Using chemical formulas

Formula Mass and Molar Mass  formula mass - mass in grams (amu) of all of the atoms of ea element in a compound  add up all the masses of individual atoms  molar mass - mass in grams of 1 mol of a substance  conversion factor

 ex: Al 2 (SO 4 ) 3  NaNO 3  Ba(OH) 2

Avogadro’s Constant  remember 1 mol of any substance contains x particles of subst  conversion factor  2 conversion factors

 How many molecules are in 56.3g of

Percent Compostion  % by mass of each element in a compound  same, no matter size of compound

 to find % comp: mass of element x 100= % comp mass compound Calculate the % comp of HNO 3

Determining Chemical Formula  once % comp is known, can determine the simplest formula of compound  aka empirical formula  consists of symbols for the elements combined w/ the smallest whole # ratios of atoms

 for ionic comp, e.f. is accepted formula  e.f. doesn’t indicate actual number of atoms present!  to solve for e.f.:  change % comp to grams  convert comp in g to mol by using molar mass  change mol ratio to smallest whole # subscripts

 to do this, divide the molar amount by the smallest #, round to the nearest whole #  EX: What is the e.f. for a compound that is 63.52% Fe and 36.48% S?

Molecular Formula  e.f. is not always the correct formula for a compound-especially covalent compounds  use info to solve for molecular formulas for covalent comp  must also have molecular mass to change from e.f. to molecular formula (GIVEN)

 x = mol form mass e.f. mass x = # by which we multiply all of the subscripts in e.f.

 ex: Glucose is 40.0% C, 6.72% H, and 53.3% O. Experiments show that its molecular weight is amu. Determine the empirical and molecular formulas.

Formula of a Hydrate  many compounds will form crystals from water soln w/ water molecules staying attached to the molecules or ions  the water molecules b/c a part of the crystal & b/c known as a HYDRATE  shown by writing chemical form followed by dot w/ # water molecules

 named by using name of comp 1 st, followed by the prefix which indicates # of water molecules attached to the word hydrate  CuSO 4 *5H 2 O  copper II sulfate pentahydrate  since the water molecules are attached they add weight

 find the formula mass for MgSO 3 *6H2O  also, can find formula of hydrated comp & the mol ratio betw the comp & the water molecules  steps:  if given mass of hydrated sample:  hydrate mass – dry mass = water mass  convert mass of dry sample & mass of water to mol ea  get mol ratio & write formula

 We have a 10.47g sample of hydrated barium iodide. The sample is heated to drive off the water. The dry sample has a mass of 9.520g. What is the mole ratio between barium iodide and water? What is the formula?

 What is the formula for a hydrate containing 0.391g Li 2 SiF 6 and g H 2 O?

 What is the formula for a hydrate that is 76.9% La 2 (CO 3 ) 3 and 23.9% H 2 O?

 THE END!