burette clamp ring stand burette erlenmeyer flask Titration

 Describe an acid/base titration.  Compare and contrast the end point and the equivalence point of a titration.  Determine the concentration of an unknown acid or base using titration data.  Determine the mass of an unknown acid or base using titration data

Titration – controlled neutralization reaction. Used to measure [ ] of unknown acid or base. A standard solution is required. Standard solution – solution of a strong acid or base of known concentration. **pH does not indicate concentration because weak acids/bases do not dissociate completely. HA + BOH  BA + H 2 O salt water

ring stand burette clamp erlenmeyer flask burette Titration Apparatus buret

Equivalence point – amount of standard added neutralizes the unknown sample. Endpoint – the indicator changes colour. Determined by adding a pH indicator. moles H + = moles OH - Eq. point - defined by reaction stoichiometry. Endpoint - determined by the choice of indicator. (pH = 7)

1.Measured amount of acid of unknown concentration is added to the flask. 2.An appropriate indicator is added to the unknown. 3. Measured amount of base of known concentration is added slowly to the flask. 4. The titration is stopped when a colour change has occurred (end point). 5.Concentration of the acid is calculated. **Titration can also be preformed with a calculated volume of standard in the flask, and the unknown in the buret.

Calculate the concentration of hydrochloric acid, if mL is just neutralized by ml of a mol/L sodium hydroxide solution. HCl (aq) + NaOH (aq) NaCl (aq) + H 2 O (l) 1:1 ratio moles of HCl = moles NaOH mol0.04 L 1 L = M HCl mol L

What volume of a mol/L solution of carbonic acid is needed to neutralize 30.0 g of sodium hydroxide. H 2 CO 3(aq) + 2 NaOH (aq) Na 2 CO 3(aq) + 2 H 2 O (l) = moles NaOH 30.0 g1 mole 40.0 g Molar mass of NaOH = 40.0 g

1.5 L of mol/L H 2 CO 3 will neutralize 30.0 g of NaOH = moles H 2 CO mol1 H 2 CO 3 2 NaOH = 1.5 L H 2 CO mol1 L mol

Determine the concentration of a hydrochloric acid solution using a mol/L solution of NaOH Indicator - phenolphthalein (endpoint turns pink) HCl (aq) + NaOH (aq) NaCl (aq) + H 2 O (l)

1. Calculate the moles of base used: 1:1 ratio moles of HCl = mol NaOH mol L 1 L = M HCl mol L 2. Use stoich to determine moles of acid used: 3. Calculate unknown value.

A solid monoprotic acid (97.09 g/mol), is dissolved in water to make mL of solution. A mL aliquot is titrated with mol/L sodium hydroxide. If mL of the base is needed to reach the endpoint, what was the mass of the sample of acid ? HX (aq) + NaOH (aq) NaX (aq) + H 2 O (l)

= mol NaOH mol L 1 L 1:1 ratio moles of HX – (aliquot) = M HX mol L Total moles in the mL sample: = mol HX (total used) mol0.1 L 1 L

= mol HX (total used) mol0.1 L 1 L = 1.07 g HX mol 1 mole g 1.07 g used to make the 100 mL solution

Titration Curves

Plotting pH values during a titration gives a titration curve. Type of curve produced accounts for 2 classes of titrations: 1. Strong acid titrated with strong base. 2. Strong base titrated with strong acid. 3. Weak acid titrated with strong base. 4. Weak base titrated with strong acid.

The equivalence point is the centre of the steepest portion of the curve. Volume of standard used in titration is taken here. SA - SB

Suitable indicators for SA - SB or SB - SA: phenolphtalein bromothymol blue (pH range = ) SB - SA

WA - SB

Phenolphthalein not suitable indicator. methyl red (pH range = ) bromocresol green (pH range = ). WB - SA

 Eqv pt of a SA-SB / SB-SA titration is at pH = 7.  Eqv pt of a WA-SB titration had a pH > 7.  Eqv pt of a WB-SA titration has a pH < 7.  Chose indicator that changes colour (end point) near the expected Eqv pt.