 g/cm3**Learning Check**The density of octane, a component of gasoline, is 0.702 g/mL. What is the mass, in kg, of 875 mL of octane? 1) 0.614 kg 2) 614 kg 3) 1.25 kg**Learning Check**The density of octane, a component of gasoline, is 0.702 g/mL. What is the mass, in kg, of 875 mL of octane? 1) 0.614 kg**Homework**Density Worksheet**Conversion Factors**Conversion factor – a ratio derived from the equality between two different units that can be used to convert from one unit to the other. Example: the conversion between quarters and dollars: 4 quarters1 dollar 1 dollar or 4 quarters**Conversion Factors**When you want to use a conversion factor to change a unit in a problem, set up the problem as follows: quantity sought = quantity given x conversion factor**Conversion Factors**Example: Determine the number of quarters in 12 dollars? Number of quarters = 12 dollars x conversion factor ? Quarters = 12 dollars x = 48 quarters 4 quarters 1 dollar**Learning Check**Write conversion factors that relate each of the following pairs of units: 1. Liters and mL 2. Hours and minutes 3. Meters and kilometers**How many minutes are in 2.5 hours?**Conversion factor 2.5 hr x 60 min = 150 min 1 hr cancel**Sample Problem**• You have $7.25 in your pocket in quarters. How many quarters do you have? 7.25 dollars 4 quarters 1 dollar =29 quarters X**Learning Check**A rattlesnake is 2.44 m long. How long is the snake in cm? a) 2440 cm b) 244 cm c) 24.4 cm**Solution**A rattlesnake is 2.44 m long. How long is the snake in cm? b) 244 cm 2.44 m x 100 cm = 244 cm 1 m**Classwork**Textbook page 45 Question 19 (a-g)**Homework**Problem Set: 4**Section 3**Using Scientific Measurements**Accuracy and Precision**Accuracy – refers to how well the measurements agree with the accepted or true value. Precision – refers to how well a set of measurements agree with each other.

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