I thought I knew how to do this problem, but I keep getting it wrong... Was wondering if anyone could give me some help. Thanks!

How many grams of 50 wt % NaOH should be diluted to 500 mL to make 0.120 M NaOH? MW=40.00 g/mol

1/23/2006 8:21:59 PM

Damn, it's amazing how quickly you forget an entire semester's worth of knowledge, when it was just a couple of months ago...

1/23/2006 8:43:24 PM

i dunno but 50 wt% would equate to 50 g NaOH in 50 g water which is what .05L...

1/23/2006 9:22:59 PM

that class gives me nightmares again w/ all those webassigns and excels for lab

1/23/2006 9:41:32 PM

50 wt% NaOH refers to 50g NaOH per 100g solution. You should be linked to a table of FW's, so you can find out how many mols of NaOH corresponds to the 50g NaOH/100g solution.

Now you've got the desired molarity and the volume used to get that. From there determine the number of mols NaOH you'll need in the solution.

Now use what you calculated earlier to determine how many grams of solution will provide you w/ that number of moles NaOH.

1/24/2006 10:05:23 PM

but she could just say 50g of NaOHin .05L of water unless we are doubting that water is 1g/ml

^and that would be the beginnings of molality not molarity

[Edited on January 24, 2006 at 10:32 PM. Reason : .]

1/24/2006 10:31:47 PM

0.120M is molarity. small m is molality.

and trust me what i did was right, i just did this problem in my assignment.

you're making this way too complicated. im looking at my work in front of me now.

50g NaOH / 100g solution.. what the solution is is not necessarily even water.. its not specified, and is virtually irrelevant so long as we know how many moles of NaOH are provided per gram of the solution-whatever it may be.

the second part of the question says we want .120 M NaOH in a 500mL flask . from ch201 or whatever, you know that .120M times 500mL will give you some number of mmol of the solute (NaOH). so you know you're gonna want X mmol of NaOH in your solution. Divide by 1000 and you need Y MOL NaOH in your solution.

how do you get Y mol from your initial sample? well Y mol desired * (100g solution/mol NaOH provided by this) = the amount of grams of solution you will need from your initial sample.

1/24/2006 10:52:56 PM

^assumed it was water since that's what solution implies to me but i get what ur saying now that u explained it out all the way

1/24/2006 11:50:28 PM