Question

A 40.15 gram sample of a hydrate of Na2SO4 was heated thoroughly in a porcelain crucible, until its weight remained constant. After heating, 21.27 grams of the anhydrous compound remained. What is the formula of the hydrate?

Answer 1

The formula of the hydrate of the sodium sulfate is :
`Na_2SO_4.7H_2O`

Step-by-step explanation:

Mass of hydrated sodium sulfate = 40.15 gram

Mass of completely dehydrated sodium sulfate = 21.27 gram

Mass of water molecules present in hydrated sodium sulfate =x

40.15 gram = 21.27 gram + x (Law of conservation of mass)

x = 40.15 gram - 21.27 gram = 18.88 g

Moles of water =
`(18.88 g)/(18 g/mol)= 1.04mol`

Moles of sodium sulfate =
`(21.27 g)/(142.04 g/mol)=0.1497 mol`

Whole number ratio of sodium sulfate and water:

Sodium sulfate =
`(0.1497 mol)/(0.1497 mol)=1`

Water =
`(1.04 mol)/(0.1497 mol)=6.9 a\\rox 7`

The formula of hydrate be
`Na_2SO_4.7H_2O`

So, the formula of the hydrate of the sodium sulfate is :
`Na_2SO_4.7H_2O`