Homework Help: Content concentration

1. Sep 6, 2010

thea831

1. The problem statement, all variables and given/known data

Nitric acid is commercially available as a 72% ( w/ w) solution ( density, 1.42 g/cm3). How many milliliters of this reagent are needed to prepare 2.00 L of a 1.00M HNO3 solution?

2. Relevant equations

C%= (g HNO3/ g Solution) * 100

3. The attempt at a solution

So basically, I started with converting a mole of the solution to mL.
1 mol HNO3=44.37ml

But I feel like I need to first use the concentration to find how much reagent is in that 72%. so its .72g I assume that the reagent is contained in that solution.

so .72g of HNO3 in the solution is .507mL

but now I am stuck. How do I use that information to figure out what the mL of the reagent is needed to get 2L of 1 mol of HNO3

2. Sep 6, 2010

epenguin

Your statements and working were frankly too unclear for me to follow. I do not know what your para 2 means.

Do not "started with converting a mole of the solution to mL" which is 2 steps. Start by converting a mole to grams.

Now 1g of your commerical nitric acid, is .72g of HNO3 .

You find the volume of the above 1g (containing your .72g of HNO3) from the density. I do not think you have got that right.

Then you work out how much volume acid contains a mole (actually 2 moles since you need to make 2l).

It should all make sense. All the chemistry is in the first step and the rest is just arithmetic to find the voume of acid/water mix that contains a mole of HNO3

3. Sep 6, 2010

thea831

i dont understand what you mean by 1g is .72g of commercial HNO3.

1g of HNO3 is 1/1.42 mL

so I need 2mols since there are 2L.

so here is what I am thinking.

Based on the w/w content concentration which is 72%, I use the formula listed above to solve for the mass of the reagent HNO3

.72*(2000ml*1.42g/mL)=2044g= mass of HNO3 reagent.

Now I need to know the volume of this reagent in a 2L of 1M concentration

So here is how I got my answer:

2044g HNO3 * (1 mol/ 63.012g) * (2L/2mol) = 32.4L which does not make sense

4. Sep 7, 2010

thea831

nvm i got the answer. I used MV=M2V2. Solved for M2 first then for V2

5. Sep 7, 2010

Staff: Mentor

At 72% w/w there is 0.72g of nitric acid in each 1g of the solution.

Now you are assuming you need 2L of the solution that will have 1.42 g/mL density - that's density of the stock solution, not the final one.

Besides, density of the final solution doesn't matter, what matters is the number of moles of nitric acid in the final solution, it must be identical to number of moles of nitric acid in stock solution volume that you will dilute. That's simple mass conservation.

Using this formula blindly won't get you far. Somehow I doubt your solution, but not seeing the numbers I can be wrong.

Last edited by a moderator: Aug 13, 2013
6. Sep 7, 2010

epenguin

Good teacher once gave me sound advice - "don't try to do several steps in your head epenguin - you are not clever enough". (Let alone am I clever enough to follow what you do in your head. Not even Borek is that clever! )

You need, OK*, 2 moles of HNO3. You need to find how many grams is that?

That is an exercise in chemistry, knowing what moles, molar masses etc. are - in general and for this substance HNO3. (BTW you cannot do it from only the information you have written, I suppose you have, or remember, some other.)

Then when you have decided that, it is an exercise in simple physics - densites, masses, volumes, to find out in what volume of this dense water-HNO3 mixture, that number of grams of HNO3 is contained.

* OK to do it that way. The reason I personally would do it for 1l and then multiply the result by 2 at the end, is that I would be working out for 1l in all cases and then multiply by appropriate factor according as I wanted 2l of IM or 250 ml. or whatever of the desired solution.