Chemistry 3/4 2013 Thread

[Mr Keshy]

They're not too bad once you start them. I mean most of the questions are the same, as in, they require the same process/working out.

They aren't bad at all, but the fact that I saw a heap of them scared me haha. Last question was what? 18? and I saw a 46 (I think)!

[teletubbies_95]

Omg . In our Unit 2 exam, they put pg 46 q 23( heinemann). i dont think anyone got it , i ended up getting consequential. like who puts a year 12 question in a year 11 exam. maybe thats a few people dropped out of chem last year.

[Chazef]

In question 2 of Chapter 6 in the Heinemann Chem 12 textbook, it asks: 'why is it necessary to measure the peak areas produced by a number of standards'

I answered: 'to accurately determine the proportionality of %ethanol to the Relative ethanol peak area, allowing the former to be determined from the latter'. but the textbook has a far more detailed response, containing many terms I'm not familiar with.

the book says: 'Techniques such as AA, GLC, HPLC, UV, etc. do not directly produce measures of concentration. Standards must be used. More than one standard should be used and the unknown sample should lie between these standards. This is because a zero standard may be contaminated with trace amounts of the chemical being tested . Calibration graphs are often non-linear and multiple standards increase the chance of detecting incorrectly prepared solutions'

I have no clue what exactly the bold text means, could somebody enlighten me? 

[thushan]

In question 2 of Chapter 6 in the Heinemann Chem 12 textbook, it asks: 'why is it necessary to measure the peak areas produced by a number of standards'

I answered: 'to accurately determine the proportionality of %ethanol to the Relative ethanol peak area, allowing the former to be determined from the latter'. but the textbook has a far more detailed response, containing many terms I'm not familiar with.

the book says: 'Techniques such as AA, GLC, HPLC, UV, etc. do not directly produce measures of concentration. Standards must be used. More than one standard should be used and the unknown sample should lie between these standards. This is because a zero standard may be contaminated with trace amounts of the chemical being tested . Calibration graphs are often non-linear and multiple standards increase the chance of detecting incorrectly prepared solutions'

I have no clue what exactly the bold text means, could somebody enlighten me? 

Your answer is fine. The book is trying to say that when you draw the calibration curve, you cannot extrapolate outside it. So if your standard solutions are from 0 ppm to 10 ppm, the calibration curve is only valid for samples between those two values.

What they are also trying to say is that you can't assume that a 0 standard will have an absorbance of 0, since the eluent itself you might be using MIGHT have a TINY amount of sample being tested (or the eluent itself may be detected by the detector to an extent, which uses a UV absorption method).

[Stick]

Omg . In our Unit 2 exam, they put pg 46 q 23( heinemann). i dont think anyone got it , i ended up getting consequential. like who puts a year 12 question in a year 11 exam. maybe thats a few people dropped out of chem last year.

Don't worry - on our Unit 2 exam there was a question where we had to work out the pH of a solution containing a weak acid at a temperature lower than 25 degrees Celsius. <_< Luckily it was only a multiple choice question so we could at least guess. I got it right! XD

[teletubbies_95]

Woah. That's like Unit 4? Yay!
This question was like a 5 part SA question!

[Mr Keshy]

Guys, is it okay if I'm writing each and every step for my answers in chem. It takes a bit of extra time but I feel as though it's worth it.

1. Because it's early stages and it's best to understand each step properly.
2. It's easy to identify errors.

But my issue is, do I have to learn the short cuts come exam time or is writing every step okay? Even if I am confident, I like to write every single step down. My worry is that time (lack of) may be an issue during the exam if I follow this. How did you go about things? Or how are you planning to go about it?

Thanks

[Stick]

I'm not sure how much time is an issue in Chemistry, but if it doesn't mean you're not finishing assessments, you should do whatever you can to minimise mistakes. This will probably be something you'll gauge as you get more practice.

[Reckoner]

I seriously wouldn't be worried about exam timing at this stage. Just get a solid understanding. If you do this, I'm sure that you will find speed comes naturally, and you'll probably end up wanting to slow yourself down come exam time.
 

[Mr Keshy]

I'm not sure how much time is an issue in Chemistry, but if it doesn't mean you're not finishing assessments, you should do whatever you can to minimise mistakes. This will probably be something you'll gauge as you get more practice.

I seriously wouldn't be worried about exam timing at this stage. Just get a solid understanding. If you do this, I'm sure that you will find speed comes naturally, and you'll probably end up wanting to slow yourself down come exam time.
 

Awesome, thanks guys Takes a lot of space in by book as well haha, looks like I've been doing heaps of work

[teletubbies_95]

What is meant by " degree of hydration"?? ie. in the question " ie . A sample of hydrated chronium (III) chloride weights 76.23 g. After drying to constant mass , the sample weights 45.23g. What is the degree of hydration?

[thushan]

The number of water of hydration in the formula - like CrCl3.xH2O or sth (how many H2O is there in the formula?) When you dry to constant mass, the H2O flies away and you're left with CrCl3. What is the value of x - that's the qn they're asking.

[Hancock]

These are equal in mole amounts because there is the same amount of in both the hydrated and anhydrous form of the molecule.

[teletubbies_95]

so you would : 
76.23-45.23= 31.00g ( mass of water)

then find the moles , divide by smallest and find the ratio?

Sorry bit confused , first time ive read this sorta question!

[Mr Keshy]

One thing I've noticed today when I got stuck in a questions is that I've been so focused on getting all the methods and formulas right that I actually had little idea behind the logic of the processes involved :O

As in, if someone asked me to calculate one of the questions, I'd have no trouble (okay maybe I would ) in doing the question and getting the answer, but I have no idea why I'm calculating the things I'm writing. If you're like me, definitely have a good look at the reasons behind every step of the question . It's all good to know the methods, but I think it's important to know why you're doing it.

If that made any sense at all