1. what is the percentage of water in MgSo4×7H20? 2. A hydrate of colbalt (II) c

Question

1. what is the percentage of water in MgSo4×7H20?

2. A hydrate of colbalt (II) chloride, CoCl2, is weigbrd out and dehydrated to determine the formula of the hydrate. Use the following data to calculate the formula
Mass of crucible: 12.635 g
Mass of crucible plus hydrate: 14.35 g
mass od crucible plus anhydrous salt after heating 14.044 g
determine:
a. mass of anhydrous salt
b. mass of water
c. moles of anhydrous salt
d. moles of water
e. ratio of moles of water to moles of anhydrous salt chemical formula of the hydrated salt.

Explanation / Answer

atomic weights : Mg=24, S=32, O=16, H=1

basis : 1 mole of MgSO4. 7H2O

Moles= mass/molar mass

when 1 mole of MgSO4. 7H2O is considered, mass= molar mass

molar mass= atomic weight of Mg+atomic weight of S+ 4* atomic weight of O + 7*(2* atomic weight of H+ atomic weight of O)= 24+32+64+7*(2+16)= 120+126= 246

mass of water= 126 , mass % of water= 100* mass of water/ total mass

Mass % of water= 100*126/246 = 51.22%

2. Molar mass of CoCl2= 99 gm/mole

from the data given, mass of hydrate (CoCl2+water)= mass of crucible + hydrate before heating - mass of crucible= 14.35-12.635=1.715 gm

during heating water is removed and hence

Mass of water removed = mass of crucible +sample before heating-(mass of crucible +sample before heating) = 14.35-14.044=0.306 gm

mass of anhydrous salt= mass of hydrated salt-mass of water removed= 1.715-0.306= 1.409 gm

moles of anhydrous salt= mass/molar mass= 1.409/99=0.0142 moles of water= 0.306/18= 0.017

moles ratio of CoCl2: H2O= 0.0142:0.017 = 1:1.1

1 mole of CoCl2= 99 gm of CoCl2

1.1moles of H2O=99*1.1=108

moles of water= 108/18=6

So formula of hydrate= CoCl2.6H2O

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