| AP Chemistry - Semester 1 : Stoichiometry of Formulas and Equations |
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Sections: Introduction
| Section 1
| Section 2
| Section 3
|  Lab
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3.1 The Mole: The mole (mol) is the SI unit identifying the number of particles in a given amount of matter. It is a dimensionless quantity equal to Avogadro's number. A mole is the amount of stuff in a system which contains as many particles as there are atoms in 12 grams of carbon 12. One mole contains 6.022×1023 entities.
Not only is there a numerical relationship, but there is also a mass relationship. One mole of a substance represents a fixed number of chemical entities and has a fixed mass. The atom mass of an element expressed in atomic mass units (amu) is numerically the same as one mole of atoms of the element expressed in grams.
The same relationship hold true with compounds. Just like calculating the molecular mass in the last chapter we can calculate the number of grams in one mole of a substance.
Molar Mass Molar mass is a basic unit in chemistry; it is the mass of one mole of a substance. Remember that carbon-12 has six neutrons and six protons for an atomic mass of 12. To calculate the molar mass of a substance you must convert the formula or molecular weight into grams. These are the masses given for the elements on the periodic table. So, the mass of a mole of C-12 is exactly 12 grams. Hydrogen, having an atomic mass of 1 amu, is one-twelve (1/12) that of Carbon and has a mole mass of 1.0 gram. Helium, with an atomic weight of 4 amu, is one-third (1/3) that of C-12 and has a molar mass of 4.0 grams. Notice that the molar mass of any element is equal to its atomic mass.
Use this equation when a chemistry lab asks for a specific amount of a substance in moles. For example: 2.2 moles of sodium. Remember that the atomic mass of sodium (Na) is 22.99 amu.
You would have to measure 50.578 grams of sodium to have 2.2 mol of sodium. Just for fun, you can calculate the number of atoms in your sample by using Avogadro's number. We know that every mole of any substance contains 6.02 x1023 atoms. There are 2.2 moles of sodium. Multiply 2.2 x (6.02 x1023) = 1.324 x1024 which corresponds to a lot of atoms! We can do the same thing for chemical formulas, such as table salt, NaCl. Na has an atomic mass of 22.99 amu and Cl has an atomic mass of 35.45 amu. Since there is one atom present of each atom NaCl has a mass of: NaCl: 22.99 amu + 35.45 amu = 58.44 amu The mass of 1 mole of NaCl: 22.99 grams + 35.45 grams = 58.44 grams The simplest sugar, glucose has a formula of C6H12O6, and its atomic mass can be calculated by using:
One mole of glucose C6H12O6 is equal to: Let's pretend that all of the sugar in a can of Pepsi is glucose. Looking at the side of a can of Pepsi, we see that it contains 41 grams of sugar, how many atoms of glucose are there in a Pepsi can?
Please notice how this equation is set up. The grams will cancel each other as will the moles, leaving us with atoms. We can also see that there is 0.23 moles of glucose in every can of Pepsi.
If you were a biologist, this number would be important as it calculates the number of atoms heading towards the mitochondria of your cells to make ATP and give you that familiar sugar rush. Our calculations show that there is 0.23 moles of glucose in every can of Pepsi. We can continue these calculations to determine the mass percent of each element in the glucose present. There are 6 moles of Carbon in every glucose molecule: Mass (g) of Mass fraction of
1. How many moles of krypton are found in 8.08 x1022 atoms? a. 8.08 moles 2. How many moles are in 3.0 grams of boron tribromide? a. 0.012 mol 3. What is the mass of 0.120 mol of glucose? a. 0.12 mol |
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