ap07_chemistry_form_b_sgs_final_complete

A student adds 010 mol of hcl to 10 l of each of

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Unformatted text preview: central.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents). AP® CHEMISTRY 2007 SCORING GUIDELINES (Form B) Question 5 (continued) (c) Two buffer solutions, each containing acetic acid and sodium acetate, are prepared. A student adds 0.10 mol of HCl to 1.0 L of each of these buffer solutions and to 1.0 L of distilled water. The table below shows the pH measurements made before and after the 0.10 mol of HCl is added. pH Before HCl Added 7.0 4.7 4.7 Distilled Water Buffer 1 Buffer 2 pH After HCl Added 1.0 2.7 4.3 (i) Write the balanced net-ionic equation for the reaction that takes place when the HCl is added to buffer 1 or buffer 2. C2H3O2− + H3O+ → HC2H3O2 + H2O One point is earned for the equation. (ii) Explain why the pH of buffer 1 is different from the pH of buffer 2 after 0.10 mol of HCl is added. Before the HCl was added, each buffer had the same pH and thus had the same [H+]. Because Ka for acetic acid is a constant, the ratio of [H+] to Ka must also be constant; this means that the ratio of [HC2H3O2 ] to [C2H3O2− ] is the same for both buffers, as shown by the following equation, derived from the...
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