Formula of a Hydrate

Safety:
Crucibles are VERY HOT; always handle them with tongs. DO NOT put hot crucibles on a balance! They cost $9.00 if you
break it. Crucibles are VERY FRAGILE. Never carry them around without a heat-proof pad under it. Wear goggles at all
times.

Introduction:
The water can easily be removed from a hydrate just by heating strongly. You will be weighing a hydrate and heating it to
remove the water (now called "anhydrous salt") and weigh it again. You can now find the percent of the anhydrous salt and the
water. By finding a mol ratio, you can find out how many moles of water there are per mol of anhydrous salt. This number goes
just before the H2O in the formula.

Sample Calculation-
An empty crucible has a mass of 12.770 grams. The crucible and hydrate have a mass of 13.454 grams. After heating, the
crucible and anhydrous salt have a mass of 13.010 grams. What is the formula of this hydrate of
MgSO4 . ?H2O?
Mass of hydrate = 13.454 - 12.770 = .684 grams
Mass of anhydrous salt = 13.010 - 12.770 = .240 grams
Mass of water = 13.454 - 13.010 = .444 grams
 
 
Moles of anhydrous salt = .240 grams MgSO4 x 1 mol MgSO4 = .00199 moles MgSO4
1 120.367 g MgSO4
Moles of water = .444 grams H2O x 1 mol H2O = .0246 moles H2O
1 18.0148 g H2O
 
 
Ratio of moles of water to moles of anhydrous salt = .0246/.00199 = 12
Therefore the formula is MgSO4 . 12H2O

Unit 8 Video 3 Empirical and Molecular Formulas

Empirical and Molecular Formula

The empirical formula of a chemical compound is the simplest whole number ratio of atoms of each element present in a compound. 
 *NOTE*: All ionic compounds are empirical formulas.

Ex:  C4H10 (molecular formula of butane) can be reduced to C2H5 (empirical formula)

Ex: What is the empirical formula of a compound containing 11.1% hydrogen and 88.9% oxygen by mass?
Assume you have 100 g.

H: 11.1% x 1 mole/ 1g = 11.1 mol   ---> 1

O: 88.9% x 1mole/ 16g = 55.6 mol --->5

Empirical formula= HO5

Now to write the molecular formula using the empirical formula!

The molecular formula is a multiple of the empirical formula that contains the actual number of atoms that combine to form a molecule.

*To calculate the multiple:  n = molar mass of the compound
                                              molar mass of the empirical formula

Ex: A molecule has an empirical formula of C2H5 and a molar mass of 58 g/mol. What is the molecular formula?

MM C2H5 = 29 g/mol

n = 58g/mol / 29g/mol =2

2 x C2H5 =  C4H10

~ The metal comes first and then the non-metals after are put in alphabetical order~