Aim # 4: How do we determine the molar mass of a compound? H.W. # 4 Study pp. 165-168 up to example 6.7 “calculating moles from mass” Complete the handout sheet

When we say the atomic mass of hydrogen is 1 When we say the atomic mass of hydrogen is 1.00794 amu, how much hydrogen are we talking about? I atomic mass unit(amu)- the unit of measurement used to express the mass of an atom. 1 amu = 1/12 the mass of one 12C atom II atomic mass- the average mass (in amu) of one atom of an element. e.g. H 1 amu Ar 40 amu What is it an average of?

III molar mass – the average mass of one mole of a substance III molar mass – the average mass of one mole of a substance. For an element, it is equal to the atomic mass expressed in grams. e.g. H 1 g Ar 40 g If the atomic mass of an element is the mass (in amu) of one atom, what do you think the molecular mass of a compound represents? IV molecular mass – the sum of all the atomic masses in a molecule. e.g. H2 1 amu + 1 amu = 2 amu H2O 2(1 amu) + 16 amu = 18 amu

V The molar mass, in grams, of one mole of molecules of an element or compound is equal to the molecular mass expressed in grams. e.g. H2 1g + 1g = 2g H2O 2(1g) + 16g = 18g VI If the compound is ionic, there are no molecules. The molar mass is the mass, in grams, of one mole of formula units. e.g. NaCl 23g + 35g = 58g

Problem: calculate the molar mass of Ca(NO3)2 Element # of moles molar mass total Ca 1 40g 40g N 2 14g 28g O 6 16g 96g molar mass = 164g

Now you try it Problem: Calculate the molar mass of Mg(OH)2 Answer: Element # of moles molar mass total Mg 1 24g 24g O 2 16g 32g H 2 1g 2g total = 58g

VII Calculating the molar mass of a hydrate BaCl2•2H2O Element # of moles molar mass total Ba 1 137g 137g Cl 2 35.5g 71g H 4 1g 4g O 2 16g 32g total = 244g

Practice makes perfect Calculate the molar mass of CuSO4•5H2O Answer Element # of moles molar mass total Cu 1 64g 64g S 1 32g 32g O 9 16g 144g H 10 10g 10g total = 250g