CHEMISTRY 161

Chapter 4

www.chem.hawaii.edu/Bil301/welcome.html

1.precipitation reactions

2. acid-base reactions

3. redox reactions

REVISION

qualitatively

quantitatively

molarity

4.5. MOLARITY

concentration of a solution

M = moles of solute

liters of solution [ ]mol

l

M = moles of solute

liters of solution [ ]mol

l

What is the Molarity of a solution of

20 g NaCl in 350 ml of water?

1. convert grams into moles

20g / [22.99 g/mol+35.45 g/mol] = 0.34 mol

2. convert volume into liters

350 ml = 0.350 l

3. calculate molarity

0.97 M

NaCl(s) → Na+(aq) + Cl-(aq)

0.97 M0.97 M +

1.94 M

(with respect to ions)

reference system in case of strong electrolytes

0.97 M

Ba(NO3)2(s) → Ba2+(aq) + 2 NO3 -(aq)

1.5 M 1.5 M 3.0 M

M = moles of solute

liters of solution [ ]mol

l

How many ml of a 4 M NaCl solution are necessary to prepare 2 l of a 4 M solution?

4 M 2 Mdilution

‘add water’

‘conservation of moles’

How many ml of a 4 M NaCl solution are necessary to prepare 2.6 l of a 0.6 M solution?

1.0 l of a 0.6M solution ------------ 0.6 moles NaCl

2.6 l of a 0.6 M solution ------------ x moles NaCl

x = 1.56 moles NaCl

1.0 l of a 4.0 M solution ------------ 4.0 moles NaCl

x l of a 4.0 M solution ------------ 1.56 moles NaCl

x = 390 ml

moles of solute before dilution = moles of solute after dilution

i: initial conditions

f: final condiotions

Mi Vi = Mf Vf

1.precipitation reactionsgravimetric analysis

2. acid-base reactions titrations

3. redox reactionstitrations

APPLICATIONS

(lab course)

1. Gravimetric Analysis

a precipitation method based on the determination of weight of a substance of

known composition

Ag+ or Cl- AgCl

Ba2+ or SO42- BaSO4

Gravimetric Analysis

• weigh sample (NaCl)• dissolve sample in water

• add precipitating reagent in excess (AgNO3)

• filtration-separate AgCl from liquid• wash precipitate

• dry and weigh to constant weight

quantitatative analysis

2. Acid-Base Titration

determine the concentration of an acid (base) by adding stepwise a solution of a known concentration of

a base (acid)

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

unknown 1 M standard

Conductivity Analysis

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

Λ (H+) >> Λ (Na+)

Λ (OH-) >> Λ (Cl-)

3. Redox Titration

determine the concentration of a sample ion by adding stepwise a solution of a known reagent

5 Fe 2+ + MnO4- + 8 H+ → Mn2+ + 5 Fe3+ + 4 H2O

unknownstandard solution

(purple)

light pink

1.precipitation reactions

2. acid-base reactions

3. redox reactions

SUMMARY

Homework

Chapter 4, p. 128-140problems