Ohhh thanks keltingmeith! I think I was confused because my teacher said the mols of diluted and undiluted are the same. And the other question which I was talking about was in the 2017 sample exam (5biii)
Ahhhh. So first, what your teacher said is correct - the amount of compound, in mol, of a diluted and undiluted sample are the same. The only difference is how much water makes up the solution. Think of a real world example - say you have dissolved 5g of copper in 50mL of water. If you add ANOTHER 50mL of water, that copper isn't going anywhere. It hasn't decided to just yeet itself out of existence - it's still there.
The difference in this question (2014s), however, isn't that we have a diluted and undiluted sample. It's that not ALL of the solution was used. Back to the copper example - after diluting to 100 mL, I then take the beaker and pour 50 mL of that solution into another beaker. Where is the copper now? Well, it should be split evenly between the two beakers. So if I started out with 5g of copper, the beakers IN TOTAL should have 5g, which would mean 2.5g of copper in each beaker. Or maybe, I poured 20mL in the second beaker, so one has 80mL of solution, and the other has 20mL - then there'd be 4g of copper in one beaker, and 1g in the other. It's the same thing with the iodine - sure, I started with 15g of it, but not all of that 15g is going to be in the 4.95 mL that I used for the titration - only some of it is. It's not that the amount of mol changed in the dilution, because it won't - if the mol changes, then that means some of the material must have reacted with something else. What's actually happened is that when I take an aliquot, or part of a solution, away from the full solution, the some of the chemical will come with the new aliquot, and some will stay behind in the full solution.
As for the 2017 question, the reason you won't use a dilution factor is because in 5bii, you don't care about the full standard yet. You only care about what's in the aliquot. The aliquot hasn't been diluted. You had to dilute the stock solution to MAKE the aliquot, but the aliquot itself was never touched. That's all that answering these questions takes - thinking about the actual chemistry that's going on, and not just plugging in the numbers you find into equations.
Is it true that you can only lose 1 mark due to significant figures in the exam??
Short answer: most likely
Long answer: everyone says something to this effect, but it's not cut and dry. Firstly, VCAA has no official stance on this - having said this, there 100% is information that teachers get that we don't, so we need to trust them a bit here. Unfortunately, every teacher says something different. Some have said that you need to use sig figs for every question, but you can only lose up to 1 mark if you get things wrong. Eg, let's say question 2 and 5 both require sig figs. You get the sig figs wrong on both of these questions. You will only lose 1 mark. Let's say you got 2 right, but 5 wrong. You will only lose 1 mark.
Another thing I've heard is that there's only one question in the whole exam where you can lose the mark. Let's go back to the previous example, but let's say that question 2 is the question where you can lose the sig fig mark. If you get the sig figs wrong in both questions, again, you will only lose 1 mark. But, if you get the sig figs right in question 2, but wrong in question 5, then you will not lose any marks for sig figs.
The problem is - if the second scenario is the true one, we have NO way of knowing what question has the sig fig marks attached to it. It's also not clear if this sig fig mark is attached to the question (eg, question is out of 3 - 1 mark for working out, 1 mark for answer, 1 mark for sig figs), or if it's just a penalty if you get the sig figs wrong (eg, question is out of 2 - 1 mark for working out, 1 mark for answer. -1 if you get sig figs wrong), so we can't even try and guess it from the mark distribution. And again, this is us trusting our teachers, because VCAA don't give this information to us (that I can find, at least. Would be very happy for someone to prove me wrong here, tbh)
So, you can rest easy knowing the yes - at most, you'll likely only lose 1 mark for sig figs. You are, of course, better off just trying to get sig figs right (they're honestly not that hard to learn if you can convert to scientific notation, because then there's just one trick for you to use). But just in case you're worried about making a mistake in the heat of the moment (because yeah, turns out, exams can be stressful - and when stressed, you often make silly mistakes you normally wouldn't. Who knew?
), don't worry - you'll probably only lose 1 mark at the most.