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pop000 · Feb 9, 2011, 11:22 AM

IN 3.0 M HCl solution contain:

A)3.0 gr of HCl for any 1 liter water?
B)3.0 gr of HCl for any 1 mol solution ?
C)3.0 mol of HCl for any 1 liter solution ?
D)3.0 mol of HCl for any 1 mol of water?
What is the way to know which answer here is correct ?is can be D or C?

And tell me if I correct Mg+2 contain 10 electron ?

Thanks. :)

Unknown008 · Feb 9, 2011, 11:34 AM

1 mol of water = 18 g of water...

[Remember H2O has an mM of 16 + 1 + 1 = 18]

which is equal to 18 cm^3 of water, of 0.018 L

Can you go from there? :)

For this one, Mg has 12 electrons...

Mg^2+ has lost 2 electrons, and is now doubly positive, hence it has 12 - 2 = 10 electrons.

Good! :)

pop000 · Feb 9, 2011, 12:03 PM

Hi thanks for answer but I have no idea.
How did you get this"1 mol of water = 18 g of water"?
How can I know how many gr of HCl there in 1 mol HCl?

Thanks again.

jcaron2 · Feb 9, 2011, 02:45 PM

The molecular weight of a compound is, by its very definition, the number of grams in 1 mol. It's analogous to the atomic weight of an element, which is the number of grams in 1 mol of that element. To find the molecular weight of water, you just add up the atomic weights of all the atoms that make it up: Water = H2O, so it has two H atoms plus one O atom. You can look up the atomic weights of H and O by just looking on the periodic table. H has an atomic weight of about 1, and O has an atomic weight of around 16. Thus water's molecular weight is 2*1 + 16 = 18.

Now can you tell us the number of grams in 1 mol of HCl?

pop000 · Feb 9, 2011, 03:10 PM

Ohh correct correct. Yes I know now that the number of 1 gr in 1 mol HCl is 36.453

pop000 · Feb 9, 2011, 03:13 PM

so 3.0M HCl=109.359 gr but how is help me here?

thanks .

jcaron2 · Feb 9, 2011, 09:24 PM

LOL! I was answering your question about molecular weight, and I never looked at your original question.

I can see why you're confused. :)

There's no calculation required for this question. It's simply asking you for the definition of molarity (M) of a solution:

M = moles of solute / 1 liter of solution.

3.0M means 3 moles of solute per 1 liter of solution, so the correct answer would be C.

It would also be correct to say 109.359 gr of HCl for 1 liter of solution (since that's the same as 3 moles), but that wasn't one of your choices. ;)

jcaron2 · Feb 9, 2011, 09:27 PM

True, but it's irrelevant what volume of water makes 1 mole. Molarity, by definition, is moles of solute per liter of solution, not per mole of solution. ;-)

Unknown008 · Feb 9, 2011, 10:23 PM

Hm... if it said instead 1 kg of water... which is equivalent to 55.6 mol of water molecules, then answers C and D would be the same, right?

What I was pointing at, is how answer D can in no way be correct.

pop000 · Feb 10, 2011, 03:40 AM

Lol yes :) but still thank you for answering.

pop000 · Feb 10, 2011, 03:41 AM

OH OK correct. Again thank you
My 2nd teacher :)

Unknown008 · Feb 10, 2011, 03:49 AM

Always pleased to help you :)

jcaron2 · Feb 10, 2011, 06:20 AM

Makes sense. I figured you probably knew the definition of molarity since you were 5 years old. :)

Unknown008 · Feb 10, 2011, 09:50 AM

Lol, nope, It was only two years ago that I learned it. I came to know it a little earlier than that, but only under the name of 'concentration in mol per dm^3'.

It is in fact on AMHD that I learned what molarity and molality mean, especially the notation for example 3.0 M

I see you're on 'Go' skin again... :p

DrBob1 · Feb 10, 2011, 06:07 PM

In this exaulted academic setting I have to point out that a liter of water and a liter of solution are NOT the same.

jcaron2 · Feb 10, 2011, 09:23 PM

I had to switch back to leave a comment. In the old skin, I can only comment if I agree or disagree, but I have to spread the rep before it will allow me to. Plus I like that I can see all new posts in the topic of science, rather than having to individually check each sub-topic. When it comes to posting, however, I usually switch back to the old skin.

Unknown008 · Feb 10, 2011, 10:32 PM

In this exaulted academic setting I have to point out that a liter of water and a liter of solution are NOT the same.

Ok, I think that this is my main problem for I was never taught how to exactly calculate the difference and pay attention to that.

I know from your previous posts, that when you dissolve a solute in a litre of water, you don't end up with a litre of solution.

But in this case, i.e. dissolving 3 mol of HCl into 1 mol of water, can we get the concentration of the HCl in molarity? Is there something we should add when calculating the concentration, or it is just too tedious to work it out?

DrBob1 · Feb 10, 2011, 10:52 PM

Molarity is a measure of the concentrations of liquid solutions. Concentrated HCl solutions (13M) are at the maximum concentration of saturated HCl in water so 100+ grams of HCl in 18 g of water will need to be pressurized to keep it from boiling violently. It isn't a calculation it is a matter of precise language and laboratory technique,

Unknown008 · Feb 10, 2011, 11:01 PM

Ok, I think I understand.

So, the correct procedure to get a standard concentration of solution remains adding the solute to some solvent, and mix thoroughly, then top up to a mark (could be 1 L, or other depending on the apparatus used)?

And in the case concerned here, the most precise concentration we would say will be 3 mol/18g or solvent, or 0.167mol/g of solvent? (not taking into consideration the solubility issue)

DrBob1 · Feb 12, 2011, 02:04 PM

Add solute, dilute to the mark and you know the amount of solute and the amount of solution. In other words you know the concentration. With moles per gram you know the molality, but this is a narrowly useful unit,(as far as I remember its only used for freezing and boiling point work.) And 3 mol/18g and .167 mol/g are WAY to concentrated for the real world, (Tthat's 58.5g 0f NaCl in 18ml of water. It will be a paste, not a solution!