Post on 27-Dec-2015
A. Percentage Composition
• the percentage by mass of each element in a compound
100mass total
element of massncompositio %
• Ex 3: How many grams of copper are in a 38.0-g sample of Cu2S? (from example 2 , we know the % Cu in Cu2S).
Cu2S is 79.852% Cu (from the last slide)
• We solved example 3 in two steps. It can be done in one longer step. Let’s look at that option.
• Ex 4: How many grams of Calcium are in a 50.0-g sample of Calcium Chloride?
B. Empirical Formula
C2H6
CH3
reduce subscripts
• Smallest whole number ratio of atoms in a compound
• If you found a molecular compound to be
Ca6(PO3)4 , what would the empirical formula for that compound be?
B. Empirical Formula Calculation1. Find mass (or %) of each element.
2. Find moles of each element.
3. Divide moles by the smallest # to find subscripts.
4. When necessary, multiply subscripts by 2, 3, or 4 to get whole #’s.
Percent to massMass to mole
Divide by smallMultiply ‘til whole
Commit this to memory and you will rock Empirical formulas!
B. Empirical Formula• Find the empirical formula for a
sample of 25.9% N and 74.1% O.
25.9 g 1 mol
14.01 g = 1.85 mol N
74.1 g 1 mol
16.00 g = 4.63 mol O
1.85 mol
1.85 mol
= 1 N
= 2.5 O
• Ex 2: A compound was found to contain 39.12% C, 8.76% H, and 52.12% O. Determine the empirical formula of the substance.
Percent to massMass to moleDivide by smallMultiply ‘til whole
C. Molecular Formula• “True Formula” - the actual number
of atoms in a compound
CH3
C2H6
empiricalformula
molecularformula
?
• Here is an analogy to use to describe the difference.• If you are at a birthday party and there are 28 friends, 7
of which are men, what is the ratio of women compared to men?
• This is like the empirical formula
• If someone asked you to tell the actual number of women compared to men, what would you tell them?
• This is like the molecular formula.
C. Molecular Formula Calculation1. Find the empirical formula.2. Calculate the empirical formula molar mass.3. Divide the molecular molar mass by the empirical molar mass.4. Multiply each subscript by the answer from step 3.
nmass EF
mass MF nEF
C. Molecular Formula• The empirical formula for ethylene is CH2. Find the molecular
formula if the molecular mass is 28.06 g/mol?
28.06 g/mol
14.03 g/mol = 2.00
empirical mass = 14.03 g/mol
(CH2)2 C2H4
• Example 4: A compound was found to contain 49.98 g of Carbon and 10.47 g of hydrogen. The molar mass of the compound was 58.12 g/mol. Determine the molecular formula.