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Inorganic chemistry Measurement in Scientific study. General Features of SI Units. Some Important SI Units in Chemistry. Assistance Lecturer Amjad Ahmed Jumaa www.soran.edu.iq 1 General Chemistry: Chemistry: A science that deals with the composition, structure, and properties of substances and with the transformations that they undergo. Measurement in Scientific study: Measurement has a history characterized by the search for exact, invariable standards. General Features of SI Units: For example, the derived unit for speed, meters per second (m/s), is the base unit for length (m) divided by the base unit for time (s). www.soran.edu.iq SI Base Units. Common Decimal Prefixes Used with Si Units. www.soran.edu.iq www.soran.edu.iq Some Important SI Units in Chemistry: Let’s discuss some of the SI units length, volume, mass, density, temperature, and time. www.soran.edu.iq Length: The SI base unit of length is the meter (m). The standard meter is defined as the distance light travels in a vacuum in 1/299,792,458 second. Biological cells are often measured in micrometers (1µm = 10-6 m). On the atomic-size scale, nanometers and picometers are used (1 nm = 10-9 m; 1 pm = 10 -12m). Many proteins have diameters of around 2 nm; atomic diameters are around 200 pm (0.2 nm). An older unit still in use is the angstrom (1 Å= 10-10m = 0.1 nm = 100 pm). www.soran.edu.iq Volume: Any sample of matter has a certain volume (V), the amount of space that the sample occupies. The SI unit of volume is the cubic meter (m3). In chemistry, the most important volume units are non-SI units, the liter (L) and the milliliter (mL) (note the uppercase L). A liter is slightly larger than a quart (qt) (1 L= 1.057 qt; 1 qt = 946.4 mL). Physicians and other medical practitioners measure body fluids in cubic decimeters (dm3), which is equivalent to liters: 1 L= 1 dm3= 10-3 m3 www.soran.edu.iq As the prefix milli-indicates, 1 mL is 1/1000 of a liter, and it is equal to exactly 1 cubic centimeter (cm3): 1 mL= 1 cm3 = 10-3 dm3 =10-3 L= 10-6 m3 Converting Units of Volume: Problem: The volume of an irregularly shaped solid can be determined from the volume of water it displaces. A graduated cylinder contains 19.9 mL of water. When a small piece of galena, an ore of lead, is added, it sinks and the volume increases to 24.5 mL. What is the volume of the piece of galena in cm 3and in L? www.soran.edu.iq Plan: We have to find the volume of the galena from the change in volume of the cylinder contents. The volume of galena in mL is the difference in the known volumes before (19.9 mL) and after (24.5 mL) adding it. The units mL and cm3 represent identical volumes, so the volume of the galena in mL equals the volume in cm3. We construct a conversion factor to convert the volume from (mL to L). The calculation steps are shown in the roadmap on the next page. www.soran.edu.iq www.soran.edu.iq Mass: The mass of an object refers to the quantity of matter it contains. The SI unit of mass is the kilogram (kg), the only base unit whose standard is a physical object—a platinumiridium cylinder kept in France. It is also the only base unit whose name has a prefix. (In contrast to the practice with other base units, however, we attach prefixes to the word “gram,” rather than to the word “kilogram”; thus, 10 -3 grams is 1 milligram, not 1 microkilogram.). Its weight, on the other hand, depends on its mass and the strength of the local gravitational field pulling on it. www.soran.edu.iq Converting Units of Mass: Problem: International computer communications are often carried by optical fibers in cables laid along the ocean floor. If one strand of optical fiber weighs (1.19 x10-3 lb/m), what is the mass (in kg) of a cable made of six strands of optical fiber, each long enough to link New York and Paris (8.84 x10 3 km)? Plan: We have to find the mass of cable (in kg) from the given mass/length of fiber (1.19x10 -3lb/m), number of fibers/cable (6 fibers/cable), and the length (8.84x 10 3 km, distance from New York to Paris). One way to do this (as shown in the roadmap) is to first find the mass of one fiber and then find the mass of cable. We convert the length of one fiber from km to m and then find its mass (in lb) by using the lb/m factor. The cable mass is six times the fiber mass, and finally we convert lb to kg. www.soran.edu.iq Solution Converting the fiber length from km to m: www.soran.edu.iq www.soran.edu.iq Density: www.soran.edu.iq Calculating Density from Mass and Length: Problem: Lithium is a soft, gray solid that has the lowest density of any metal. It is an essential component of some advanced batteries, such as the one in your laptop. If a small rectangular slab of lithium weighs 1.49 X 10 3 mg and has sides that measure (20.9 mm) by (11.1 mm) by (11.9 mm), what is the density of lithium in g/cm 3? Plan: To find the density in (g/cm3), we need the mass of lithium in (g) and the volume in (cm3). The mass is given in mg (1.49 x 103 mg), so we convert (mg) to (g). Volume data are not given, but we can convert the given side lengths (20.9 mm, 11.1 mm, 11.9 mm), from (mm to cm), and then multiply them to find the volume in cm 3. Finally, we divide mass by volume to get density. The steps are shown in the roadmap. www.soran.edu.iq Solution Converting the mass from (mg to g): www.soran.edu.iq www.soran.edu.iq Temperature: There is a common misunderstanding about heat and temperature. Temperature (T) is a measure of how hot or cold a substance is relative to another substance. Heat is the energy that flows between objects that are at different temperatures. We can convert between the Celsius and Kelvin scales by remembering the difference in zero points: since 0°C = 273.15K: T(in K ) = T (in 0°C ) + 273.15 T(in 0°C) = T(in K ) -273.15 www.soran.edu.iq The Fahrenheit scale differs from the other scales in its zero point and in the size of its unit. Water freezes at (32°F) and boils at (212°F). Therefore, 180 Fahrenheit degrees (212°F- 32°F) represents the same temperature change as (100 Celsius degrees (or 100 kelvins). Because 100 Celsius degrees equal 180 Fahrenheit degrees, 1 Celsius degree = 180/100 Fahrenheit degrees = 9/5 Fahrenheit degrees. To convert a temperature in °C to °F, first change the degree size and then adjust the zero point: T (in °F) = 9/5T (in °C) + 32 www.soran.edu.iq To convert a temperature in °F to °C, do the two steps in the opposite order; that is, first adjust the zero point and then change the degree size. In other words, for T (in °C): T(in °C)= [T(in °F)- 32]5/9 (The only temperature with the same numerical value in the Celsius and Fahrenheit scales is -40°; that is, -40°F = -40°C.) Converting Units of Temperature Problem: A child has a body temperature of 38.7°C. (a) If normal body temperature is 98.6°F, does the child have a fever? (b) What is the child’s temperature in kelvins? www.soran.edu.iq Plan: (a) To find out if the child has a fever, we convert from °C to °F and see whether 38.7°C is higher than 98.6°F. (b) Convert the temperature in °C to K. Solution (a) Converting the temperature from °C to °F: T (in °F) = 9/5T (in °C) +32 = 9/5(38.7°C) +32= 101.7 °F (b) Converting the temperature from °C to K: T (in K) =T (in °C) +273.15 =38.7°C +273.15 = 311.85 www.soran.edu.iq