Molecular Weights, Formulae and Use of Chemical Equations, Oxides;
Percentage Composition and Formula; Calculation of a Recipe from a Simple Glaze Formula; Given the Recipe of a Glaze Calculate the Formula
CHEMICAL equations are often used in ceramic calculations.
The great importance of chemical equations lies not so much in their being a convenient symbol to express the nature of the reagents and products of a chemical change, but in the fact that all chemical calculations are based on the use of formulae and equations in conjunction with atomic weights.
It is important to understand the use of these fundamental calculations as applied to ceramic materials. Once the formula of a compound is known, then it is a simple matter to calculate the molecular weight.
To calculate the molecular weight of soda ash, Na2CO3. Using the atomic weights of each atom,
two atoms of sodium = 2× 23 = 46
one atom of carbon = 1×12 = 12
three atoms of oxygen = 3×16 = 48
The molecular weight = 106
To calculate the molecular weight of white lead, Pb( O H ) 2 . 2PbCO 3,
3 atoms of Pb = 3× 207 = 621
8 atoms of O = 8×16 = 128
2 atoms of H = 2×1 = 2
2 atoms of C = 2×12 = 24
The molecular weight of white lead = 775
Use of Chemical Equations
Chemical equations describe not only the nature of the atoms and molecules of substances taking part in a chemical change, but also the quantitative relationship between them.
For example, consider the equation
CaCO3 = CaO+CO2
The weight of one molecule of calcium carbonate expressed in grams = 40+12+(3×16) = 100. The gram-molecular weight of Ca0 = 56 and the gram-molecular weight of CO2 = 44.
This means that 100 gm of calcium carbonate on heating will yield 56 gm of lime and 44 gm of carbon dioxide.
The equation also indicates the volume of gas liberated. This is based on the fact that the molecular weight of any gas, expressed in grams occupies 22·4 litres at N.T.P. (Normal Temperature and Pressure), -i.e. 0°C and 760 mm pressure (reference Avogadro’s Hypothesis). In the above equation 100 gm of calcium carbonate on heating would liberate 22>4.litres of carbon dioxide at N.T.P.
These principles are used in the following examples: if one ton of china clay is calcined to 1,100°C, what is the loss in weight in Ib?
The equation for this reactión may be written
Al203.2SiO2.2H2O = Al2O3+2SiO2+2 H 20
The loss in weight is due to the vaporization of the chemically combined water.
258 parts by weight of china clay suffers a loss in weight of 36 parts by weight of chemically combined water ;
258 tons of china clay loses 36 tons of water ;
1 ton of china clay loses 36/258 tons of water or 36/258 ×2,240 lb = 312·6 lb
Note. A list of important ceramic materials, together with their formulae and molecular weights is given in the Appendix.
A frit mixture contains 100 lb of calcium carbonate. Calculate the volume of carbon dioxide (cu ft ) which will be liberated on fritting at a temperature of 1,000°C. (28·32 litres = I cu ft ; 453·6 gm = I lb.)
44 gm CO2 = 22·4 litres at N.T.P.
44 lb CO2= (22.4X453.6 cu ft) : 28.32
Applying Charles’ Law:
( (22·4×453·6/28.32) X (1,000+273/ 273)) = 1,673 cuft CO2 at 1,000°C
This article was taken from the book “Calculations in ceramics” by R. Griffiths and C. Radford