Multiple Choice
Determine the pH of a solution that is 0.125 M in CO3^2−. For carbonic acid (H2CO3), Ka1=4.3×10^−7 and Ka2=5.6×10^−11. What is the pH of the solution?
17. Acid and Base Equilibrium
pH of Weak Bases
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If 0.365 g of sodium carbonate (Na2CO3) is dissolved in 1.5 L of water, what is the pH of the solution? (Kb for CO3^2- = 1.8 x 10^-4)
A
11.23
B
7.00
C
10.52
D
8.34
Verified step by step guidance1
Calculate the molarity of the sodium carbonate (Na2CO3) solution by dividing the mass of Na2CO3 by its molar mass to find the number of moles, and then divide by the volume of the solution in liters.
Write the hydrolysis reaction of the carbonate ion (CO3^2-) in water: CO3^2- + H2O ⇌ HCO3^- + OH^-.
Use the given Kb value for CO3^2- to set up the expression for the equilibrium constant: Kb = [HCO3^-][OH^-] / [CO3^2-].
Assume that the initial concentration of CO3^2- is equal to the molarity of the Na2CO3 solution, and that the change in concentration of CO3^2- is 'x', which is also the concentration of OH^- produced.
Solve for 'x' using the Kb expression, which represents the concentration of OH^- ions. Then, calculate the pOH of the solution using the formula pOH = -log[OH^-], and finally, find the pH by using the relation pH + pOH = 14.
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