Transcript Chapter 3 Molecules, Compounds, & Chemical Equations

Chapter 3 Molecules, Compounds, &
Chemical Equations
CHE 123: General Chemistry I
Dr. Jerome Williams, Ph.D.
Saint Leo University
Overview
• Empirical vs. Molecular Formulas
• Mass Percent
• Determining Chemical Formulas
Empirical vs. Molecular Formulas
• Empirical Formula represents the simplest whole
number ratio of atoms in a formula.
• Molecular Formula represents the true formula for a
substance and is usually a multiple of the empirical
formula.
Mass Percent
• Empirical Formulas are determined from mass
percent data.
• To find the mass percent of an element use the
following equation.
Mass % of X = (mass X / total mass of compound) x 100%
• The mass percentages for all elements in a
compound must sum to 100.00%.
Mass Percent
• Find the mass % oxygen in sodium nitrate (NaNO3).
Molar mass of NaNO3 is 85.07 g/mole.
Mass Percent
• Given:
sodium nitrate NaNO3
Molar Mass = 85.07 g/mole
• Unknown: ? Mass % oxygen
• Relevant Information:
1 mole Na, 1 mole N, and 3 moles O are found in NaNO3
Mass Percent X = (mass X / total mass of compound) x 100%
Mass Percent
• Solution:
mass O = (3 mol x 16.00 g/mol) = 48.00 g
Total mass (NaNO3) = (1 mol x 23.00 g/mol) + (1 mol x 14.07 g/mol) + (3 mol
x 16.00 g/mol) = 85.07 g
Mass % O = (48.00 g / 85.07 g) x 100% = 56.42% O
• Check: Answer is reasonable.
Mass Percent
• For additional practice, find the % Na and % N in sodium
nitrate.
Answers:
27.04% Na
16.54% N
Determining Chemical Formulas
• To find an empirical formula, one would use the
following method (Works Every Time).
Determining Chemical Formulas
Determining Chemical Formulas
• Example Problem
– 71.65% Cl
– 24.27% C
– 4.07% H
Mol. Wt. = 98.96 g/mol
– Find the empirical & molecular formulas for this
compound.
Determining Chemical Formulas
• The Traditional Approach
– Step 1: Assume 100.00 g sample
• We pick this sample size so that our percentage data will translate
directly into mass in grams.
71.65% Cl
24.27% C
4.07% H
71.65 g Cl
24.27 g C
4.07 g H
Determining Chemical Formulas
• The Traditional Approach
– Step 2: Convert mass values to moles (Axiom #3)
71.65 g Cl x ( 1 mol Cl / 35.45 g Cl) = 2.021 mol Cl
24.27 g C x ( 1 mol C / 12.01 g C) = 2.021 mol C
4.07 g H x ( 1 mol H / 1.008 g H) = 4.04 mol H
Determining Chemical Formulas
Determining Chemical Formulas
• Traditional Approach (Works Every Time)
– Step 3: Divide number of moles obtained from step 2 by
smallest number of moles present. This gives the mole
ratios for each component (look at this value as being the
same thing as the number of atoms present).
Determining Chemical Formulas
• The Traditional Approach
– Step 3: Convert moles to mole ratios (# atoms)
2.021 mol Cl / 2.021 mol = 1.000 Cl atoms
2.021 mol C / 2.021 mol = 1.000 C atoms
4.04 mol H / 2.021 mol = 2.00 H atoms
Determining Chemical Formulas
Determining Chemical Formulas
• Traditional Approach (Works Every Time)
– Step 4: Examine values obtained from step 3. The mole
ratios should be whole numbers. If you have whole
numbers, then you have the subscripts for the empirical
formula directly. If not, proceed to step 5.
• Note: Values like 2.98, 4.03, etc. translate into 3.00 and 4.00,
respectively. Do not round numbers like 2.5, 3.67, 1.33, etc. to
whole numbers.
Determining Chemical Formulas
• The Traditional Approach
– Step 4: Values are whole numbers, so we can now write
the empirical formula
1.000 Cl atoms
1.000 C atoms
2.00 H atoms
Empirical Formula = CH2Cl
Determining Chemical Formulas
Determining Chemical Formulas
• Traditional Method (Works Every Time)
– Step 5: If mole ratios are not whole numbers, then one
must introduce a factor that will convert the mole ratios
into whole numbers.
– Once you have found the whole number ratios, then the
subscripts for the empirical formula are then known.
Simply write out the empirical formula.
Determining Chemical Formulas
Determining Chemical Formulas
• To find a molecular formula, one would use the same
procedure as the empirical formula with the
following additional steps.
– Step 6: Once you have empirical formula, calculate the
empirical molar mass (EMM).
Determining Chemical Formulas
• The Traditional Approach
– Step 6: Calculate Empirical Molar Mass (EMM)
Empirical Formula = CH2Cl
EMM = (1 x 12.01 g/mol) + (2 x 1.01 g/mol) + (1 x 35.45
g/mol) = 49.48 g/mol
Determining Chemical Formulas
• To find a molecular formula
– Step 7: Using the molecular weight (this must be given in
problem), find the factor (x) that must be multiplied
through to get molecular formula.
EMM (x) = Mol. Wt.
(Empirical Formula) (x) => Molecular Formula
Determining Chemical Formulas
• The Traditional Approach
– Step 7: Determine Factor (x)
EMM (x) = Mol. Wt.
49.48 g/mol (x) = 98.96 g/mol
(x) = 2.000 = 2
Molecular Formula = (CH2CL) (2) => C2H4Cl2