What is the value of x in this chem problem?

Chemistry
Tutor: None Selected Time limit: 1 Day

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?

Jul 23rd, 2015

Thank you for the opportunity to help you with your question!

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.
Jul 23rd, 2015

Thank you so much for your thorough and quick answer :) Just one question... Is 10 the answer in the correct number of sig figs?

Jul 24th, 2015

Shouldnt it be in 3 sig figs?

Jul 24th, 2015

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.

If you have any more questions on this problem or in general feel free to contact me.   

Jul 24th, 2015

Thank you so much! You are one of the best tutors ever!

Jul 24th, 2015

An absolute pleasure.  Thank you for the sweet compliment ! :)

Jul 24th, 2015

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