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THINK ABOUT IT

Mercury is a toxic substance that accumulates in the body and damages the central nervous system and other organs. Which do you think would be worse for you, 1 mole of mercury or 10 grams of mercury? To answer this question, it is necessary to understand the relationship between these two measures.

What is the relationship between mass and moles?

To answer this question, you will explore

EXPLORING THE TOPIC

Moles and Grams

In the world around you, many different measures are used to specify amounts. For example, at the store you might buy 10 apples or a 5 lb bag of apples. One measure is a counting unit and the other is a unit of weight. To compare the prices, you would probably want to know how many apples are in the 5 lb bag.

Likewise, chemists use both number and mass to specify the amount of a substance. For example, they might know the effect of either 1 mol of mercury atoms or of a 10 g sample of mercury. To compare these two quantities, they would need to know either how many moles of atoms are in each gram of mercury or the mass in grams of one mole of mercury.

The mole, abbreviated mol, is a counting unit used to count a large number of atoms. A mole of atoms is equal to 602 sextillion atoms. This number is also referred to as Avogadro’s number.

The table compares the average mass of one atom with the mass of 1 mol of atoms for five elements. Examine the table.

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Notice that the numerical value on the periodic table for the average atomic mass of an element is identical to the mass of one mole of atoms of that element. In other words, both values are represented by the same number. What is different about the two measurements is the units. The average mass of one atom of arsenic is 74.92 amu, and the mass of one mole of arsenic atoms is 74.92 grams. How convenient!

MOLAR MASS

The mass of one mole of atoms of an element is called the molar mass. You can find the molar mass of any element by looking up the average atomic mass on the periodic table. For example, the average atomic mass of mercury is 200.6 amu. The mass of 1 mol of mercury atoms is 200.6 g.

Scientific Notation

Consider the table on the next page. It shows the mass, in grams, of different numbers of atoms of mercury. Notice that the number of atoms and the mass are shown both in longhand and in scientific notation. Scientific notation is a shorthand method that uses an exponent to keep track of where the decimal point should be. Small numbers, between 0 and 1, have negative exponents. Numbers greater than 1 have positive exponents. The exponent indicates where the decimal point belongs in the longhand number.

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In scientific notation, ten thousand is written 1.0 × 10⁴. One ten-thousandth is written 1.0 × 10^–4.

10,000 = 1.0 × 10⁴

[For a review of this topic, see MATH Spotlight: Scientific Notation on page A-14.] The mass of 1000 atoms of arsenic is only 0.00000000000000000012 g or 1.2 × 10^–19 g. This many arsenic atoms wouldn’t even be visible to the naked eye. On the other hand, 74.92 g of arsenic contains 1 mol, or 6.02 × 10²³ atoms.

When you are dealing with atoms, numbers tend to be either very tiny or very large. So it is more convenient to use scientific notation.

LESSON SUMMARY

What is the relationship between mass and moles?

Chemists keep track of the number of molecules and atoms they are working with by using a unit called the mole. They also use scientific notation to express numbers that are very small or very large. One mole is equal to 602 sextillion. Using scientific notation, this number is written as 602,000,000,000,000,000,000,000, or 6.02 × 10²³. The mass (in grams) of one mole of atoms of an element can be found on the periodic table. This is called the molar mass and has the same numerical value as the average atomic mass.