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[PDF] CHAPTER 4 Calculations Used in Analytical Chemistry

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LABORATORY TECHNIQUE, MATERIALS AND FUNDAMENTAL OPERATION 7 31 Analytical chemistry – science field evolving and adapting methods, devices and Calculation of solution's molar equivalent concentration (normality )

CHAPTER 4 Calculations Used in Analytical Chemistry

In this chapter, we describe several methods used

to compute the results of a quantitative analysis

Mass: g = 103 mg = 106 ȝg

Volume: L = 103 mL = 106 ȝL

Temperature: K = 273.15 + ୅

Example

590 nm = 590109 m = 5900 Å

20 L = 20 106 L

6 pmol = 61012 mol

20 Gbite = 20109 bite

1 mL = 106 m3 = 1 cm3

The angstrom unit A is a non-SI

unit of length that is widely used to express the wavelength of very short radiation such as X-rays (1 A

5 0.1 nm 5 10210 m). Thus,

typical X-radiation lies in the range of 0.1 to 10 A. 3

4A-2 The Distinction between Mass and Weight

Mass : An invariant measure of the quantity of matter in an object.

Mass is constant.

Weight : The force of attraction exerted between an object and the earth. Weight(w) equals mass(m) times the gravitational attraction (g=980.665 cm/s2). w = g m 4 nx:number of mole of X (mol),

Mx: mole mass of X (g/mol)

massx: mass of X (g)

4A-3 The Mole

The mole (abbreviated mol) is the SI unit for the amount of a chemical substance. It is the amount of the specified substance that contains the same number of particles as the number of carbon atoms in exactly 12 grams of 12C. This important number is

NA = 6.022 x 1023.

Los Alamos National Lab http://pearl1.lanl.gov/periodic/ 7

4A-5 Calculating the amount of a substance in moles or millimoles

4B Solutions and their Concentration

1) Molarity : molar concentration : M

M = (no. moles of solute) / [volumes (liter) of solution] = (mmoles) / (mL) A mole is defined as the number of atoms of 12C in exactly 12 g of 12C (12 amu or daltons). 1023.
A mole of a chemical species is 6.02214381023 atoms, molecules, ions, electrons, ion pairs, or subatomic particles. 10

2) Formality ; formal or analytical concentration

F = M

Strong electrolyte : completely dissociated

Ex. NaCl mol wt 58.44 g/L = 1 M

2.7g/100 mL sea water = x M

x = 0.46 M = 0.46F

Weak electrolyte : partially dissociated

Ex. HAC 0.1 F (1.3% dissociated)

3) Molality m = (mol solute) / (kg solvent)

11 Analytical molarity is the total number of moles of a solute, regardless of its chemical state, in 1 L of solution. The analytical molarity describes how a solution of a given molarity can be prepared. Equilibrium molarity is the molar concentration of a particular species in a solution.

Ex. 1.0 M H2SO4

[H2SO4] = 0.00 M [H+] = 1.01 M [HSO4] = 0.99 M [SO42 ] = 0.01 M Analytical molarity of H2SO4 is given by c H2SO4 = [HSO4] + [SO42 ] 12 Ex. Calculate the analytical and equilbrium molar concentrations of the solute species in an aqueous solution that contains 285 mg of trichloroacetic acid, Cl3CCOOH (163.4 g/mol), in 10.0 mL (the trichloroacetic acid is 73% ionized in water).

Cl3CCOOH = HA 163.4 g = 1 mol

285 mg = x mol x = 1.744 10-3 mol

Analytical concentration :

163.4 g / 1000 mL = 1 M

285 mg / 10.0 mL = y M y = 0.174 M

Equilibrium concentration:

HA H+ + A

initial 1 0 0

Final 10.73 0.73 0.73

[HA] = y M (1 0.73) = 0.174 0.27 = 0.047 M [H+] = [A] = y M 0.73 = 0.127 M 13

Percent concentration (%) Solution

= solute + solvent Parts per million (ppm) and parts per billion (ppb)

Solution-diluent volume ratios

p-function The density of a substance is its mass per unit volume, and its specific Density has units of kilograms per liter or grams per milliliter in the metric system. Specific gravity is dimensionless and so is not tied to any particular system of units. 25

[PDF] [PDF] CHAPTER 4 Calculations Used in Analytical Chemistry

CHAPTER 4

Calculations Used in Analytical Chemistry