 # Section 10-1 Counting Particles Chemists need a convenient method for accurately counting the number of atoms, molecules, or formula units of a substance.

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Section 10-1 Counting Particles Chemists need a convenient method for accurately counting the number of atoms, molecules, or formula units of a substance. The mole is the SI base unit used to measure the amount of a substance.mole 1 mole is the amount of atoms in 12 g of pure carbon-12, or 6.02  10 23 atoms. The number is called Avogadro’s number.Avogadro’s number

Section 10-1 Converting Between Moles and Particles Conversion factors must be used. Moles to particles

Section 10-1 Converting Between Moles and Particles (cont.) Particles to moles Use the inverse of Avogadro’s number as the conversion factor.

End of Section 10-1

Section 10-2 The Mass of a Mole 1 mol of copper and 1 mol of carbon have different masses. One copper atom has a different mass than 1 carbon atom. Molar mass is the mass in grams of one mole of any pure substance.Molar mass The molar mass of any element is numerically equivalent to its atomic mass and has the units g/mol.

Section 10-2 Using Molar Mass Moles to mass Convert mass to moles with the inverse molar mass conversion factor. Convert moles to atoms with Avogadro’s number as the conversion factor.

End of Section 10-2

Section 10-3 Section 10.3 Moles of Compounds (cont.) The molar mass of a compound can be calculated from its chemical formula and can be used to convert from mass to moles of that compound. Chemical formulas indicate the numbers and types of atoms contained in one unit of the compound.

Section 10-3 The Molar Mass of Compounds The molar mass of a compound equals the molar mass of each element, multiplied by the moles of that element in the chemical formula, added together. The molar mass of a compound demonstrates the law of conservation of mass.

Section 10-3 Converting Moles of a Compound to Mass For elements, the conversion factor is the molar mass of the compound. The procedure is the same for compounds, except that you must first calculate the molar mass of the compound. The conversion factor is the inverse of the molar mass of the compound.

Section 10-3 Converting the Mass of a Compound to Number of Particles Convert mass to moles of compound with the inverse of molar mass. Convert moles to particles with Avogadro’s number.

End of Section 10-3

Section 10-4 Percent Composition The percent by mass of any element in a compound can be found by dividing the mass of the element by the mass of the compound and multiplying by 100.

Section 10-4 Percent Composition (cont.) The percent by mass of each element in a compound is the percent composition of a compound.percent composition Percent composition of a compound can also be determined from its chemical formula.

Section 10-4 Empirical Formula The empirical formula for a compound is the smallest whole-number mole ratio of the elements.empirical formula You can calculate the empirical formula from percent by mass by assuming you have 100.00 g of the compound. Then, convert the mass of each element to moles. The empirical formula may or may not be the same as the molecular formula. Molecular formula of hydrogen peroxide = H 2 O 2 Empirical formula of hydrogen peroxide = HO

Section 10-4 Molecular Formula The molecular formula specifies the actual number of atoms of each element in one molecule or formula unit of the substance.molecular formula Molecular formula is always a whole-number multiple of the empirical formula.

Section 10-4 Molecular Formula (cont.)

End of Section 10-4

Section 10-5 Naming Hydrates A hydrate is a compound that has a specific number of water molecules bound to its atoms.hydrate The number of water molecules associated with each formula unit of the compound is written following a dot. Sodium carbonate decahydrate = Na 2 CO 3 10H 2 O

Section 10-5 Analyzing a Hydrate When heated, water molecules are released from a hydrate leaving an anhydrous compound. To determine the formula of a hydrate, find the number of moles of water associated with 1 mole of hydrate.

Section 10-5 Analyzing a Hydrate (cont.) Weigh hydrate. Heat to drive off the water. Weigh the anhydrous compound. Subtract and convert the difference to moles. The ratio of moles of water to moles of anhydrous compound is the coefficient for water in the hydrate.

Section 10-5 Use of Hydrates Anhydrous forms of hydrates are often used to absorb water, particularly during shipment of electronic and optical equipment. In chemistry labs, anhydrous forms of hydrates are used to remove moisture from the air and keep other substances dry.

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