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5-1 Molecular and Formula Mass
The symbol for an element represents one atom of the element or one mole of the element The formula for a compound can represent one molecule or formula unit/or one mole of the compound The atomic mass scale is used to compare the masses of atoms. This scale has the atomic mass unit(u) as a standard. Molecular mass is the sum of the atomic masses of the atoms in a molecule. Formula mass is the sum of the atomic masses of the atoms in formula unit. 5-2 The Avogadro Constant The mass of one atom of H is 1.0079u/ the mass of 6.02X10(23) atoms is 1.0079g 5-3 The Mole One mole of a substance contains 6.02X10(23) particles The mass of one mole of molecules, atoms, ions, or formula unit is called the molar mass of that species. The concept of a mole is central in chemistry. The number of moles in a given mass of substance can be found by dividing the total mass by the molar mass expressed in units of g/mole 5-4 Moles in Solution Molarity(M) is the ratio between the moles of dissolved substanc and the volume of solution in cubic decimeters M=mol of solute/dm(3) of solution (1 cubic decimeter=1L=1000cc) 5-5 Percentage Composition Percentage composition may be used in the identification of some compounds The total percent of all elements in a compound equals 100% % composition is a statement of the relative mass of each element contributes to the mass of the compound as a whole 5-6 Empirical Formulas Elements in compounds are in simple whole number ratios Empirical formula is the simplest ratio of atoms in a compound The ratio of moles is the same as the ratio of atoms for a give formula unit because there are exactly one avogadro atoms in one mole of any element. To determine an empirical formula, change grams to moles and find the ratio of the moles 5-7 Molecular Formulas A molecular formula shows the actual number of atoms of each element in a molecule The molecular mass of a substance is a whole number multiple of the empirical formula mass Hydrates are crystals that contain water molecules |
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