% reaction yield and theoretical yield calculations
and why you can't actually get 100% yield in practice
14.2b atom economy calculations *
14.3 dilution of solutions calculations
water of crystallisation
much of a reactant is needed? calculation of quantities required, limiting
Pharmaceutical Industry Economics & Sustainability, Life Cycle
14.1 Percentage purity of a chemical reaction product
% Purity is very
e.g. for analytical standards in laboratories or
pharmaceutical products where impurities could have dangerous side
effects in a drug or medicine.
However in any chemical process it is
almost impossible to get 100.00% purity and so samples are always
analysed in industry to monitor the quality of the product.
The more a product is
processed e.g. by distillation or crystallisation, the more costly the
process, but the purer the product gets.
Somewhere there has to be a
compromise, so it is important that before sale, the product is assayed
or analysed as to its percentage purity.
It would not be acceptable
e.g. in the pharmaceutical industry to manufacture a drug for treating
us, with impurities in it, that may have harmful effects.
Similarly in fuels for road
vehicles, which themselves have additives in to enhance engine
performance, you wouldn't want other impurities that may cause engine
You can apply the same sort
of argument to thousands of domestic and industrial products from the
chemical and pharmaceutical industries.
An assay is any procedure used to
analyse and test for its purity of the % content of a specified
component in a mixture of a % of an element or ion etc.
% purity is the
percentage of the material which is the actually desired chemical in a sample
MASS of USEFUL PRODUCT
= 100 x
|| in TOTAL MASS of
Example 14.1 (Q1) Purity calculation
A 12.00g sample of a
crystallised pharmaceutical product was found to contain 11.57g of the
Calculate the % purity of the sample of the drug.
% purity = actual
amount of desired material x 100 / total amount of material
% purity =
11.57 x 100 / 12 =
Example 14.1 (Q2) Purity calculation
Sodium chloride was
prepared by neutralising sodium hydroxide solution with dilute
hydrochloric acid. The solution was gently heated to evaporate most of
the water and allow the salt to crystallise. The crystals were separated
from any remaining solution and dried on a filter paper. However, the
crystals are not necessarily completely dry.
The salt maybe required to be completely anhydrous, that is, not containing
The prepared salt
was analysed for water by heating a sample in an oven at 110oC
to measure the evaporation of any residual water.
The following results
were obtained and from them calculate the % purity of the salt.
Mass of evaporating
dish empty = 51.32g.
Mass of impure salt
+ dish = 56.47g
Mass of dish + salt
after heating = 56.15g
Therefore the mass
of original salt = 56.47 - 51.32 = 5.15g
and the mass of pure
salt remaining = 56.15 - 51.32 = 4.83g
% salt purity
= 4.83 x 100 / 5.15 =
(3 sf, 1dp)
Example 14.1 (Q3) Purity calculation
- an assay calculation is sketched out below for A Level students + link to
Titrations can be used to
analyse the purity of a substance e.g. here an acid (aspirin) is
titrated with standard sodium hydroxide solution of concentration 0.1000
The aspirin is dissolved in
ethanol solvent, diluted with deionised water and titrated with standardised 0.100 mol/dm3
sodium hydroxide solution using phenolphthalein indicator, the end-point
is the first permanent pink colour.
An assay calculation is 'sketched
See also ...
TOP OF PAGE
OTHER CALCULATION PAGES
What is relative atomic mass?,
relative isotopic mass and calculating relative atomic mass
formula/molecular mass of a compound or element molecule
Law of Conservation of Mass and simple reacting mass calculations
Composition by percentage mass of elements
in a compound
Empirical formula and formula mass of a compound from reacting masses
(easy start, not using moles)
Reacting mass ratio calculations of reactants and products
moles) and brief mention of actual percent % yield and theoretical yield,
and formula mass determination
Introducing moles: The connection between moles, mass and formula mass - the basis of reacting mole ratio calculations
(relating reacting masses and formula
moles to calculate empirical formula and deduce molecular formula of a compound/molecule
(starting with reacting masses or % composition)
Moles and the molar volume of a gas, Avogadro's Law
Reacting gas volume
ratios, Avogadro's Law
and Gay-Lussac's Law (ratio of gaseous
Molarity, volumes and solution
concentrations (and diagrams of apparatus)
How to do acid-alkali
titration calculations, diagrams of apparatus, details of procedures
Electrolysis products calculations (negative cathode and positive anode products)
e.g. % purity, % percentage & theoretical yield, dilution of solutions
(and diagrams of apparatus), water of crystallisation, quantity of reactants
required, atom economy
Energy transfers in physical/chemical changes,
Gas calculations involving PVT relationships,
Boyle's and Charles Laws
Radioactivity & half-life calculations including
keywords and phrases:
revision study notes for AQA Edexcel OCR IGCSE/GCSE
chemistry science topics modules on how to do percentage purity of the
product of a chemical reaction, the definition percent percentage purity,
what is the importance of knowing the purity of a substance product of a
chemical reaction, how do you do percent purity calculations questions
exercises practice chemical calculations for IB chemistry AQA Edexcel OCR
Salters advanced A level chemistry question exercise on calculating % purity
of a chemical product
Website content © Dr
Phil Brown 2000+.
All copyrights reserved on revision notes, images,
quizzes, worksheets etc.
Copying of website material is NOT
permitted. Exam revision summaries & references to science course specifications
Online quizzes to test yourself