Chapter 6 ­ Chemical Proportions in Compounds
In the late 1700’s Joseph Proust discovered that samples of copper (II) carbonate always contained the same proportion of Cu, C and O.
The mass of an element in a compound, expressed as a % of the total mass is and elements MASS PERCENT.
Ex H2O
This led to the LAW OF DEFINITE PROPORTIONS ­ the elements in a chemical compound are always present in the same proportion by mass.
The law of definite proportions does not imply that the elements in compounds are always the same.
CO
CO2
If we state all of the mass percents for a compound we call it the PERCENTAGE COMPOSITION
A compound with a mass of 48.72g is found to contain 32.69g of Zn and 16.03g of S. What is the % composition?
How many atoms of nitrogen are in 3.62 mol of ammonium phosphide?
What is the percent composition of lithium chloride?
Calculating % composition from a formula
Pyrite (aka fool’s gold) is a compound with the chemical formula FeS2. If you wanted to extract the iron and know how much to expect, you first need to know the percentage composition. Calculate the % composition for FeS2.
Cinnamaldehyde C9H8O is the molecule responsible for the odour of cinnamon. Find the % composition of this compound.
Warmup:
Empirical Formula of a Compound
What is the molar mass of 73.4g of sodium phosphate?
Find the percent composition of 50g of lithium chloride.
What is the mass of a single molecule of sodium chloride?
The empirical formula of a compound shows the lowest whole # ratio of the elements in the compound.
Ex H2O2
How many atoms are found in 2.75 grams of magnesium fluoride
The molecular formula of a compound shows the actual # of atoms of each element in the compound.
The relationship between the 2 types of formulas is:
Molecular Formula Subscripts = n x Empirical Formula Subscripts
Determining Empirical Formulas
To determine an empirical formula you need to use the concepts of the mole and percentage composition.
Ex2. The percentage composition of a fuel is 81.7% carbon and 18.3% hydrogen. Find the empirical formula.
Ex1. Calculate the empirical formula for a compound that is 85.6% C and 14.4% H.
PRACTICE NOW!
Try #9 pg. 209
# 13 pg. 211
# 2 pg. 214
Molecular Formula of a Compound
Chemists use a MASS SPECTROMETER to determine the molar mass of a substance. Then:
Being able to identify unknown substances has many uses. Think forensic science.
Since many substances have the same empirical formulas, we need to be able to determine the MOLECULAR FORMULA for a substance. To do this we use MOLAR MASS.
Ex. 1 The empirical formula for ribose (a sugar) is CH2O. The molar mass of ribose is 150g/mol (by experiment). What is the molecular formula of ribose?
MCompound = n x MEmpirical Formula
PRACTICE NOW!
Try #17 pg. 218
# 20 pg. 218
# 2 pg. 218
# 5 pg. 218
Finding Formulas by Experiment
There are many ways to determine formulas by experiment.
The Carbon­Hydrogen Combustion Analyzer
This is a device used to determine formulas of CHO containing compounds.
How it works
• Stream of O 2 is passed over CH containing compounds.
• CH compound will react completely to produce CO 2 and H2 O
• All the H2O is absorbed by Mg(ClO4)2. Final mass ­ initial mass = mass of H2O. All the H ends up here.
• Use % composition of H in H2O to determine mass of H
• All the CO2 is absorbed by NaOH. All C ends up here.
• Use % composition of CO2 to determine mass of C.
• If original compound contained a third element, then:
• m3rd = msample ­ massH ­ massC Ex1. A 1.000g sample of a pure compound containing only C and H was combusted. 0.6919g of water and 3.338g of carbon dioxide were produced.
a. Calculate the masses of C and H in the sample.
b. Find the empirical formula of the compound.
Hydrated Ionic Compounds
Many ionic compounds crystallize from an aqueous solution, with H2O molecules incorporated into their crystal structure. These are called HYDRATES.
Hydrates have a specific # of H2O molecules chemically bonded to each formula unit.
Chemical Name
Formula
Compounds that have no H2O molecules are called ANHYDROUS
Just remember: when doing calculations you must include the mass of any H2O molecules.
Ex 1.
A 50.0g sample of a hydrate of barium hydroxide
Ba(OH)2 XH2O contains 27.2g of Ba(OH) 2.
a. Calculate the percent by mass of water in
Ba(OH)2 XH2O
b. Find the value of x in Ba(OH)2 XH2O .