December 04, 2024, 09:30:41 PM
Forum Rules: Read This Before Posting


Topic: Assistance with son's double-displacement precipitate problems  (Read 4766 times)

0 Members and 1 Guest are viewing this topic.

Offline rdb

  • Very New Member
  • *
  • Posts: 2
  • Mole Snacks: +0/-0
Okay, haven't looked at chemistry since about 1990. I can usually remember how to do stuff after I look around online for reminders, but this one, I just can't quite remember.

The problem:

Solutions of aluminum sulfate and calcium hydroxide are mixed. What is the mass of precipitate formed when 250ml of 3.4M aluminum sulfate is mixed with 100ml of 7.4M calcium hydroxide.


So I balance the equation, and I'm pretty sure it ends up being

Al2(SO4)3 + 3Ca(OH)2 ---> 3Ca(SO4) + 2Al(OH)3

with 2Al(OH)3 being the precipitate.

The problem is that I can't remember where to go from here, so I'll throw down what I've done and hopefully someone can continue to point me in the right direction.

.250L * { 3.4mol Al2(SO4)3 / L } * { 342g Al2(SO4)3 / mol  Al2(SO4)3  } * { 2 mol Al(OH)3 / 1 mol Al2(SO4)3 } =  581.4g Al(OH)3

so that's what I have. I don't know if it's correct. I don't know if I even performed the operation properly, although the units and such seem to cross out and leave me with the appropriate thing at the end 

So even assuming all of the above was performed correctly and is relevant, how do I incorporate the 100ml of 7.4M Calcium hydroxide?  Do I need to do so? How can I tell if the amount of my precipitate will be limited by the amount of the Al "supplier" or the hydroxide "supplier"?

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27875
  • Mole Snacks: +1813/-412
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Assistance with son's double-displacement precipitate problems
« Reply #1 on: April 18, 2008, 03:18:29 AM »
This is a little bit tricky question. If it is HS level you probably have to assume BOTH products are insoluble, and think in terms of limiting reagents.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline shelanachium

  • Chemist
  • Full Member
  • *
  • Posts: 137
  • Mole Snacks: +12/-0
Re: Assistance with son's double-displacement precipitate problems
« Reply #2 on: April 18, 2008, 03:54:37 PM »
Sure you copied the question out right? 7.4 M calcium hydroxide is quite impossible, this substance has very limited water-solubility!

Offline rdb

  • Very New Member
  • *
  • Posts: 2
  • Mole Snacks: +0/-0
Re: Assistance with son's double-displacement precipitate problems
« Reply #3 on: April 18, 2008, 04:08:50 PM »
yep, sure copied the question correctly. I don't think the issue is so much whether or not 7.4M is possible, I think the instructor just wanted them to practice.

out of what I wrote, did I at least balance the equation and write the substances properly?

Was the way I wrote the mathematics understandable?

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27875
  • Mole Snacks: +1813/-412
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Re: Assistance with son's double-displacement precipitate problems
« Reply #4 on: April 18, 2008, 04:24:55 PM »
Equation is balanced OK, although it is CaSO4, not Ca(SO4).

Your result is off - check your units, they don't cancel out the way you want them to cancel, your final answer is not in grams of Al(OH)3.

And as I wrote earlier, you have to check what is a limiting reagent.

Finally, I hate it when teachers don't pay attention to such details as possible/not possible concentrations of reagents (good catch shelanachium, I missed that one; snake for you). You never know what students will remember, why give them wrong information, when there are zillions of correct examples?
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Sponsored Links