Making Measurements: The SI System (THE SECOND MOST
Making Measurements: The SI System (THE SECOND MOST AWESOME THING EVER!) Tools of Quantitative Chemistry At its core, chemistry is a quantitative science. In the laboratory, chemists take a variety of measurements. These measurements include: mass, volume, time, and temperature. Scientists rely on common units of measurement for such quantitative studies. The scientific community has chosen a modified version of the metric system for the recording and reporting of measurements.
This decimal system is called the International System of Units, or SI System. SI System: Modified Metric System Like the metric system, the SI units are derived from base units Quantity SI base unit Symbol Length meter m Mass kilogram kg
Temperature Kelvin K Time second s Amount of substance mole mol Luminous intensity candela
cd Electric current ampere A The most common units used by chemists are the meter, gram, second, Kelvin, and mole. Temperature Scales Several scales have been developed for measuring temperature: Fahrenheit, Celsius, and Kelvin. The Celsius scale was developed using the freezing point and boiling point of water as points of reference. The Kelvin scale is also known as the absolute scale. The lowest measure on this scale is 0 K, also known as absolute zero. Converting from one temperature scale to
another: Measuring Length The SI unit for length is the meter, but chemists are typically studying objects which are much smaller than a meter. Measurements are often reported in centimeters (cm), millimeters (mm), or micrometers (m). Objects on the atomic and molecular scales are even smaller. These have dimensions of nanometers (nm) or picometers (pm). Measuring Volume The SI unit for volume is the cubic meter (m 3), but this unit is too large for convenient laboratory use. Chemists use the liter (L) as the base unit for volume measurements. The milliliter (mL) is also a convenient unit to use in the laboratory. 1 L = 1000 mL = 1000 cm3 = 0.001 m3 This means that 1 mL = 1 cm3 (cc). This conversion factor enables us to
convert between units. Measuring Mass When chemists prepare chemicals for chemical reactions, they take the mass of the quantity of matter. The SI unit for mass is the kilogram, which is unusual since it uses the prefix, kilo-, instead of gram alone. The instruments used to measure mass are platform balances, scales, or analytical balances. SI System: Modified Metric System Larger and smaller quantities are expressed using prefixes with the base unit. Example: The kilometer is 1000 or 103 times larger than a meter. The centimeter is 100 times smaller than a
meter, or 1/100 of a meter! Why is this system so awesome? It is a base 10 system. We can easily convert from unit to another! To do so, we need to use a ratio of equivalent measures called a conversion factor. LARGE Small kilo- hecto- deka- base deci- centi-
km hm dam m dm cm 10 10 10 10 10
milli- micro- nano- m nm mm 10 1000 1000 pico- pm 1000
When a measurement is multiplied by a conversion factor, the numerical value changes, but the actual quantity remains the same. Conversion Factors kilo- hecto- deka- base deci- centi- km hm dam
m dm cm 10 10 10 10 Examples: 10 milli- micro-
nano- m nm mm 10 1000 1000 **Common Bases** 1 km = 1000 m or 1 km = 10 3 m Mass: gram (g)
1 m = 0.001 km or 1 m = 10-3 km Volume: liter (L) Time: second (s) pico- pm 1000 Conversion Factors kilo- hecto- deka-
base deci- centi- km hm dam m dm cm 10 10
10 10 10 milli- micro- nano- m nm mm 10 1000 1000
pico- pm 1000 Lets Practice: 1m= 1 kg = nm mg 1 cL = L 1 pm = m 1s=
ms 1 dm = cm Conversion Factors Scientists usually write conversion factors as fractions For example: Why do scientists need conversion factors? Dimensional Analysis! Dimensional analysis is a problem solving technique used to analyze and solve problems using the units, or dimensions, of the measurements. Conversion factors are used to convert from one unit to another. Example: The length of a paperclip measures 2.0 cm. What is its length in mm? Dimensional Analysis
Problem: The length of a paperclip measures 2.0 cm. What is its length in mm? Step 1: Analyze (List what you know and what you need to know) Step 2: Strategy (How can we solve the problem?) Dimensional Analysis Step 3: Calculate (Use the conversion factor to solve for your unknown) Remember the Rules of Significant Figures! You can also express your answer in scientific notation: Dimensional Analysis Step 4: Evaluate (Does this answer make sense?) Did the units cancel properly? Is your final answer(s) in the correct unit? Does the result make sense? Dimensional Analysis Using the problem solving steps, see if you can solve this problem: The distance between an O atom and H atom in a water
molecule is 95.8 pm. What is this distance in meters? In nanometers? Dimensional Analysis Using the problem solving steps, see if you can solve this problem (Tricky!): The area of triangle can be found using the formula: . If the base measures 1.2 m and the height measures 2.65 m, what is the area of the triangle? What is this area in cm 2 ? Dimensional Analysis Dimensional analysis is not limited to the SI system. Use dimensional analysis to solve the following problem: Jaime studies chemistry 5 nights a week. If he studies 2 hours per night, how many hours per week does he spend studying chemistry? How many minutes? Seconds? Dimensional Analysis The SI System is not the only system of measurement. In the United States, we use the English System of measurement. Conversion factors allow us to convert measurements between systems.
Examples: 1 mile = 1.609 km 1 in = 2.54 cm 1 lb = 454 g 1 gallon = 3.785 L Alice was driving 55 miles per hour (mph). What speed was she driving in m/s?
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