Calculate the pH of the resultant mixtures:
a) 10 mL of 0.2M Ca(OH)2 + 25 mL of 0.1M HCl
b) 10 mL of 0.01M H2SO4 + 10 mL of 0.01M Ca(OH)2
c) 10 mL of 0.1M H2SO4 + 10 mL of 0.1M KOH
a) 10 mL of 0.2M Ca(OH)2 + 25 mL of 0.1M HCl
b) 10 mL of 0.01M H2SO4 + 10 mL of 0.01M Ca(OH)2
c) 10 mL of 0.1M H2SO4 + 10 mL of 0.1M KOH
Solution and Explanation
(a) Moles of H3O+ = \(\frac{25 × 0.1}{1000}\) = .0025 mol
Moles of OH- = \(\frac{10 × 0.2 × 2}{1000}\) = .0040 mol
Thus, an excess of OH- = .0015 mol
[OH-] =\(\frac{ .0015}{35 × 10^{-3}}\) mol/L = .0428
pOH = -log[OH] = 1.36
pH = 14 - 1.36 = 12.63 (not matched)
(b) Moles of H3O+ =\(\frac{10 × 0.01 × 2}{1000}\) = .0002 mol
Moles of OH- = \(\frac{10 × 0.1 × 2}{1000}\) = .0002 mol
Since there is neither an excess of H3O+ or OH-, the solution is neutral. Hence, pH = 7.
(c) Moles of H3O+ = \(\frac{10 × 0.1 × 2}{1000}\)= .002 mol
Moles of OH- = \(\frac{10 × 0.1 }{1000}\) = 0.001 mol
Excess of H3O+ = .001 mol
Thus, [H3O+] = \(\frac{.001}{20 × 10^{-3}}\) = \(\frac{10^{-3}}{20 × 10{-3}}\) = 0.5
∴ pH = -log(0.05) = 1.30
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Concepts Used:
Equilibrium Constant
The equilibrium constant may be defined as the ratio between the product of the molar concentrations of the products to that of the product of the molar concentrations of the reactants with each concentration term raised to a power equal to the stoichiometric coefficient in the balanced chemical reaction.
The equilibrium constant at a given temperature is the ratio of the rate constant of forwarding and backward reactions.
Equilibrium Constant Formula:
Kequ = kf/kb = [C]c [D]d/[A]a [B]b = Kc
where Kc, indicates the equilibrium constant measured in moles per litre.
For reactions involving gases: The equilibrium constant formula, in terms of partial pressure will be:
Kequ = kf/kb = [[pC]c [pD]d]/[[pA]a [pB]b] = Kp
Where Kp indicates the equilibrium constant formula in terms of partial pressures.
- Larger Kc/Kp values indicate higher product formation and higher percentage conversion.
- Lower Kc/Kp values indicate lower product formation and lower percentage conversion.
Medium Kc/Kp values indicate optimum product formation.
Units of Equilibrium Constant:
The equilibrium constant is the ratio of the concentrations raised to the stoichiometric coefficients. Therefore, the unit of the equilibrium constant = [Mole L-1]△n.
where, ∆n = sum of stoichiometric coefficients of products – a sum of stoichiometric coefficients of reactants.