Molarity..ish

[looneych00n]

Hi,

Im stuck doing some calculations, and I was wondering if anyone could help me? The question is: If 200ml of 0.1M sodium cholride solution was evaporated to dryness, how much NaCl would be present? (a) In moles and (b) In g?

I dont know where to start with this question, so if the workings out could be shown I would be very grateful!
Thanks

[brachiopod]

OK. Firstly;

A mole of s substance is the relative molecular mass 'formulaic mass', or that of the simplest ionic lattice in grams.

A 1M solution contains I mole of solute dissolved in a litre of solute.

So, looneych00n, how many grams of sodium chloride (NaCl) are there in a litre of 1M sodium chloride solution?

(R.A.M Na = 23
R.A.M Cl = 35)

[looneych00n]

Thanks Brachipod, I knew that, but it doesnt seem to answer my question. Does that mean that it doesnt matter if the solution was evaporated or not?? The moles and g would still be the same??

[brachiopod]

Well, yes, the sodium chloride is still there whether you evaporate the solution or not.
But by doing so, you can demonstrate that your calculations are correct by weighing just the sodium chloride directly -otherwise you would only be able to weigh the sodium chloride and the water- which wouldn't tell you much.

So come on, how many grams of NaCl in a litre of 1M?

[looneych00n]

is it 0.994g?

[brachiopod]

No.

A mole of NaCl = RAM (Na) + RAM (Cl) in grams

= 23 + 35.5 = 58.5g

A 1M solution of NaCl is a mole of NaCL dissolved in a litre of water.
So a litre of 1M NaCl solution contains 58g of NaCl (which you could weigh for yourself in a lab if you evaporated off the water.)

Your solution is only 0.1M, so how many grams of NaCl in that?

[brachiopod]

Sorry, how many grams of NaCl in a litre of 0.1M NaCl solution?

[looneych00n]

This is where I get confused!!! for a litre I would have to convert that to 1000ml, as the calculation says 200ml.

To find the mass do i do moles x the RAM? which would be 0.1x58.5 = 5.85g

but where does the 200ml come into the equation????????

[brachiopod]

That's right. Moles x RAM = mass in grams

Ok, here's the easy way of looking at it;

There is 1 mole of NaCl in 1 litre (1l.) of 1M NaCL solution,

Therefore, there is 0.1 moles of NaCl in 1l. of 0.1M solution

You only have 200ml of 0.1M NaCl solution, (There are 1000 ml in a litre, so 200ml is 200/1000 or 0.2l.).
So there must be;

0.1 x 200/1000
= 0.02 moles NaCl in 200ml of 0.1M NaCl solution. (part a. answer)


So if a mole of NaCl = 58.5g (RAM (Na) + RAM (Cl) = 23 + 35.5 = 58.5 g)

then how much is 0.02 moles of NaCl in grams?

[looneych00n]

is it the moles x RAM again?

meaning that 0.02 x 58.5 = 1.17g

or have i got the wrong end of the stick?

[brachiopod]

Yay - that's it!

A mole of NaCl is 58.8g

So 0.02 moles is 58.5 x 0.02 = 1.17g

and a million moles (NaCl) is 58.5 x 1000000g

and X moles is 58.5 x X g

Well done. Hope you see it a bit cleaarer now.

[brachiopod]

...or even clearer !!!!

[looneych00n]

Brachiopod, thank you very much!! you must be some kind of chemistry guru!!

Thanks very much, because this has been doing my head in!!!

Cheers
looneych00n

[brachiopod]

Glad to help !

B. Pod

[drestie]

This thread has made me so happy!! This is what AB is all about :)

You are such a star, brachiopod.

[China Doll]

I agree Drestie... made me feel all fuzzy inside! (And I only clicked in cause I was curious!) Nice one Brachi.

[Gef]

Just to recap the previous answers from brachi.

You need to use the following formulae:

number of moles = volume in litres x molarity

mass = number of moles x formula mass.

[looneych00n]

Brachiopod is ace!

Thanks again
looneych00n

[whowhatwhy]

Nice work Brachiopod. If only my chemistry teacher had the patience and the inclination to teach that clearly. I failed my BioChem modules twice at university! Got it in the end thank God. Now how about having a stab at the Middle East 'peace' process? :-)

[adar]

couldn,t help popping over brachiopod

are you some sort of proffessor or something?

your a star awlrite

you should have your own show lol