Chemistry help: Hydrants and percentages. Why is my answer incorrect ?

Problem: A mixture of BaCl2 x 2H2O (hydrant) and NaCl with a mass of 1.6992 grams is placed in a vial weighing 3.3531.  (Ignore vial weight).  The mixture is heated at 105 degrees Celsius (also irrelevant) to drive off the water.  The residue of the mixture now has a mass of 1.4804 grams.  Use this info to calculate the percentage of NaCl in the mixture. 

HERE'S HOW I DID IT (WRONG): Well, there are 1.4804 grams of BaCl2 and NaCl.  So, I multiplied that value by 58.44 grams NaCl over the molar mass of BaCl2 and NaCl (which is 266.67 g/mol) in order to find the amount of NaCl in that sample.  I got 0.3244 grams.  Then I did: 0.3244 grams NaCl divided by the total mass of mixture which was given (1.6992 g BaCl2 x 2H2O (hydrant) and NaCl).  I got 19.09 % NaCl, which is incorrect.

CORRECT ANSWER: They did 1.6992 minus 1.4804 to find the mass of the 2 water molecules.  Got 0.2188 g water.  Then,  used 0.2188 g H2O to find the grams of BaCl2 x 2H2O.  Basically just multiplied 0.2188 by molar mass of water, then by 1 mol hydrant divided by 2 mol water, and then by molar mass of the hydrant (1 mol BaCl2 x 2 H2O = 244.26 g/mo) to find the grams of hydrant in relation to that amount of water.  The answer is 1.483 grams of hydrant.  Then, they did 1.483 grams of hydrant divided by 1.6992 g BaCl2 x 2H2O (hydrant) and NaCl, and got 87.27%.  Then that percent was subtracted by 100% to find percentage of NaCl: 12.72%.  

Why am I not getting same answer? Both methods make sense!

Update:

I realize I am wrong, and I don't care.  What I do care about is understanding.  It bothers me when I cannot get why something doesn't work out.  :)

Update 2:

Thanks so much, but why wouldn't the first way I listed work as well?  I know it's wrong, I just want to understand WHY :)

2 Answers

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  • 1 month ago
    Favourite answer

    So really, neither of your two methods makes sense. You have to think about chemical composition, masses and moles, and the kinds of steps you took mix masses and moles in ways that don't work.

    So here is how you should approach this:

    1. The loss of mass of the sample after heating tells you how much H2O was present in the sample. 

    2. Converting that mass to moles H2O and dividing by 2 (because the formula of the hydrate is BaCl2.2H2O) gives you the number of moles of BaCl2.2H2O in the sample. 

    3. Multiplying moles of BaCl2.2H2O by its molar mass gives you the mass of BaCl2.2H2O in the sample.

    4. Subtracting that mass from the mass of the sample gives you mass NaCl in the sample.

    5. Dividing (mass NaCl / mass sample) X 100 give you % NaCl in the sample.

    (Also, the word is "hydrate" not "hydrant".)

  • Anonymous
    1 month ago

    Maybe you think so, but your method makes no sense at all.

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