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Empirical formula word problem?

Washing soda, a compound used to prepare hard water for washing laundry, is a hydrate, which means that a certain number of water molecules are included in the solid structure. Its formula can be written as Na2CO3·x H2O, where x is the number of moles of H2O per mole of Na2CO3. When a 2.480 g sample of washing soda is heated at 125°C, all the water of hydration is lost, leaving 0.919 g of Na2CO3. What is the value of x?

Using relative atomic masses you will need to find the molar mass of Na2CO3 which will be approx 23x2 + 12 + 16x3 = 106 g (use values for your course as required - I have used 23 for Na, 12 for C and 16 for O, and later I use 1 for H)

Find how many moles this is by dividing 0.919g by 106g (= w moles)

The difference in mass is the mass of water lost = 2.480g - 0.919 g = 1.561g H2O

Find how many moles of water this is 1.561/approx 18g (= y moles)

So you know that w moles of Na2CO3 combined with y moles of H2O
So                      1 mole of Na2CO3 combined with (y/w) x 1 moles of H2O

This will be the mole ratio of Na2CO3 : H2O in the formula

Na2CO3.xH2O ----> Na2CO3 + xH2O

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