Question

What is the mass percent of oxygen in $A{l_2}{(S{O_4})_3}.18{H_2}O$?
[The molar mass of the substance is 666.43g/mol]
A.9.60
B.128.8
C.143.2
D.72

Answer 1

Mass percentage is the method to represent the concentration of an element in a compound or a component in a mixture. The mass percentage can be calculated by dividing the mass of the component by the total mass of the solution.
Mass percentage = $\dfrac{\text{mass component}}{\ext{mass of mixture}}$

Complete step by step answer:
Aluminium sulphate is a chemical compound with the formulation $A{l_2}{(S{O_4})_3}$. It is water soluble and is basically used as a coagulating agent (selling particle collision by neutralizing fee) which is helped in purification of drinking water and wastewater remedy flora, and also in paper manufacturing.
The anhydrous form takes place evidently as a rare mineral millosevichite, located e.g. In volcanic environments and on burning coal-mining waste dumps. Aluminium sulphate is not often, if ever, encountered as the anhydrous salt. It forms some of special hydrates, of which the hex decahydrate $A{l_2}{(S{O_4})_3}.16{H_2}O$ and octa decahydrate $A{l_2}{(S{O_4})_3}.18{H_2}O$ are the most commonplace. The hepta decahydrate, whose components can be written as, $A{l_2}{({H_2}O)_6}2{(S{O_4})_3}.5{H_2}O$ occurs apparently as the mineral alunogen.
There are 30 oxygen atoms present in 1 mol of $A{l_2}{(S{O_4})_3}.18{H_2}O$
Mass of oxygen in the compound = 30 x 16
Mass % of oxygen = $\dfrac{{480}}{{666.43 \times 100}} = 72\%$
Hence option (4) is correct.

Additional Information:
Aluminium sulphate is once in a while known as alum or papermaker's alum in certain industries. However, the name "alum" is more usually and nicely used for any double sulphate salt with the popular system $XAl{(S{O_4})_2}.12{H_2}O$, wherein X is a monovalent cation which include potassium or ammonium.

Note:
Aluminium sulphate is used in water purification and as a mordant in dyeing and printing textiles. In water purification, it causes suspended impurities to coagulate into larger particles and then settle to the lowest of the box (or be filtered out) greater easily. This procedure is referred to as coagulation or flocculation.