Chapter 4
Calculations in Analytical Chemistry
SI UNITS
In science generally use SI units are used but in Analytical
Chemistry, due to the very wide range of quantities (from
very small to very large) we sue derived SI orr sometimes
non SI units.
For example visible radiation is given in terms of nm; eg
the wavelenght of the yellow light of sodium flame is 590
nm
Volumes can vary from micro liters to liters.
Mass can be anywhere from micrograms to kilograms.
SI and other Units,
Amount
Concentration
Density
Stoichiometry
You must learn the units, the prefixes and unit conversions
using conversion factors!
The Mole (mol)
This is the amount of chemical matter. I mol contains
Avagadro’s number of particles.
Millimol and micromol are derived units of mol and are
10-3 and 10-6 mol respectively.
Relationship between mass (m), molar mass (MM) and
amount in mols (n) is given by the following eqn:
Amount in mols =
n=
Example
How many grams of C is present in a 72.8 g sample of
Iron(III) carbonate.
Solution Concentrations - MOLARITY
Molar concentration cX of a solution of a chemical species X
is the number of moles of that species contained in 1 liter of
the solution (not one liter of the solvent).
The unit of molar concentration is molarity, M. It has the
dimensions of mol/L or mol·L-1
cX =
.
Example
Calculate the molar concentration of ethanol in an aqueous
solution that contains 5.80g ethanol in 2.50 L solution
At times it is necessary to distinguish between two types of
concentrations.
Analytical Molarity is the total number of moles of a solute,
irrespective of its chemical state, in 1 L of solution. It
describes how a solution of a given molarity can be prepared.
Equilibrium Molarity is the molar concentration of a particular
species in a solution. This can either be calculated if the
relevant equilibrium constants are known or measured
experimentally.
Example
Calculate the analytical and equilibrium molar concentrations
of the solute species in an aqueous solution that contains 285
mg of trichloroacetic acid in 10.0 mL(the acid is 73% ionized
in water).
Other concentration units
Other concentration units
Try and avoid the use of these units. Only w/w is temperature
independent
Eg Nitric acid is sold as a 70% w/w solution. What mass
should you take to prepare a 1 L 2 M HNO3(aq) solution
Other concentration units
Other concentration units
p-functions
Frequently in chemistry the p-function or p-value is used to
express the concentration of certain species. This is the
negative logarithm (to the base 10) of the molar concentration
of that species. Thus for species X:
pX=-log10[X]
Useful for expressing concentrations that are small and vary
over ten or more orders of magnitude.
APPROXIMATION
For dilute aqueous solutions whose densities are approximately
1.00 g/mL.
1ppm= 1.00 mg/L
1ppb= 1.00 µg/L
Example
Calculate the molar concentration of Ag+ in a solution that has
a pAg+ of 6.372.
DENSITY AND SPECIFIC GRAVITY OF SOLUTIONS
Example
STOICHIOMETRY
Quantitative relationship between reacting chemicals.
Stoichiometric calculations
Empirical and Molecular Formula
1. Write a balanced chemical equation
1. A 25.00-g sample of an orange compound contains 6.64 g
of potassium, 8.84 g of chromium, and 9.52 g of oxygen.
Find the empirical formula.
2. From the elemental analysis of 5.00-g acetic acid, we get
7.33 g CO2 and 3.00 g H2O. The compound is known to
contain C, H and O only. Find the empirical formula.
3. Analysis by a mass spectrometer gives the molar mass of
acetic acid as 60 g/mol. Determine its molecular formula.
2. Establish stoichiometric factors between the reactant and
product species
3. Use these factors to calculate unknown quantities from
known quantities.
Example