Calculating the Formula of a Hydrated Compound

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In summary, the conversation discusses the process of calculating the value of x in the equation Na2CO3 + 2HCl --> 2NaCl + CO2 + H2O, given a sample of sodium carbonate and a titration with HCl. The conversation includes a discussion of molar ratios and molar mass calculations.
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Homework Statement



Sodium carbonate exists in hydrated form, Na2CO3.xH2O, in the solid state. 3.5 g of a sodium carbonate sample was dissolved in water and the volume made up to 250 cm3. 25.0 cm3 of this solution was titrated against 0.1 moldm-3 HCl and 24.5 cm3 of the acid were required. Calculate the value of x given the equation:

Na2CO3 + 2HCl --> 2NaCl + CO2 + H2O

Homework Equations



NO. of Moles = Conc. x volume/1000

The Attempt at a Solution


I have attempted this question in this way,
Moles of HCI = 0.1 x 24.5 / 1000 = 0.00245
Then looking at the molar ratio, I see 1:2 ratio of Sodium carbonate and HCI.
So, Moles of Na2CO3, is 0.00245/2 = 0.001225 moles in 25 cm3
Then I did the NO. of moles for Na2CO3 in 250cm3 so 250cm3/25cm3 = 10 which is the multiplier, then i did 10 x 0.001225 = 0.01225 mol
I did the Molar Mass of Na2CO3 which is 3.5/0.01225 = 285.7142.. which rounds to 286.
I assumed the Mr of Na2CO3.xH2O is 286, as the mark scheme assumed that
I don't know what to do after this, can someone please help me, as this is Homework!
Thank you!
 
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  • #2
Many ways to skin that cat, but if you got to the molar mass... What is the molar mass of anhydrous Na2CO3? Why is your number larger? By how much? What does the difference man?
 
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  • #3
Borek said:
Many ways to skin that cat, but if you got to the molar mass... What is the molar mass of anhydrous Na2CO3? Why is your number larger? By how much? What does the difference man?
The molar mass of Na2CO3 is 106..
Is my method right, or have I done it differently/wrong?
 
  • #4
Try to answer my other questions. You did everything OK so far.
 

1. How do you calculate the formula of a hydrated compound?

To calculate the formula of a hydrated compound, you need to know the mass of the anhydrous compound and the mass of the water in the compound. You can then use the following formula: Molar mass of anhydrous compound / Molar mass of water = Ratio of anhydrous compound to water. This ratio will give you the subscripts for the formula of the hydrated compound.

2. Can you give an example of calculating the formula of a hydrated compound?

For example, let's say we have a sample of copper (II) sulfate pentahydrate (CuSO4 * 5H2O). We weigh out 2.5 grams of the compound and determine that 0.9 grams of that is water. Using the formula, we get: Molar mass of CuSO4 = 159.609 g/molMolar mass of H2O = 18.015 g/molRatio of CuSO4 to H2O = 159.609 / 18.015 = 8.86Therefore, the formula for the hydrated compound is CuSO4 * 8.86H2O.

3. How do you determine the mass of water in a hydrated compound?

To determine the mass of water in a hydrated compound, you can use a process called dehydration. This involves heating the compound until all the water has evaporated, leaving behind the anhydrous compound. The difference in mass between the original compound and the anhydrous compound will give you the mass of water present.

4. What is the significance of calculating the formula of a hydrated compound?

Calculating the formula of a hydrated compound is important because it allows us to determine the exact composition of the compound and how many water molecules are attached to each molecule of the anhydrous compound. This information is crucial for understanding the physical and chemical properties of the compound.

5. Are there any limitations to calculating the formula of a hydrated compound?

Yes, there are some limitations to calculating the formula of a hydrated compound. This method assumes that all the water has been removed from the compound during the dehydration process. However, some compounds may undergo decomposition or other chemical reactions during heating, which can affect the accuracy of the results. Additionally, this method may not be suitable for compounds that have a high degree of hydration or that are unstable at high temperatures.

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