# Calculating the Formula of a Hydrated Compound

## 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!

Borek
Mentor
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?

• Bystander
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?

Borek
Mentor
Try to answer my other questions. You did everything OK so far.