2: Units and Measurements
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- 2.1: Taking Measurements
- A quantity is an amount of something and consists of a number and a unit. The number tells us how many (or how much), and the unit tells us what the scale of measurement is. For example, when a distance is reported as “5 kilometers,” we know that the quantity has been expressed in units of kilometers and that the number of kilometers is 5.
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- 2.2: Scientific Notation
- Chemists often work with numbers that are exceedingly large or small. For example, entering the mass in grams of a hydrogen atom into a calculator would require a display with at least 24 decimal places. A system called scientific notation avoids much of the tedium and awkwardness of manipulating numbers with large or small magnitudes.
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- 2.3: Significant Figures
- Some error or uncertainty always exists in any measurement. The amount of uncertainty depends both upon the skill of the measurer and upon the quality of the measuring tool. While some balances are capable of measuring masses only to the nearest 0.1g , other highly sensitive balances are capable of measuring to the nearest 0.001g or even better.
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- 2.4: Accuracy and Precision
- Counting is the only type of measurement that is free from uncertainty, provided the number of objects being counted does not change while the counting process is underway. The result of such a counting measurement is an example of an exact number. Quantities derived from measurements other than counting, however, are uncertain to varying extents due to practical limitations of the measurement process used.
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- 2.5: The Basic Units of Measurements
- How long is a yard? It depends on whom you ask and when you asked the question. Today we have a standard definition of the yard, which you can see marked on every football field. If you move the ball ten yards, you get a first down and it does not matter whether you are playing in Los Angeles, Dallas, or Green Bay. But at one time that yard was arbitrarily defined as the distance from the tip of the king's nose to the end of his outstretched hand.
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- 2.6: Measured Units vs. Derived Unit
- Measurements provide the macroscopic information that is the basis of most of the hypotheses, theories, and laws that describe the behavior of matter and energy in both the macroscopic and microscopic domains of chemistry. Every measurement provides three kinds of information: the size or magnitude of the measurement; a standard of comparison for the measurement; and an indication of the uncertainty of the measurement.
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- 2.7: Conversion between Units with Conversion Factor (Single-step Conversion Problems)
- During your studies of chemistry (and physics also), you will note that mathematical equations are used in many different applications. Many of these equations have a number of different variables with which you will need to work. You should also note that these equations will often require you to use measurements with their units.
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- 2.8: Conversion Between Units with Conversion Factor (Multi-step Conversion Problems)
- Sometimes you will have to perform more than one conversion to obtain the desired unit. For example, suppose you want to convert 54.7 km into millimeters. We will set up a series of conversion factors so that each conversion factor produces the next unit in the sequence.
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- 2.9: Convert between %, ppm, and ppb
- To define a solution precisely, we need to state its concentration: how much solute is dissolved in a certain amount of solvent. Words such as dilute or concentrated are used to describe solutions that have a little or a lot of dissolved solute, respectively, but these are relative terms with meanings that depend on various factors.
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- 2.10: Mole and Avagadro’s Number Conversions
- We need a unit of amount that relates quantities of substances on a scale that we can interact with. Chemistry uses a unit called mole. A mole (mol) is a number of things equal to the number of atoms in exactly 12 g of carbon-12. Experimental measurements have determined that this number is very large - 1 mol = 6.02214179 × 1023 things
Thumbnail: Measuring Tape (Inch+Centimeters), US-Standard (but far less common than only Inch). ( public domain ; No machine-readable author provided. Stilfehler assumed via Wikimedia )