Buffers
What are buffers made of?
weak acid/base and its conjugate base/acid
Write the Ksp expression for silver chloride (PbCl2)
Ksp=[Pb2+][Cl-]2
In the following reversible reaction, identify the acid and its conjugate base
NH3 + H2O ⇌ NH4+ + OH−
H2O and OH−
Which one is the base and its conjugate acid?
NH3 + H2O ⇌ NH4+ + OH−
NH3 and NH4+
what do buffers do?
resist changes in pH when small amount of strong acids/bases are added
Write the Ksp expression for magnesium phosphate, Mg3(PO4)2.
Ksp=[Mg2+]3[PO43−]2
pH of 0.001 NaOH solution
11
The Ka for a weak acid, HCN, is 6.2x10−10. Calculate the pKa for this acid.
9.21
What makes a buffer effective?
High concentration and relatively equal concentration of the acid/base + conjugate base/acid components
The Ksp of lead(II) chloride (PbCl2) is 1.7×10−5. What is the molar solubility of lead(II) chloride in pure water?
1.62×10−2 M
You add a strong acid to a buffer and discover the pH does not drastically change. why? (think about what is reacting)
because the conjugate base in the buffer neutralizes the added H⁺
A solution of a strong acid, HNO3, has a concentration of 0.0050 M. What is the pH of this solution?
2.3
What happens to pH when the concentrations of the acid and base are equal?
pH = pKa
How does adding a common ion affect solubility?
It decreases the solubility because it shifts the equilibrium to the left, reducing the amount of compound that can dissolve
AgCl is slightly soluble. What will happen if you add NaCl to its solution?
solubility will decrease due to common ion effect
You mix 50.0 mL of 0.0100 M HNO₃ (strong acid) with 30.0 mL of 0.0200 M NaOH (strong base) at 25 °C.
Will the mixture be acidic or basic?
Basic
A buffer solution is prepared by mixing 0.50 L of 0.20 M acetic acid (CH3COOH) with 0.50 L of 0.20 M sodium acetate (CH3COONa). What is the pH of this buffer solution?
The Ka for acetic acid is 1.8×10−5.
4.74
if a compound contains a basic ion, how will the addition of a strong acid affect its solubility?
adding a strong acid will increase the solubility because it will react with the base to neutralize it, shifting equilibrium to dissolve more compound
A solution is made with 0.5 M acetic acid and 0.5 M sodium acetate. if the pH is 4.1, what is the pKa?
pKa = 4.1 because pH = pKa when mols of A and mols of B are equal
The solubility product of AgCl is Ksp=1.8×10−10.
Calculate the solubility of AgCl in a 0.10 M NaCl solution.
1.8×10−9 mol dm-3