Chemistry Titrations

scherzo!

I'm okay with everything for titrations except calculations. The big thing I'm having trouble with is concentrations and how one book lists one method and then another book lists another, or maybe both methods are the same and I'm just missing something. I have three questions on 'em.

Anyway, in 2009, calculating the conc of the iron solution, why doesn't the marking scheme take into account that the iron solution was only made to 250cm? Shouldn't there be a multiplication by 4 to get the molarity of it, since in 250cm it would be 4 TIMES as concentrated?
Va*Ma Vb*Mb
na = nb

That gets per litre, and then multiplying by 4 gets concentration per 250cm. In the marking scheme, it gets the molarity, but it stops before the multiplication of four. Why's that? I think it does this in all of the titrations actually. In the "Chemistry Live!" book, it always took into account any soluition that wasn't made up to exactly 1 litre.

My second question is, sometimes in the textbook, instead of taking the concentration into account BEFORE converting from moles to grams, it converts from moles to grams and THEN takes into account the concentration. It does that on page 164, for example, where it converts from moles to grams first and divides by four to get number of grams per 250cm, in the percentage purity calculation. It does the same thing again for the Iron(II) sulfate experiment on page 199.

My third question's a bit like the second.
Sometimes instead of multiplying to get the concentration of a solution per 250cm, it divides. On page 170, it's found out that the conc of the NaCO3 solution is 0.121 and instead of multiplying by 4 to get the concentration in 250cm3, he divides. I thought you'd only divide if it were in grams, not moles.

Seriously, I've been trying to figure this out for the longest time and I'm getting nowhere. I don't want to have answered the first question on the chemistry paper, with it constantly being in the back of my mind afterwards, wondering if I made the right moves. I'd appreciate it so much if someone could tell me where I'm going wrong.
Thanks for listening to my nonsense. XD

zam

I'm really sorry, I tried to help but it's hard to understand what you're saying! You know that thing when it makes perfect sense to you but it's hard to explain to others :P
Basically I just study from marking schemes at this stage... hope that's some help! If you know anyone who's good at chemistry, or even get one session with a grind the weekend before the chemistry exam, that could help answer some questions.

scherzo!

zam wrote: »

I'm really sorry, I tried to help but it's hard to understand what you're saying! You know that thing when it makes perfect sense to you but it's hard to explain to others :P
Basically I just study from marking schemes at this stage... hope that's some help! If you know anyone who's good at chemistry, or even get one session with a grind the weekend before the chemistry exam, that could help answer some questions.

Haha! I always thought I was really clear. 0____0 Thanks very much for trying anyway!
And I tried to get help from my teacher, except I think there's some sort of language barrier between us, and now I'm out of school so I don't think I can contact her, not that I think it would be of much help.

I guess I'll just leave the calculations to luck. Unless someone else can help me (or if you get an epiphany :B)

lc2010

I get what you mean but it really would be near impossible to explain it over boards. I used to have similar problems to you but thankfully I began to understand it over time. What you really need to do is practice and if there is a step in it that you dont understand, dont continue on with the calculation until you figure it out.
It really sounds as if you might need a grind or two as zam suggested just to straighten out what seem to pretty, easy to fix problems. Perhaps, if you have eircom broadband you could try the studyhub tutorials which can be very useful sometimes.
Hope this helps!!!!

scherzo!

lc2010 wrote: »

I get what you mean but it really would be near impossible to explain it over boards. I used to have similar problems to you but thankfully I began to understand it over time. What you really need to do is practice and if there is a step in it that you dont understand, dont continue on with the calculation until you figure it out.
It really sounds as if you might need a grind or two as zam suggested just to straighten out what seem to pretty, easy to fix problems. Perhaps, if you have eircom broadband you could try the studyhub tutorials which can be very useful sometimes.
Hope this helps!!!!

Well the message is comprehensible at least, ha.

Really, I can understand every mathsy bit of chemistry, and I have been trying to figure them out, it's just the inconsistencies between those three that have me stumped.

Victoria.

I'll try to help you think I know what you're talking about! Would really be a shame not to be able to do this Q fully!

Victoria.

scherzo! wrote: »

My second question is, sometimes in the textbook, instead of taking the concentration into account BEFORE converting from moles to grams, it converts from moles to grams and THEN takes into account the concentration. It does that on page 164, for example, where it converts from moles to grams first and divides by four to get number of grams per 250cm, in the percentage purity calculation. It does the same thing again for the Iron(II) sulfate experiment on page 199.

Well the usual sequence of events for a titration calc is:

• Get moles per litre

• Calc Mr of thing you have (iron/crystals or whatever else)

• Divide the number of moles in litreby that Mr

• Now you will have grams per litre

• But you dont have a litre so you will need to get grams in whatever amount you have by division

You can do the dividing to get your 250 after you get the moles per litre if you want but sometimes it asks you to get grams per litre. If you divided then you would have to get the grams in your 250 and also in a litre anyway to answer the question.

Either way will work, same thing.

Victoria.

scherzo! wrote: »

Anyway, in 2009, calculating the conc of the iron solution, why doesn't the marking scheme take into account that the iron solution was only made to 250cm? Shouldn't there be a multiplication by 4 to get the molarity of it, since in 250cm it would be 4 TIMES as concentrated?
Va*Ma Vb*Mb
na = nb

That gets per litre, and then multiplying by 4 gets concentration per 250cm. In the marking scheme, it gets the molarity, but it stops before the multiplication of four. Why's that? I think it does this in all of the titrations actually. In the "Chemistry Live!" book, it always took into account any soluition that wasn't made up to exactly 1 litre.

Try printing off a few of the marking scheme answers for Q1 because it might make it more clear if you're getting nowhere with the textbook.


I think I understand what you're saying about the 250 situation. Hopefully I can help...

I have another text book and we use:

V1 X M1 / N2 = V2 X M1 /N1 (prob same as yours)

That gives you moles per litre.

At this point after the formula here you would always divide your value to get how much you have of that litre rather than multiply, say by four.

When you work out the molarity it takes it is as if you have a whole litre of it exactly the same concentration, the answer is written as moles per litre. Its like you have an imaginary extra 3/4 just for the purpose of the calc.

You don't have a litre, only a quarter of it so you must divide by 4.

If you multiplied by 4 you would actually be calculating how many moles per 4 litres something you don't want to do. If you do that so early on in the calc every other part of the question will be wrong.

Hope that helps!

Victoria.

scherzo! wrote: »

My third question's a bit like the second.
Sometimes instead of multiplying to get the concentration of a solution per 250cm, it divides. On page 170, it's found out that the conc of the NaCO3 solution is 0.121 and instead of multiplying by 4 to get the concentration in 250cm3, he divides. I thought you'd only divide if it were in grams, not moles.

Yeh I would divide by 4 too in this case.

It's like what I said in my other answer that if you have say .12g per litre and want to get that in 250 you would definitely divide by four not multiply because if you multiplied by four you would get how much in 4 litres.

You can divide both grams and moles by 4 to get value for 250

You could do it first moles per litre / 4.
or
grams per litre / 4

scherzo!

Victoria. wrote: »

If you multiplied by 4 you would actually be calculating how many moles per 4 litres something you don't want to do. If you do that so early on in the calc every other part of the question will be wrong.

Hope that helps!

But when you're calculating moles, you're calculating concentration, so the concentration INCREASES when the solution gets smaller (that is, goes from 1 litre to 250cm3). If you divided when the solution got smaller, you're saying that the concentration is getting smaller, but the concentration of the solution is going UP if you're taking the solvent away.

It's like, if you have a full cup of tea with a spoon of sugar, you'd say the tea isn't really that concentrated in sugar, but if you took some of the tea away, the tea in the cup would have a HIGHER sugar concentration than before. So the number would go up. If you divide, the number goes down, so shouldn't you multiply?

My reasoning is:

Solution goes from 1 litre to 250cm3 and you're calculating conc (moles) then you multiply (concentration is going UP)
And Solution goes from 1 litre to 250cm3 and you're calculating grams/litre, you divide (number of grams is going DOWN).

This is all just a big mess DDD: