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Q.15 g of a compound `Na_2SO_4.xH_2O`contains `2.36 * 10^23`molecules of water.Find...

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user8235304 | Student, Grade 11 | (Level 1) Valedictorian

Posted August 5, 2013 at 5:12 PM via web

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Q.15 g of a compound `Na_2SO_4.xH_2O`contains `2.36 * 10^23`molecules of water.Find the value of x.

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jeew-m | College Teacher | (Level 1) Educator Emeritus

Posted August 6, 2013 at 1:26 AM (Answer #1)

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According to the law of Avogadro 1mol of water contains `6.022xx10^23` molecules.

Molar mass of water `= 18g/(mol)`

Amount of water molecules present `= 2.36xx10^23`

Amount of water moles present `= (2.36xx10^23)/(6.022xx10^23)`

Amount of water moles present `= 0.392mol`

Mass of water moles `= 18xx0.392 = 7.054g`

Mass of `Na_2SO_4 = 15-7.054 = 7.946g`

Molar mass of `Na_2SO_4 = 142.043g/(mol)`

Amount of `Na_2SO_4 = 7.946/142.043 = 0.056mol`

Mole ratio

`Na_2SO_4:H_2O = 0.056:0.392 = 1:0.392/0.056= 1:7`

`Na_2SO_4:H_2O = 1:x = 1:7`

So the value of x = 7

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