Chemistry help: solutions, concentrations, and molarity

[Nanobyte]

I can't understand how to solve this problem:
How many milliliters of 16.0 M HNO3 would be required to prepare 750. mL of a 0.500 M solution?

It says to see Sample Problem 13-3, but it includes grams into the equation, when all I have is the volume and concentration. Can someone help me w/ this problem?

Alright, I think I have it.
Would you take .5Mx.75L= .38mol/16M= .02L
.38mol/.02L= 19M.. hmm, that isn't right.

.5Mx750mL= 375mol/16M= 23.44mL
375mol/23.44mL= 16M.. but, the molarity equation says that you have to do the process in L..

Darn it.

[ 11-23-2002, 02:16 PM: Message edited by: Nanobyte ]

[Staralfur]

Just use Concentration(M) = moles/volume (in litres);

750ml of a 0.5M solution contains 0.375moles (0.750 x 0.5)

The solution of acid is 16 Molar, and vol.= moles/conc.;

So the volume of 16M HNO3 solution containing 0.375moles is 0.0234 Litres (23.4mL) (0.375 divided by 16)

You can converts the number of moles used into mass, but this just adds in a couple of extra steps that aren't needed in this case.

[WillowIX]

Ok n=V*C right. And you want 750ml solution that is 0,5M. Then you need 0,75*0,5=0,375 mole. Now you know how much you need from the original HNO₃. Using the first formula V=n/C --> V=0,325/16= 0,023L or 23ml.



Edit: 3 answers. Should help you

[ 11-23-2002, 02:46 PM: Message edited by: WillowIX ]

[Davros]

Gee, this takes me back many a year :
OK, so you need to make 750 mls of 0.5 molar solution

Molarity = no of moles per litre, so your new solution needs 0.5M * .75 litres = 0.375 moles (often referred to as gram moles)

You have 16M nitric - ie 16 moles per litre

What volume of that solution will contain 0.375 moles ======>

16M = 0.375 moles / V litres of solution - rearrange to get V = 0.375 / 16 = 0.0234 litres = 23.4 mls - your answer.

You can do everything on a gram basis instead of a number of moles basis, but that just means additional steps of multiplying and dividing by molecular weight (not needed).

OK, does this mean that I can place you as non-American (what with the metric values) - although I am wondering if molarity actually means the same thing in all countries, and can I also deduce that you, Nanobyte are doing Grade 8 or 9 chemistry?

[Davros]

Originally posted by WillowIX: Ok n=V*C right. And you want 750ml solution that is 0,5M. Then you need 0,75*0,5=0,375 mole. Now you know how much you need from the original HNO3. Using the first formula V=n/C --> V=0,325/16= 0,023L or 23ml. Edit: 3 answers. Should help you

Damn - you edited before I could quote you - I was gunna suggest you get yourself a new calculator, because 0.75 * 0.5 is NOT 0.325 , and 0.325 / 16 is NOT 0.024

But your method is good

Edit :
Hold on, I see you only changed the first to 0.375, not the second (still 0.325) and now 0.325 / 16 is NOT 0.023

[ 11-23-2002, 02:55 PM: Message edited by: Davros ]

[WillowIX]

Originally posted by Davros: Damn - you edited before I could quote you - I was gunna suggest you get yourself a new calculator, because 0.75 * 0.5 is NOT 0.325 , and 0.325 / 16 is NOT 0.024 But your method is good

The problem is I have no calculator! And maths when dead tired is like pulling an elephant out of a mudbank I noticed when I had posted it though. Half of 750=325? Oh deary

[Davros]

Dead tired - but I thought I was the one that was up at 3am cos he couldn't sleep (damn cold), and for you it should be early afternoon?

Have to admit I started to do things mentally, but then when that ugly divide came along I used the windows calculator - now where did I hide that old slide rule

[Nanobyte]

Originally posted by Davros: OK, does this mean that I can place you as non-American (what with the metric values) - although I am wondering if molarity actually means the same thing in all countries, and can I also deduce that you, Nanobyte are doing Grade 8 or 9 chemistry?

American and 11th grade. But, I do think this is 8th or 9th grade chemistry, I just don't understand how to figure the equation; I'm also a little braindead after seeing Jackass last night.. [img]tongue.gif[/img]

[Blind_Prophet]

I would help you if i could but i don't remeber anything from chemistry last year besides the fact that i barely passed [img]tongue.gif[/img]

BTW jackass was so funny!!

[Davros]

So that's interesting - the American system of weights and measures is not used when working in chemical terms like Molarity - so you get a little exposure to the metric system of grams and litres - kewl