Prince Mohammad Bin Fahd University Week 4 Henderson Hasselbalch Equation HW knowledge check for week 4 i will attach a word doc with all the questions tot

Prince Mohammad Bin Fahd University Week 4 Henderson Hasselbalch Equation HW knowledge check for week 4 i will attach a word doc with all the questions total 12Q, and the PP with it. Remaining topics from acidbase equilibria
Week 4_Part 1
Polyprotic Acids and Bases
• Polyprotic acids such as H2SO4, H3PO4, and H2CO3 contain more than one
ionizable proton
• The protons are lost in a stepwise manner
• The fully protonated species is always the strongest acid
• It is easier to remove a proton from a neutral molecule than from a negatively charged ion
• H2SO4 is more acidic than HSO4-
16-2
ALEKS problem on polyprotic acids
Sulfuric acid H2SO4 is a polyprotic acid. Write balanced chemical
equations for the sequence of reactions that sulfuric acid can undergo
when it’s dissolved in water.
Knowledge Check 1
• Write balanced chemical equations for the 2nd sequence of reaction
of phosphoric acid when it’s dissolved in water. (Hint: Use the 2nd
sequence in the previous slide as a template to guide you.)
ALEKS (Solving a polyprotic acid equilibrium
composition problem)
Knowledge Check 2
• What will be the equilibrium molarity of the sulfate ion if the
prepared concentration of sulfuric acid was 0.015 M instead of 0.031
M. (Hint: You will have to use the quadratic formula. Also, use the
same Ka1 and Ka2 values from the previous problem.)
Molecular Structure and Acid–Base Strength
• Bond Strengths: The stronger the A–H or B–H+ bond
• The less likely the bond is to break to form H+ ions and thus the less acidic the substance
• The trend in bond energies is due to a steady decrease in overlap between the
orbital of hydrogen and the valence orbital of the halogen atom as the size of
the halogen increases
16-7
Knowledge Check 3
• Here are some hypothetical acids and their bond strength in kJ/mol.
Rank these acids from the lowest to the highest pKa. (Assume the
element A, B, C, X, and Y are in the same group in the periodic table.)
H-A = 412 kJ/mol
H-B = 345 kJ/mol
H-C = 555 kJ/mol
H-X = 320 kJ/mol
H-Y = 575 kJ/mol
Acid Base Titrations
An acid–base titration is a method of quantitative analysis for determining the
concentration of an acid or base by exactly neutralizing it with a standard solution
of base or acid having known concentration.
End Point
• The point at which the
indicator changes its color
during titration
Equivalence Point
• The point in a titration where
a stoichiometric amount of
the titrant has been added
End point and choice of an indicator
Properties of a good indicator:
•
The color change must be easily detected
•
The color change must be rapid
•
The indicator molecule must not react with the substance being
titrated
•
The indicator must change color within a specific narrow pH range.
Figure: The Titration of (a) a Strong Acid with a Strong
Base and (b) a Strong Base with a Strong Acid
For the titration of a monoprotic
strong acid (HCl) with a monobasic
strong base (NaOH),
How much 0.20 M NaOH is required to completely neutralize 50.0 mL of a 0.10 M
solution of HCl?
Ans: 25 mL
16-11
Figure: The Titration of (a) a Weak Acid with a Strong Base and (b) a
Weak Base with a Strong Acid
ALEKS (Calculating the pH of a weak acid titrated with a strong base)
In a titration experiment, 5.00 mL of 0.200 M NaOH was added to 50.00 mL of 0.100 M CH3CO2H,
calculate the pH of the resulting solution. Ka for acetic acid is 1.74 × 10−5.
Step 1: Write the equation CH3CO2H(aq) + OH−(aq) ⇌ CH3CO2−(aq) + H2O(l)
Step 2: Using ICE-table, calculate the moles and finally the concentration of reactants and
products in the mixture after the addition is complete.
initial
change
final
[CH3CO2H]
5.00 mmol
−1.00 mmol
4.00 mmol
[OH−]
[CH3CO2−]
1.00 mmol 0 mmol
−1.00 mmol +1.00 mmol
0 mmol
1.00 mmol
Step 3: Use Ka for acetic acid to determine [H]+ ion concentration, and then pH.
Finally, pH = −log[H+]
= −log(6.95 × 10−5)
= 4.158
Buffers
• Solutions that maintain a relatively constant pH when an acid or a base is added.
• Buffers contain either a weak acid (HA) and its conjugate base (A−) OR a weak base (B)
and its conjugate acid (BH+).
• Buffer Capacity: The amount of strong acid or strong base that a buffer solution can
absorb before the pH changes
Example of buffers
• Example: Acetate buffer (a solution of 1.0 mole of acetic acid and 1.0 mole of
sodium acetate in water)
CH3COOH + H2O ⇌ CH3COO– + H3O+
Case 1: If we add a strong acid, such as HCl, the added H3O+ ions react with
acetate ions:
CH3COOH + H2O ← CH3COO– + H3O+
Case 2: If we add a strong base, such as NaOH, the added OH– ions react with
acetic acid:
CH3COOH + OH- → CH3COO– + H2O
Henderson-Hasselbalch equation
Used to estimate the pH of the buffer solution.
H = +
[ ]
log
[ ]
Question: What is the pH of a phosphate buffer solution containing 1.0 mole of
NaH2PO4 and 0.50 mole of Na2HPO4 dissolved in enough water to make 1.0 liter
of solution? (ALEKS: Calculating the pH of a buffer solution)
The equilibrium and its pKa are:
H2PO4– + H2O ⇌ HPO4–2 + H3O+
pKa = 7.21
Knowledge Check 4
A solution is prepared at 25oC that is initially 0.42 M in a “X” base, a
weak base with Kb = 7.4X10^-4, and 0.42 M in conjugate acid “HX”.
Calculate the pH of the solution. Round your answer to 2 decimal
places. (Hint: pKw = pKa + pKb)
Lewis Acids and Bases
ALEKS (Identifying Lewis acids and bases)
Knowledge Check 5
• In the reaction below which species are the Lewis acids (LA) and
which are the Lewis bases (LB) in the forward and reverse direction?
Reaction:
H2O (l) + NH3 (aq) NH4+ (aq) + OH- (aq)
In the forward direction: LA = ?
In the reverse direction: LA = ?
LB = ?
LB = ?

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