2Number of Atoms in a Formula In writing the formula of a substance, subscript numbers are used to indicate the number of atoms or groups of atoms of each element in the formula unit.
3Number of Atoms in a Formula How many atoms of each element are in a formula unit of ammonium carbonate?The formula of the ammonium ion is NH4+The formula of carbonate is CO32–The formula of the ammonium carbonate is (NH4)2CO3.Each element in the parentheses is multiplied by two.Total number of atoms of each element:2 nitrogen atoms, 8 hydrogen atoms,1 carbon atom, 3 oxygen atoms
4Atomic Mass Atomic Mass The average mass of atoms of an element By definition the mass of a carbon-12 atom is 12 u.
5Molecular & Formula Mass Molecular MassThe sum of the atomic masses of each atom in the molecule.Formula massThe sum of atomic masses in the formula unit
6Molecular & Formula Mass What is the formula mass of calcium phosphate?Calcium ion: Ca2+ Phosphate ion: PO43–Calcium phosphate: Ca3(PO4)2Ca 3 × u = uP 2 × u = uO 8 × u = uCa3(PO4) u
7Defenition of MoleThe mole is the amount of substance that contains as many elementary entities as there are atoms in exactly 12 grams of carbon-12.In 12 g of carbon-12 there are x atoms1 mole of any substance = x units of that substance.When the mole is used the elementary entities must be specified: atoms, molecules, ions…
8Avogadro’s Number NA The number of elementary units in one mole 1023 units/molThe Avogadro constant is a conversion factor between units and mole.
9Conversion of Mole to Molecules How many carbon dioxide molecules are in 2.0 moles of carbon dioxide?2.0 mol CO2 × =1.2 × 1024 molecules CO2
10Molar MassMolar mass of a substance is the mass in grams of one mole of the substance.Units: g/molMolar mass of an element is the mass of the element per mole of its atoms.
11Atomic mass unit and gram The mass of one atom of carbon-12 is exactly 12 atomic mass units.The mass of one mole of carbon-12 atoms (6.022 x 1023 atoms of carbon-12) is exactly 12 grams(6.022 x 1023 atoms) x (12 u/atom) = 12 grams6.022 x 1023 u = 1 gram
12Molar MassThe mass of one atom of carbon-12 is exactly 12 atomic mass units.The mass of one mole of carbon-12 atoms is exactly 12 grams.This leads to the conclusion:The molar mass of any substance in grams per mole is numerically equal to the atomic, molecular or formula mass of that substance in atomic mass units.
13Molar MassCalculate the mass of one NH3 molecule and the mass of one mole of NH3 molecules.14.01 u + 3x1.008 u = uThe mass of one ammonia molecule is u.To change from molecular mass to molar mass, change the units from u to g/mol: g/mol.One mole of ammonia molecules has a mass of17.03 g.
21The simplest ratio of atoms of the elements in a compound. Empirical FormulaEmpirical FormulaThe simplest ratio of atoms of the elements in a compound.The empirical formula of C2H4 is CH2.Likewise, the empirical formula of C3H6 is CH2.All compounds with the general formula CnH2nhave the same empirical formula andtherefore the same percentage composition.
22Empirical FormulaWrite the empirical formulas of benzene, C6H6, and octane, C8H18.Look for the simplest whole-number ratio of elements:For C6H6, the 6/6 ratio can be reduced to 1/1: CH.For C8H18, the 8/18 ratio can be divided by 2 on top and bottom to be reduced to 4/9: C4H9.
23Find Empirical Formula To find empirical formula, you need to find the ratio of atoms of the elementsratio of atoms = ratio of moles of atoms
24How to Find an Empirical Formula Find the masses of different elements in a sample of the compound.Convert the masses into moles of atoms.Determine the ratio of moles of atoms.Express the moles ratio as the smallest possible ratio of integers.Write the empirical formula, using the number in the integer ratio as the subscript in the formula.
25Find Empirical Formula What is the empirical formula of a compound that has 85.6% carbon, 14.4 % hydrogen?Solution:It is usually helpful to organize the calculations in a table with the following headings:Mole Formula EmpiricalElement Grams Moles Ratio Ratio Formula
26Find Empirical Formula Mole Formula EmpiricalElement Grams Moles Ratio Ratio FormulaCH CH2
27Find Empirical Formula What is the empirical formula of a compound that analyzes as 20.0% carbon, 2.2% hydrogen, and 77.80% chlorine?Mole Formula EmpiricalElement Grams Moles Ratio Ratio FormulaCHCl C3H4Cl4
28Molecular Formula The molecular formula of a compound can be found by determination of the number ofempirical formula units in the molecule.
29How to Find the Molecular Formula Determine the empirical formula of the compound.Calculate the molar mass of the empirical formula unit.Divide the molar mass of the compound by the molar mass of the empirical formula unit to get n, the number of empirical formula units per molecule.
30Molecular FormulaWhat is the molecular formula of a compound with the empirical formula C2H5and a molar mass of g/mol?The molar mass of the empirical formula unit is2(12.01 g/mol C) + 5(1.008 g/mol H) =29.06 g/molThe number of empirical formula units per molecule is(C2H5)2 = C4H10
31Find Molecular Formula An unknown compound is found to be 40.0% of carbon, 6.71% of hydrogen and the remainder is oxygen. The molar mass of the compound is g/mol. Find the empirical and molecular formulas of the compound.
32First Find Empirical Formula Mole Formula EmpiricalElement Grams Moles Ratio Ratio FormulaCHO CH2O
33Calculate Molar Mass/Empirical Formula Mass The molar mass of the empirical formula unit is(12.01 g/mol C) + 1(1.008 g/mol H) + (16.00 g/mol O) = g/molThe number of empirical formula units per molecule is(CH2O) 6 = C6H12O6
34Molecular FormulaAnother way to find molecular formula is to consider one mole of the compound ( g of compound). The numbers of moles are also numbers of atoms in the molecule.The masses of carbon, hydrogen and oxygen are :40.0% x g = g of C6.71% x g = g of H53.3% x g = g of O
35Molecular Formula C 72.06 6 H 12.09 12 O 96.03 6 C6H12O6 Element Grams Moles Molecular FormulaCHO C6H12O6