A hydrate of Na2CO3 has a mass of 4.31 g before heating. After heating, the mass of the anhydrous compound is found to be 3.22 g. Determine the formula of the hydrate and then write out the name of the hydrate.

Question

anonymous · Answer

I found the mass of the water, it's 1.09 g, but I am lost from there on.

aaronq · Answer

find how many moles there are of the anhydrous compound and moles of water, separately. Compare them

anonymous · Answer

So I would start with 22.989g/3.22g, and then multiple the that answer by 2, to get the moles of the Na2? Or would I multiple 22.989 by 2 and then divide?

anonymous · Answer

Or does the 2 not matter?

anonymous · Answer

@aaronq

aaronq · Answer

i meant find the moles of $Na_2CO_3$ and the moles of $H_2O$, not of the individual atoms

anonymous · Answer

I'm sorry, moles have always confused me a bit. How would I start with finding the moles of Na2Co3?

aaronq · Answer

you need the molar mass (which is the sum of the individual molar masses of the atoms in their proportions, so like 2 times the molar mass of Na + molar mass of C + 3 times molar mass of O).

use this formula:  $\sf moles=\dfrac{mass}{Molar~mass}$

anonymous · Answer

Okay so molar mass of$$Na _{2}CO _{3}$$ is 105.985 g?

aaronq · Answer

yes

anonymous · Answer

and then of H2O it would be 18.0148

anonymous · Answer

Thank you for being patient with me aha

aaronq · Answer

yes, you can just use 18 g/mol  

and no problem!  haha

anonymous · Answer

So from here, where do I go ?

anonymous · Answer

Divide the mass of original compound by the mass?

aaronq · Answer

divide the mass of the anhydrous compound by the molar mass of Na2CO3 

divide the mass of the water by it's molar mass

anonymous · Answer

3.22/ 106g = .030 g
1.06/18g= .060 g

anonymous · Answer

Divide by the smallest number in the ratio to get a whole number ratio , right?

aaronq · Answer

good stuff! 

remember that this is moles not grams, 

$\sf \dfrac{3.22~g}{ 106~g/mol}$ = .030 mol

$\sf \dfrac{1.06~g}{18 ~g/mol}$= .060 mol

aaronq · Answer

yeah

anonymous · Answer

Oh yeah right , thanks! 
so from here, it's .030 mol/ .030 mol = 1 mol
                            .060 mol/.030 mol = 2 mol

anonymous · Answer

Na2CO3 * 2H2O
or Sodium carbonate dioxide?

anonymous · Answer

*dihydrate

aaronq · Answer

haha yes i was gonna say thats wrong but you corrected yourself. good job

anonymous · Answer

Awesome! This problem killed me but we finally finished it. Thanks so much for your help!!!

aaronq · Answer

no problem !