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 + H2O 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)

http://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/flashfiles/stoichiometry/acid_base.html

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 25.00 mL is just neutralized by 40.00 ml of a 0.150 mol/L sodium hydroxide solution.

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

1:1 ratio 0.00600 moles of HCl

= 0.006 moles NaOH0.150 mol 0.04 L 1 L

= 0.240 M HCl0.006 mol0.025 L

What volume of a 0.250 mol/L solution of carbonic acid is needed to neutralize 30.0 g of sodium hydroxide.

H2CO3(aq) + 2 NaOH(aq) Na2CO3(aq) + 2 H2O(l)

= 0.750 moles NaOH30.0 g 1 mole40.0 g

Molar mass of NaOH = 40.0 g

1.5 L of 0.250 mol/L H2CO3 will neutralize 30.0 g of NaOH

= 0.375 moles H2CO30.750 mol 1 H2CO3

2 NaOH

= 1.5 L H2CO30.375 mol 1 L

0.250 mol

Determine the concentration of a hydrochloric acid solution using a 0.100 mol/L solution of NaOH

Indicator - phenolphthalein (endpoint turns pink)

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

1. Calculate the moles of base used:

1:1 ratio 0.001383 moles of HCl

= 0.001383 mol NaOH0.100 mol 0.01383 L 1 L

= 0.121 M HCl0.001383 mol0.01144 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 100.0 mL of solution.

A 12.00 mL aliquot is titrated with 0.0985 mol/L sodium hydroxide. If 13.38 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) + H2O(l)

= 0.001318 mol NaOH0.0985 mol 0.01338 L 1 L

1:1 ratio 0.001318 moles of HX – (aliquot)

= 0.1098 M HX0.001318 mol0.01200 L

Total moles in the 100.0 mL sample:

= 0.01098 mol HX (total used)0.1098 mol 0.1 L 1 L

= 0.01098 mol HX (total used)0.1098 mol 0.1 L 1 L

= 1.07 g HX0.01098 mol1 mole97.09 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 = 6.0 - 7.6)

SB - SA

WA - SB

Phenolphthalein not suitable indicator.methyl red (pH range = 4.8 - 6.0)bromocresol green (pH range = 3.6 - 5.2).

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.