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A 30.7 g sample of Strontium nitrate, Sr(NO3)2•nH2O, is heated to a constant mass of 22.9 g. Calculate the hydration number.

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Edufirst

Answer:

  • Hydration number: 4

Explanation:

1) Mass of water in the hydrated compound

Mass of water = Mass of the hydrated sample - mass of the dehydrated compound

Mass of water = 30.7 g - 22.9 g = 7.8 g

2) Number of moles of water

  • Number of moles = mass in grams / molar mass

  • molar mass of H₂O = 2×1.008 g/mol + 15.999 g*mol = 18.015 g/mol

  • Number of moles of H₂O = 7.9 g / 18.015 g/mol = 0.439 mol

3) Number of moles of Strontium nitrate dehydrated, Sr (NO₃)₂

  • The mass of strontium nitrate dehydrated is the constant mass obtained after heating = 22.9 g

  • Molar mass of Sr (NO₃)₂ :  211.63 g/mol (you can obtain it from a internet or calculate using the atomic masses of each element from a periodic table).

  • Number of moles of Sr (NO₃)₂ = 22.9 g / 211.63 g/mol =  0.108 mol

4) Ratio

  • 0.439 mol H₂O / 0.108 mol Sr(NO₃)₂ ≈  4 mol H₂O : 1 mol Sr (NO₃)₂

Which means that the hydration number is 4.

Answer: The hydration number is 4

Explanation:

We are given a chemical compound having formula [tex]Sr(NO_3)_2.nH_2O[/tex]

Mass of [tex]Sr(NO_3)_2.nH_2O[/tex] = 30.7 g

Mass of [tex]Sr(NO_3)_2[/tex] = 22.9 g

Mass of water released = (30.7 – 22.9) = 7.8 g

Further Explanation:

Molar mass of strontium nitrate = [tex][87.62+2(14.01+(3\times 16))]=211.64g/mol[/tex]

Molar mass of water = [tex][(2\times 1)+16]=18g/mol[/tex]

Mass of water in [tex]Sr(NO_3)_2.nH_2O=18\times n[/tex]

To calculate the mass of water released by heating strontium nitrate, we apply unitary method:

When [tex](211.64 + (18\times n))[/tex] grams of [tex]Sr(NO_3)_2.nH_2O[/tex] is heated, [tex]18\times n[/tex] grams of water is released

So, when 30.7 grams of [tex]Sr(NO_3)_2.nH_2O[/tex] is heated, [tex]\frac{(18\times n)}{(211.64+(18\times n))}\times 30.7[/tex] grams of water will be released

But, mass of water released = 7.8 grams

Equating the mass of water released:

[tex]\frac{(18\times n)}{(211.64+(18\times n))}\times 30.7=7.8\\\\412.2\times n=1650.714\\\\n=\frac{1650.714}{412.2}=4[/tex]

The hydration number is 4

Learn More

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Keywords: Hydration number, Water of crystallization

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