[rdb]

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"?

[Borek]

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.

[shelanachium]

Sure you copied the question out right? 7.4 M calcium hydroxide is quite impossible, this substance has very limited water-solubility!

[rdb]

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?

[Borek]

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?