March 28, 2024, 05:43:34 PM
Forum Rules: Read This Before Posting


Topic: Can someone check these stoichiometry problems?  (Read 8498 times)

0 Members and 1 Guest are viewing this topic.

Offline bob12

  • New Member
  • **
  • Posts: 7
  • Mole Snacks: +0/-0
Can someone check these stoichiometry problems?
« on: September 21, 2007, 09:15:40 AM »
I have done the following problem, but I don't think they are correct:

1) Commercial bleach solution contains 5.25%(by mass) of NaClO in water. It has a density of 1.08g/mL. Calculate the molarity of this solution.  [hints: assume you have 1L of solution(mass 1080g); molar mass of NaClO is 74.4 g/mol.

I get 7.056 x 10^-4 but I think the number is suppose to be higher than this!


2) A mixture is made of 20.0g of sodium chloride and 30.0g of magnesium chloride.  Calculate the mole fraction of sodium chloride and of magnesium chloride in the mixture.

I get .521 for sodium chloride and .479 for magnesium chloride.  I'm pretty sure this is correct.

Can someone check these over for me?

Thanks.

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27633
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Can someone check these stoichiometry problems?
« Reply #1 on: September 21, 2007, 09:59:19 AM »
I get 7.056 x 10^-4 but I think the number is suppose to be higher than this!

http://www.chembuddy.com/?left=concentration&right=percentage-to-molarity

Check your units - mL vs L.

Quote
.521 for sodium chloride and .479 for magnesium chloride

Looks OK.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline bob12

  • New Member
  • **
  • Posts: 7
  • Mole Snacks: +0/-0
Re: Can someone check these stoichiometry problems?
« Reply #2 on: September 21, 2007, 10:30:08 AM »
Converting from mL to L is the first thing I did.  I'm not sure I understand why the density even plays a role in this problem.  Since you know the mass % and the mass in 1 L, then why not multiply and get 56.7g.  Divide that by 74.4 to get .76622 moles and that's over 1 L so you get the molarity = .77... Is that right or am I still missing something?  The website was a little bit confusing, but from what I understood it yielded a value of 8.231, which seems like i did something wrong.


Offline bob12

  • New Member
  • **
  • Posts: 7
  • Mole Snacks: +0/-0
Re: Can someone check these stoichiometry problems?
« Reply #3 on: September 21, 2007, 10:39:58 AM »
Sorry to double post, but I think the answer is 14.5... Is that correct?

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27633
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Can someone check these stoichiometry problems?
« Reply #4 on: September 21, 2007, 10:52:57 AM »
You know solution concentration as w/w percentage - so you know there is 52.5g per 1000g of solution - but what you DON'T know is what volume this solution occupy - and by definition of molar concentration you MUST know the volume. That's where the density comes into play.

ChemBuddy lecture puts emphasis on the universal approach to this type of problems, so that you will know how to convert between any concentration types, but it also gives you - at the very bottom of the page - simple formula that you can use in this particular case.

However, you are given enough information in your question so that you can find molarity just with simple reasoning. 1L of solution is 1080 g, 5.25% of that is NaClO. Calculate NaClO mass, convert that to moles and that's your answer (you have to divide it by 1L which won't change the number, just units)
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline bob12

  • New Member
  • **
  • Posts: 7
  • Mole Snacks: +0/-0
Re: Can someone check these stoichiometry problems?
« Reply #5 on: September 21, 2007, 12:36:29 PM »
Okay, I think I see now.  So the answer is M=.733.  Can you double check to make sure I'm right?

Thanks for all your help and patients!

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27633
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Can someone check these stoichiometry problems?
« Reply #6 on: September 21, 2007, 04:31:44 PM »
Close - but wrong. Show how you got there.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline bob12

  • New Member
  • **
  • Posts: 7
  • Mole Snacks: +0/-0
Re: Can someone check these stoichiometry problems?
« Reply #7 on: September 21, 2007, 07:12:42 PM »
I multiplied 1080 by .0525 to get the grams used.  I then divided that by 74.4 (molar mass) and got .733 moles.  M=moles/volume.  Divide .733 by 1 and you got the molarity.  Seems resonable, but if it's wrong I'm not sure what else to do!  I don't know why I'm having so much trouble with this problem.  I've done these type before...

Thanks.

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27633
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Can someone check these stoichiometry problems?
« Reply #8 on: September 22, 2007, 03:45:41 AM »
I multiplied 1080 by .0525 to get the grams used.  I then divided that by 74.4 (molar mass) and got .733 moles.

Check your math. IMHO you did 1080*0.0505 instead of 1080*0.0525.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Sponsored Links