Writing Conversion Factors

Many problems in chemistry, engineering, and medicine require you to change from one nnit to another unit. For example, suppose you spent 2.0 h on your homework, and someone asked you how many minutes that was. You would answer 120 min. You must have multiplied 2.0 h X 60 min/h because you knew that one hour is equal to 60 minutes (1 h = 60min). The relationship between the units, hours and minutes, is called an equality, which means they both measure the same quantity. When you expressed 2.0 h as 120 min, you did not change the amount of time. You changed only the unit of measurement nsed to express the time. Any equality can be written as fractions called conversion factors with one of the quantities in the numerator and the other quantity in the denominator. Be sure to include the units when you write the conversion factors. Two conversion factors are always possible from any equality. 

These conversion factors are read as 60 minutes per 1 hour and 1 hour per 60 minutes. The term per means divide. The relationship may also be written as 60 min/h. Some 

We can write conversion factors for any of the metric relationships. For example, from the equahty for meters and centimeters, we can write the following factors  

Both of these conversion factors represent the same equality one is just the inverse of the other. The usefulness of conversion factors is enhanced by the fact that we can turn a conversion factor over and use its inverse. The number 100 and 1 in this equahty and its conversion factors are both exact numbers. 

Identify one or more correct conversion factors for the equality of gigagrams and grams. 

LEARNING GOAL Use the numerical values of prefixes to write a metric equality. 

29 The speedometer is marked in both km/h and mi/h, or mph. What is the meaning of each abbreviation  

233 Write the complete name for each of the following units  

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Mole ratios You have seen that the coefficients in a chemical equation indicate the relationships among moles of reactants and products. For example, return to the reaction between iron and oxygen described in Table 12-1. The equation indicates that four moles of iron react with three moles of oxygen. It also indicates that four moles of iron react to produce two moles of iron(III) oxide. How many moles of oxygen react to produce two moles of iron(III) oxide You can use the relationships between coefficients to write conversion factors called mole ratios. A mole ratio is a ratio between the numbers of moles of any two substances in a balanced chemical equation. As another example, consider the reaction shown in Figure 12-2. Aluminum reacts with bromine to form aluminum bromide. Aluminum bromide is used as a catalyst to speed up a variety of chemical reactions. 

Write conversion factors for the units given. From Table 3, you know that 100 cm = 1 m. The conversion factors are... 

Write conversion factors that relate the metric base units to units derived from the metric prefixes—for example,... 

Given a formula for a compound, write conversion factors that convert between moles of atoms of each element in the compound and moles of compound. 

Writing conversion factors As you just read, conversion factors are ratios of equivalent values. Not surprisingly, these conversion factors are derived from equality relationships, such as 12 eggs = 1 dozen eggs, or 12 inches = 1 foot. Multiplying a quantity by a conversion factor changes the units of the quantity without changing its value. 

CONCEPT CHECK 2.7 Writing Conversion Factors from Equalities... 

The units of density can be used to write conversion factors that convert between the mass and volume of a substance. 

Determine the moiar mass and write conversion factors. 

Writing Conversion Factors from Equalities 42 Using Conversion Factors 48 Using Density as a Conversion Factor 58 Classifying Matter 70... 

Using Significant Figures in Calculations (2.3) Writing Conversion Factors from Equalities (2.5) Using Conversion Factors (2.6)... 

STEP0 Use Avogadro s number to write conversion factors. 

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