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Chemistry, like all sciences, is quantitative. It deals with quantities, things that have amounts and units. Dealing with quantities is very important in chemistry, as is relating quantities to each other. In this chapter, we will discuss how we deal with numbers and units, including how they are combined and manipulated.
- 2.1: Taking Measurements
- Chemists measure the properties of matter and express these measurements as quantities. 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.
- 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 requires 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.
- 2.3: Significant Figures
- Uncertainty exists in all measurements. The degree of uncertainty is affected in part by the quality of the measuring tool. Significant figures give an indication of the certainty of a measurement. Rules allow decisions to be made about how many digits to use in any given situation.
- 2.4: Significant Figures in Calculations
- To round a number, first decide how many significant figures the number should have based on calculation rules. Once you know that, round to that many digits, starting from the left. If the number immediately to the right of the last significant digit is less than 5, it is dropped and the value of the last significant digit remains the same. If the number immediately to the right of the last significant digit is greater than or equal to 5, the last significant digit is increased by 1.
- 2.5: The Metric System
- Metric prefixes derive from Latin or Greek terms. The prefixes are used to make the units manageable. The SI system is based on multiples of ten. There are seven basic units in the SI system. Five of these units are commonly used in chemistry.
- 2.6: Problem Solving and Unit Conversions
- During your studies of chemistry, you will note that mathematical equations are used in a number of different applications. Many of these equations require specific units for the variables. Thus, you will often need to convert measurements from one unit to another. This is easily accomplished using conversion factors and dimensional analysis. Algebra skills become very important here!
- 2.7: Solving Multi-Step Conversion Problems
- Sometimes you will have to perform more than one conversion to obtain the desired unit.
- 2.8: Units Raised to a Power
- Conversion factors for area and volume can also be produced by the dimensional analysis method. Just remember that if a quantity is raised to a power of 10 both the number and the unit must be raised to the same power of 10.
- 2.9: Density
- Density is a physical property found by dividing the mass of an object by its volume. Regardless of the sample size, density is always constant. It can be used as a conversion factor between mass and volume for a particular substance.
- 2.10: For Future Use
- 2.E: Exercises
Thumbnail Chapter 2: Balance scale (winnifredxoxo via Flickr)