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

The difference of the sample mass and the heated sample mass is the amount of water which evaporated by heating process, then we can calculate:  

mass of H₂O = 30.7 -  22.9 = 7.8 g

then calculate the number of moles:  

number of moles of H₂O =  mass / molecular mass

number of moles of H₂O = 7.8 / 18 = 0.43 moles

So the hydration number is 0.43.

Sr(NO₃)₂ • 0.43 H₂O

Answer 2

The hydration number "n" in Sr(NO3)2•nH2O is 4.

Further Explanation:

Hydrates

• Hydrates are compounds that contain water of crystallization with a definite mass.
• These compounds are in the form of crystals and can be heated to remove water and form an anhydrous form of the compound.
• Unknown hydrates with an "n" before H2O. For example;  Sr(NO3)2•nH2O.
• "n" represents the hydration number or the moles of water of crystalization in hydrates and can be determined using some steps.

Steps in determining hydration number in hydrates;

1. Determining the mass of water produced when hydrates are heated
2. Converting the mass of water to moles
3. Convert the mass of anhydrate that is left over to moles.
4. Find the mole ratio of anhydrate to water
5. Use the mole ratio to write the formula

Therefore:

We can determine the hydration in  Sr(NO3)2•nH2O

Step 1: Mass of water

Mass of water = Mass of hydrate - mass of anhydrate

                       = 30.7 g - 22.9 g

                       = 7.8 g

Step 2: Moles of water

Number of moles = mass/ molar mass

                            = 7.8 g / 18 g/mole

                             = 0.433 moles

Step 3; Moles of anhydrate

Mass of anhydrate = 22.9 g

Number of moles = mass/molar mass

Molar mass of anhydrate  Sr(NO3)2 is 211.63 g/mol

Thus;

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

                                                  = 0.108 moles

Step 4: Mole ratio of anhydrate to water

Moles of water = 0.433 moles

Moles of anhydrate = 0.108 moles

Mole ratio = 0.108 : 0.433

                = 1 : 4

Therefore; The value of n is 4

Hence the formula of the hydrate is  Sr(NO3)2• 4H2O

Keywords: Hydrates, water of crystallization, steps in finding the formula of hydrates.

Learn more about:

• Hydrates : https://brainly.com/question/10665473
• Formula of hydrates example: https://brainly.com/question/10665473
• Empirical formula: https://brainly.com/question/12061700
• Example on empirical formula: https://brainly.com/question/12061700
• Molecular formula: https://brainly.com/question/12061700

Level : High school

Subject: Chemistry

Topic: Moles

Sub-topic: Formula of hydrates