Question

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.

Answer 1

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 2

Answer: The hydration number is 4

Explanation:

We are given a chemical compound having formula $Sr(NO_3)_2.nH_2O$

Mass of $Sr(NO_3)_2.nH_2O$ = 30.7 g

Mass of $Sr(NO_3)_2$ = 22.9 g

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

Further Explanation:

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

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

Mass of water in $Sr(NO_3)_2.nH_2O=18\times n$

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

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

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

But, mass of water released = 7.8 grams

Equating the mass of water released:

$\frac{(18\times n)}{(211.64+(18\times n))}\times 30.7=7.8\\\\412.2\times n=1650.714\\\\n=\frac{1650.714}{412.2}=4$

The hydration number is 4

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Keywords: Hydration number, Water of crystallization