1. What are the two pK a values for carbonic acid? What is the pH of a 0.100M so
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1. What are the two pKa values for carbonic acid? What is the pH of a 0.100M solution of potassium bicarbonate? What is the pH of a 0.100M solution of potassium carbonate? 2. Student Bobby Sue weighed out a 2.3453 g sample of a mixture that was 50.00% potassium carbonate and 50.00% potassium bicarbonate. The sample was dissolved in 250 mL of water and treated with excess NaOH and BaCl2 as described in the procedure. What is mass of the BaCO3 could be precipitated from the entire solution? 3. Assume that the precipitation reaction between Ba2+ and CO32- is exothermic. Would heating the solution tend to make the analysis more or less accurate? Explain. 4. The concentration of the BaCl2 solution added added to the carbonate solution was triple the expected value! Would this likely present a problem with the analysis? Explain what possible effect it could have on the reaction and analysis. 1. What are the two pKa values for carbonic acid? What is the pH of a 0.100M solution of potassium bicarbonate? What is the pH of a 0.100M solution of potassium carbonate? 2. Student Bobby Sue weighed out a 2.3453 g sample of a mixture that was 50.00% potassium carbonate and 50.00% potassium bicarbonate. The sample was dissolved in 250 mL of water and treated with excess NaOH and BaCl2 as described in the procedure. What is mass of the BaCO3 could be precipitated from the entire solution? 3. Assume that the precipitation reaction between Ba2+ and CO32- is exothermic. Would heating the solution tend to make the analysis more or less accurate? Explain. 4. The concentration of the BaCl2 solution added added to the carbonate solution was triple the expected value! Would this likely present a problem with the analysis? Explain what possible effect it could have on the reaction and analysis.Explanation / Answer
1) pka1 = 6.367 , pka2 = 10.32 ,
HCO3- + H2O <---> CO32- + H3O+
Ka2 = 10^ -10.32 = [H3O+][CO32-]/[HCO3-]
4.786 x10^ -11 = x^2/( 0.1-x)
x = 2.188 x10^ -6 =[H+] , pH = -log ( 2.188x10^-6) = 5.66
2)KHCO3 = 2.3453 /2 gm = 1.17265 gm = 1.17265/110.115 = 0.01065 moles
K2CO3 moles = 1.17265/138.2 = 0.0085 ,
moles of CO32- produced after complete neutralisation with NaOH
= 0.01065+0.0085 = 0.019135
moles of BaCO3 = 0.019135 ( since Ba is excess)
BaCO3 mass = 0.019135 x 197.34 = 3.776 gn
3) In exothermic reactions as temp increases reaction favours backward direction according to Lechatliers principle. Hence analysis will be less accurate .
4) Even BaCl2 is excess there will be no problem in analysis since BaCO3 moles formed depend on number of moles of carbonate reacted which is limiting reagent
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