{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

Quiz-2-07F-Solutions

# Quiz-2-07F-Solutions - 1 CHEM 119 H Quiz 2 USE PENCIL ERASE...

This preview shows pages 1–2. Sign up to view the full content.

1 CHEM 119 H Quiz 2 11/06/07 ____________________________________ USE PENCIL! ERASE CLEANLY! If we cannot I have neither given nor received aid while follow your work quickly, it is wrong. taking this quiz. 1. (50) A weak acid, HZ, is dissolved in water. pK A {HZ} = 2.810 ε {HZ, 450 nm} = 2348 L/cm*mol ε {Z - , 450 nm} = 0 a. (20) Calculate the absorbance, A, of 0.00167 F{HZ} in a 1.15 cm cell. A = 1.782 [ ] + 3 - + + A 3 3 2 + 3 -2.810 + 3 + 3 Only HZ absorbs, but some HZ dissociates and [HZ] F{HZ} H O Z K = and standard approximation gives [Z ] = [H O ] and [HZ] = F- [H O ]; HZ H O Therefore, 10 = H O .00167- H O ⎤ ⎡ ⎦ ⎣ = - -4 [Z ]=0.00101M and [HZ] = (0.00167 - 0.00101) = 6.6*10 M ( ) 2348L 0.00066mol A = 1.15cm = 1.782 cm*mol L b. (20) Calculate the % Transmittance, in a 11.5 cm cell, of the solution prepared when 1.00 mL of 0.00167 F{HZ} is diluted to a final volume of 10.0 mL. %T = 39.4 % 1.00 mL Dilution gives a new formality: F(diluted) = F{Original} 0.000167M 10.0 mL = If the extent of ionization of the acid did not change, the absorbance and % T would be the same because A a = 2348*1.15*C a and A b = 2348*(11.5)(C a /10). Hence, % T = 100*10 -1.782 = 1.7% (1.65 %). BUT the extent of dissociation does depend on concentration and the extent of dissociation increases with decreasing concentration (Le Chatelier, again). Hence 2 2.81 -5 [Z ] 10 [Z ] 0.000152 M & [HZ] = 0.000167-0.000152 = 1.5*10 M 0.000167 [Z ] = = A = 2348*11.5*1.5*10 -5 = 0.405 %T = 10 -0.405 = 39.4%

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

### What students are saying

• As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

Kiran Temple University Fox School of Business ‘17, Course Hero Intern

• I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

Dana University of Pennsylvania ‘17, Course Hero Intern

• The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

Jill Tulane University ‘16, Course Hero Intern