Lecture 24 10/31/05

Finding endpoint with pH electrode

Titration of H 6 A with NaOH

Gran Plot Advantage is that you can use data before the endpoint to find the endpoint

V b never goes to 0 because 10 -pH never gets to 0 Also slope doesn’t stay constant as V b nears 0

Indicator Weak acid or base Different protonated forms have different colors Need a ratio greater than ~ 10:1 to see color Seek indicator whose color change is near equivalence point

Indicator errors Difference between endpoint (color change) and true equivalence point If you use too much  can affect reaction

Problem 7-11 The carbonate content of g of powdered limestone was measured by suspending the powder in water, adding mL of M HCl, and heating to dissolve the solid and expel CO 2 : CaCO 3 (s) [FM ] + 2H +  Ca 2+ + CO 2 (g) + H 2 O The excess acid required mL of M NaOH for complete titration to a phenolphthalein end point. Find the weight % of calcite in the limestone.

Problem 7-11 (solutions) Moles OH - = (39.96 mL)*( M) = mmol Moles H + = (10 mL)*(1.396 M) = mmol Moles H + used to titrate CaCO 3 = mmol Moles CaCO 3 = mmol H*(1 mol CaCO 3 / 2 mol H) Moles CaCO 3 = mmol Mass CaCO 3 = mmol *( g/mol) = g Weight % = g / * 100 = 92%

Quiz 4 A sample was analyzed using the Kjeldahl procedure. The liberated NH 3 was collected in 5.00 mL of 0.05 M HCl, and the remaining acid required 3 mL of M NaOH for a complete titration. How many moles of Nitrogen were in the original sample?