% PURITY CALCULATIONS
and ASSAY CALCULATIONS
Doc
Brown's Chemistry  GCSE/IGCSE/GCE (basic A level)
O Level Online Chemical Calculations
14.
Other GCSE chemical calculations % PURITY OF A PRODUCT from a chemical
preparation reaction
Quantitative chemistry calculations Help for problem solving in doing % purity calculations.
To 'assay' means to analyse a sample for its purity. This page
includes of fully worked
examples of percentage purity calculations. Online practice exam
chemistry CALCULATIONS and solved problems for KS4 Science GCSE/IGCSE
CHEMISTRY and basic starter chemical calculations for A level AS/A2/IB
courses. These revision notes and practice questions on how to do
percent purity and assay chemical calculations and worked examples
should prove useful for the new AQA, Edexcel and OCR GCSE (9–1)
chemistry science courses.
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14.
Other GCSE chemical calculations
14.1
% purity of a product * 14.2a
% reaction yield * 14.2b
atom economy * 14.3
dilution of solutions
14.4
water of crystallisation
calculation * 14.5
how much reactant is needed? & limiting
reactant calculations
14.1 Percentage purity of a chemical reaction product

Purity is very
important 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
damage.

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
of it.

MASS of USEFUL PRODUCT 
PERCENT PURITY
= 100 x 
 

in TOTAL MASS of
SAMPLE 

Example 14.1 (Q1) Purity calculation

A 12.00g sample of a
crystallised pharmaceutical product was found to contain 11.57g of the
active drug.

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 = 96.4% (to 1dp)



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
any water.

The prepared salt
was analysed for water by heating a sample in an oven at 110^{o}C
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 = 93.8% (to 1dp)



Example 14.1 (Q3) Purity calculation
 an assay calculation is sketched out below for A Level students + link to
others.

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
mol dm^{3}.

The aspirin is dissolved in
ethanol solvent, diluted with deionised water and titrated with standardised 0.100 mol/dm^{3}
sodium hydroxide solution using phenolphthalein indicator, the endpoint
is the first permanent pink colour.

An assay calculation is 'sketched
out' below.

See also ...
Above is typical periodic table used in GCSE sciencechemistry specifications in
doing chemical calculations,
and I've 'usually' used these values in my exemplar calculations to cover most
syllabuses
OTHER CALCULATION PAGES

What is relative atomic mass?,
relative isotopic mass and calculating relative atomic mass

Calculating relative
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
from equations
(NOT using
moles) and brief mention of actual percent % yield and theoretical yield,
atom economy
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
mass)

Using
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 GayLussac's Law (ratio of gaseous
reactantsproducts)

Molarity, volumes and solution
concentrations (and diagrams of apparatus)

How to do acidalkali
titration calculations, diagrams of apparatus, details of procedures

Electrolysis products calculations (negative cathode and positive anode products)

Other calculations
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,
exothermic/endothermic reactions

Gas calculations involving PVT relationships,
Boyle's and Charles Laws

Radioactivity & halflife calculations including
dating materials
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