{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

CHE134 Summer Lecture 7

# CHE134 Summer Lecture 7 - HO O OH SA A ASPIRIN PURITY BY T...

This preview shows pages 1–14. Sign up to view the full content.

ASPIRIN PURITY BY TITRATION pH O HO OH ASA SA NaOH C OH O O C O CH 3

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

View Full Document
Finishing Last Exercise Your synthesized aspirin should be completely dry by now. You can weigh it ( on the top-loading balance ) and calculate your percent yield. If your melting point last week was less than 120 oC, you should REPEAT the melting point determination
Purpose: To study pH variation during an acid/base titration and use this to determine the purity of your synthesized aspirin Concepts: pH Nernst Equation Calibration and use of the pH meter Molar Mass Effective Molar Mass Titration Stoichiometry Equivalence Point Titration of a mixture Techniques: Weighing by Difference Using a pH Meter Graphing Apparatus: pH Meter Buret

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

View Full Document
pH and the pH METER pH Definition pH = -log [ H+ ] pH METER : Measures voltage difference between two electrodes each in contact with the same aqueous solution Electrodes are selected so that relative voltage depends only on HYDROGEN ION concentration, [ H+ ] ( and temperature ) Other electrodes can measure concentration of other ions. { Have read SUPL-006 which discusses types, use and care of pH electrodes }
NERNST EQUATION describes dependence of electrode voltages on concentration and temperature For normal pH electrodes, 2.303 R T E = E0 - ----------- log [ H+ ] NA e where: E = VOLTAGE DIFFERENCE between reference and indicator electrodes E0 = CONSTANT depending only on TYPES of ELECTRODES T is Absolute Temperature R is gas constant NA is Avogadro’s number e is absolute value of electron charge 2.303 R T = E0 + ------------ pH NA e

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

View Full Document
At 298 K, 2.303 R T / NA e = 0.0592 volts Obj108 0 0.2 0.4 0.6 0.8 2 4 6 8 10 12 E0 + E0 + E0 + E0 + 298 K 273 K At constant T , voltage varies linearly with pH. Voltage, \ (volts) pH E0 + E = E0 + B x pH At 273 K, = 0.0542 volts
CALIBRATION ( STANDARDIZATION ) Like all measuring devices, must calibrate pH METER before using it. Immerse pH electrode into one or more (buffer) solutions of accurately known pH. Set Calibration Control on meter so reading agrees with pH of standard buffer pH 7.00 Buffer How? Remember to rinse electrode after standardization

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

View Full Document
Obj1 0 How does pH vary with added NaOH for a weak acid? How sharp is the pH rise at the equivalence (end) point?
Obj1 1 Full Range Note Scale End Point

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

View Full Document
Obj1 2 ~6 mL Range Note Scale Change Note Range
Obj1 3 ~1 mL Range Note Scale Change Note Range 1 drop

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

View Full Document
Obj1 4 ~0.5 mL Range Note Scale Change Note Range 1 drop
SAMPLE TABLE FOR pH TITRATION Buret Rdg Cum Vol pH Incr Buret Rdg Cum Vol pH Incr 4.35 0.00 2.94 27.10 22.75 6.35 6.46 2.11 3.15 2 mL 27.20 22.85 7.00 8.72 4.37 3.31

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 ]}

### Page1 / 42

CHE134 Summer Lecture 7 - HO O OH SA A ASPIRIN PURITY BY T...

This preview shows document pages 1 - 14. Sign up to view the full document.

View Full Document
Ask a homework question - tutors are online