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 2.396-g sample of washing soda is heated at 125°C, all the water of hydration is lost, leaving 0.887 g of Na2CO3. What is the value of x?
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The first thing we need to do is find out the molecular weight of Na2CO3. When we add it up from the periodic table, we arrive at 106 g/mol. To see how many mols are left over at the end, we can do a quick calculation of: .887g/ 106g/mol = .00837 mols of Na2CO3.
Too see how much weight the water contributed, all we do is subtract the amount of Na2CO3 from the total.
Weight of H20= 2.396g - .887g = 1.509 g of water
Next, we need to find out how many mols of water are in 1.509g. We do this by dividing it by its molecular weight: 1.509 g/ 18.02 g/mol = .0837 mols of water
lastly, in order to find the the coefficient that goes in front of H20, we divide by the smallest number:
.0837/.00837 = 10
therefore, x =10
Please let me know if you need any clarification. I'm always happy to answer your questions.
Not a problem. The numbers that go in front of compounds or elements in a reaction are written without decimals or as fractions. In this case, we solved for x as the coefficient before H20, so I would say to leave it as 10.
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