F. 4 Chemistry Mole Concept

1.
For anhydrous sodium carbonate:
Cost of pure Na2CO3 = $2/kg

For Na2CO3•H2O:
Molar mass of Na2CO3 = 23x2 + 12x1 + 16x3 = 106 g/mol
Molar mass of Na2CO3•H2O = 106 + 1x2 + 16 = 124 g/mol
Cost of pure Na2CO3 = $1 / (106/124) = $1.17/kg

Hence, B is the cheaper source.


2.
Mole ratio Al : S : O : H
= 8.11/27 : 14.41/32.1 : 72.07/16 : 5.41/1
= 0.300 : 0.449 : 4.50 : 5.41
= 1 : 1.5 : 15 : 18
= 2 : 3 : 30 : 36

The empirical formula of Al2S3O30H36.
Formula mass of the salt = 27x2 + 32.1x3 + 16x30 + 1x36 = 666.3

Molar mass of H2O = 1x2 + 16 = 18
Let n be the number of moles of water in each mole of the salt.
Mole ratio of Al2S3O30H36 : H2O = 20/666.3 : 9.73/18 = 1 : n
n(20/666.3) = 1(9.73/18)
n = 18

In the hydrated salt:
No. of H atoms in water of crystallization = 18 x 2 = 36
No. of O atoms in water of crystallization = 18
No. of H atoms in the part of anhydrous salt = 36 - 36 = 0
No. of O atoms in the part of anhydrous salt = 30 - 18 = 12

The formula of the crystalline salt = Al2S3O12•18H2O = Al2(SO4)3•18H2O

Wendy 的答案沒有錯，有兩個可能：
(1) 本書答案答。
(2) 題目是 10H2O 而不是 H2O。