Engineering Chemistry (BAS102) - AKTU Question Paper 2025-26

Plaster of Paris (POP) is a white powder of calcium sulphate hemihydrate, with formula CaSO₄·½H₂O.

It is prepared by heating gypsum (CaSO₄·2H₂O) at about 373 K (100°C). During heating, gypsum loses water and forms POP.

CaSO_4\cdot2H_2O \xrightarrow{\text{heat}} CaSO_4\cdot\frac{1}{2}H_2O + \frac{3}{2}H_2O

So, Gypsum → heating → Plaster of Paris.

Using Beer-Lambert law,

A=\varepsilon c l

Given,

A=0.75,\qquad c=5\times10^{-4}\text{ mol L}^{-1},\qquad l=1\text{ cm}

Therefore,

\varepsilon=\frac{A}{cl}

\varepsilon=\frac{0.75}{(5\times10^{-4})(1)}

\varepsilon=1500\text{ L mol}^{-1}\text{ cm}^{-1}

Hence,

\boxed{\varepsilon=1.5\times10^{3}\text{ L mol}^{-1}\text{ cm}^{-1}}

Calgon is sodium hexametaphosphate,(NaPO3)6(NaPO3​)6​. It prevents scale formation by converting Ca²⁺ and Mg²⁺ ions into soluble complex compounds, so they cannot form hard deposits.

For calcium sulphate:

Na2[Na4(PO3)6]+2CaSO4→Na2[Ca2(PO3)6]+2Na2SO4Na_2[Na_4(PO_3)_6] + 2CaSO_4 \rightarrow Na_2[Ca_2(PO_3)_6] + 2Na_2SO_4Na2​[Na4​(PO3​)6​]+2CaSO4​→Na2​[Ca2​(PO3​)6​]+2Na2​SO4​

The formed calcium-Calgon complex is soluble in water, therefore scale formation is prevented.

Ca2+/Mg2++Calgon→Soluble complex\boxed{\text{Ca}^{2+}/\text{Mg}^{2+} + \text{Calgon} \rightarrow \text{Soluble complex}}Ca2+/Mg2++Calgon→Soluble complex​

G.C.V. (Gross Calorific Value) includes the heat released when the water vapour formed during combustion condenses.

N.C.V. (Net Calorific Value) does not include this latent heat.

Therefore,

      NCV=GCV−Latent heat of steam formedNCV=GCV−Latent heat of steam formed

So, GCV = NCV when no water vapour is formed during combustion, i.e. when the fuel contains no hydrogen and no moisture.

   GCV=NCV when no latent heat is lost in water vapour\boxed{GCV = NCV \text{ when no latent heat is lost in water vapour}}GCV=NCV when no latent heat is lost in water vapour​

Polymer blends are mixtures of two or more different polymers combined to obtain better properties.

Example: ABS + Polycarbonate (PC) blend is used to improve strength and impact resistance.