Chemistry 3/4 2013 Thread

[curry_bro]

A sample of blue copper sulfate crystals weighing 2.55g is heated and decomposes to produce 1.63g of anhydrous copper (II) sulfate. Show that the formula of the blue crystals is Cu204.5H20.

this is a pretty straight forward question, but do you only have to prove that 5 H20 molecules accompany a single CuS04? Is that all i need to show?

EDIT: never mind i get it haha! its been while since ive actually sat down and done chemistry haha blonde moment

[Stick]

Apply the empirical formula method.

[teletubbies_95]

In units 3/4 , do we need to know about element mass spectrometry and organic compounds spectrometry , or only organic compound spectrometry?

This is about mass spectrometry~~

[#1procrastinator]

^ Pretty sure you need to be familiar with all the analytical chemistry techniques for Unit 3

[Mr Keshy]

sig figs are getting my head in D:



Should it not be 4 sig figs? I looked at the answers and it was only to 3 sig figs.

Sorry if my question is basic, but I just want some clarification thats all

1. I see that final mass of jam is 6.27 (that's 3 sig figs). But since that was derived from 2 figures that were of 4 sig figs, shouldn't that make it accurate to 4 sig figs then? (is it 6.270 or something)
Then subsequently, m(H₂O), is to 3 sig figs as well.

2. If you multiply by 100 for a percentage, does that automatically make it a 3 sig fig question (given 3 is as low as the question got), like this question?

3. Of all the points in the question, where does it become a  "3 sig fig question"


OR should I only be looking at the data for sig figs?? (in which case it's 4) 20.22, 30.95, 26.49

Do I have no idea about sig figs?

[Reckoner]

1. When adding/subtracting you look at the number of decimal places. As 26.49 and 20.22 both are to two d.p, the result must be to two d.p as well (6.27). As you know this new figure is 3 sig figs, so the lowest number of significant figures is now 3.

2. I don't think so. I think the 100 is considered an exact value, so would not impact on significant figures.

3. It becomes 3 sig figs at m_f(jam). But if that value was (for example) 0.27 instead of 6.27, when you calculate the m(H₂O) you would get 10.46 grams, and the question would be back to 4 sig figs (as you are subtracting, so decimal places are used)

At least I think that's correct...   

[Mr Keshy]

1. When adding/subtracting you look at the number of decimal places. As 26.49 and 20.22 both are to two d.p, the result must be to two d.p as well (6.27). As you know this new figure is 3 sig figs, so the lowest number of significant figures is now 3.

2. I don't think so. I think the 100 is considered an exact value, so would not impact on significant figures.

3. It becomes 3 sig figs at m_f(jam). But if that value was (for example) 0.27 instead of 6.27, when you calculate the m(H₂O) you would get 10.46 grams, and the question would be back to 4 sig figs (as you are subtracting, so decimal places are used)

At least I think that's correct...

Thanks a lot! That's cleared up a hug gap in my sig figs understanding.

[Stick]

Hmm... The only thing I can think of is that your molar mass is too precise. My periodic table only goes to one decimal place.

[KevinooBz]

Hmm... The only thing I can think of is that your molar mass is too precise. My periodic table only goes to one decimal place.

He is most likely using the periodic table from the back of the chem book. If you use the chem data booklet instead, sometimes it messes up the sig figs + accuracy of the answer at the back of the book. For now using the chem book for information is alright but later in the year it would be recommended that you get used to using the vcaa data booklet.

[Mr Keshy]

Hmm... The only thing I can think of is that your molar mass is too precise. My periodic table only goes to one decimal place.

I think either way, 6.27 had the lowest sig figs so that shouldn't have changed it? But yeah, I used a periodic table from the internet which I'm certain it had the same values as per the book.

He is most likely using the periodic table from the back of the chem book. If you use the chem data booklet instead, sometimes it messes up the sig figs + accuracy of the answer at the back of the book. For now using the chem book for information is alright but later in the year it would be recommended that you get used to using the vcaa data booklet.

You're right. I should be using the chem data booklet one though. I just didn't have access to it at that time. I'll print on off.

[FlorianK]

As stated above for addition you look only at the decimels.
However, the safest way to go is just looking at the lowest number of significant figures in your given information. So lets say you get:
1.234 A; 1.2 B; 1.23 C; 1.23456789 D

Then the lowest number of sig fics would be 2, so you should write your answer in 2 sig figs.

[Mr Keshy]

As stated above for addition you look only at the decimels.
However, the safest way to go is just looking at the lowest number of significant figures in your given information. So lets say you get:
1.234 A; 1.2 B; 1.23 C; 1.23456789 D

Then the lowest number of sig fics would be 2, so you should write your answer in 2 sig figs.

Yeah, it was on a power point I looked at as well. Thanks!


The problem I'm facing now is that I'm taking far too long for each question.. But I've found in the past day or two, I've been getting quicker. So practice is helping

[Mr Keshy]

For this thread, I've taken at least 3 pictures of my work. Written what I think I've done right, asked what I've done wrong.. And before I click post. I realised what mistake I've made and delete the whole thing haha.

And I realise dumb I'd look

[Lasercookie]

For this thread, I've taken at least 3 pictures of my work. Written what I think I've done right, asked what I've done wrong.. And before I click post. I realised what mistake I've made and delete the whole thing haha.

I'll admit to doing that a lot too I think sometimes just setting out all the information you have while writing out the post and what you're having trouble with ends up making things clear for yourself.

Anyway, LaTeX might save you time scanning/taking photographs of all this handwritten stuff in, and also make things easier to read (especially if you have a low quality camera).

For just calculations it's easy enough to type that stuff out - the big part is probably doing the fractions. For most things it's pretty much how you'd type it out normally, where it really changes around is when you want special symbols and such.

Code: [Select]

[tex]n = \frac{m}{M}[/tex]

Code: [Select]

[tex]\frac{V_1}{T_1} = \frac{V_2}{T_2}[/tex]
which will give you and

For typing out some of the stuff in a previous post in this thread (just picked one at random)

Code: [Select]

[tex]m_f(dish + jam) =  26.44[/tex]

[tex]m_i(jam) = 30.95 - 20.22 = 10.73[/tex]

[tex]m(H_2O) = 10.73 - 6.27 = 4.46[/tex]

[tex]\% H_2O = \frac{4.46 \times 100}{10.73}[/tex]

The big part where LaTeX helps is for the chemical symbols / reactions:

You can use an underscore for the subscript. e.g. CO_2 and H_2O


If you need to include more than a single character in the subscript, e.g. for showing states, you can wrap things in curly braces { }

Code: [Select]

[tex]H_2O_{(l)}[/tex]


If you want to include charges etc. just use superscript H_3O^+ which gives you
Again the curly braces thing applies if you need to include more than a single character, Zn^{2+} gives you

If we want to write out a reaction, then we can use \rightarrow for the arrow

Code: [Select]

[tex]CH_4_{(g)} + 2O_2_{(g)} \rightarrow 2H_2O_{(g)} + CO_2_{(g)}[/tex]


For equilibrium arrows you can use

Code: [Select]

[tex]\leftrightharpoons[/tex]Which gives you

Code: [Select]

[tex]Fe^{3+}_{(aq)} + e^{-} \leftrightharpoons Fe^{2+}_{(aq)}[/tex]
(that's not entirely perfect if you want to be picky with formatting I guess)

That probably covers a lot of things and once you get your head around it it might save some time when it comes to making posts. There'd be a few things like oxidation numbers which I guess you could do using subscripts and other tricks you can use here and there.

[Mr Keshy]

I'll admit to doing that a lot too I think sometimes just setting out all the information you have while writing out the post and what you're having trouble with ends up making things clear for yourself.

Anyway, LaTeX might save you time scanning/taking photographs of all this handwritten stuff in, and also make things easier to read (especially if you have a low quality camera).

For just calculations it's easy enough to type that stuff out - the big part is probably doing the fractions. For most things it's pretty much how you'd type it out normally, where it really changes around is when you want special symbols and such.

Code: [Select]

[tex]n = \frac{m}{M}[/tex]

Code: [Select]

[tex]\frac{V_1}{T_1} = \frac{V_2}{T_2}[/tex]
which will give you and

For typing out some of the stuff in a previous post in this thread (just picked one at random)

Code: [Select]

[tex]m_f(dish + jam) =  26.44[/tex]

[tex]m_i(jam) = 30.95 - 20.22 = 10.73[/tex]

[tex]m(H_2O) = 10.73 - 6.27 = 4.46[/tex]

[tex]\% H_2O = \frac{4.46 \times 100}{10.73}[/tex]

The big part where LaTeX helps is for the chemical symbols / reactions:

You can use an underscore for the subscript. e.g. CO_2 and H_2O


If you need to include more than a single character in the subscript, e.g. for showing states, you can wrap things in curly braces { }

Code: [Select]

[tex]H_2O_{(l)}[/tex]


If you want to include charges etc. just use superscript H_3O^+ which gives you
Again the curly braces thing applies if you need to include more than a single character, Zn^{2+} gives you

If we want to write out a reaction, then we can use \rightarrow for the arrow

Code: [Select]

[tex]CH_4_{(g)} + 2O_2_{(g)} \rightarrow 2H_2O_{(g)} + CO_2_{(g)}[/tex]


For equilibrium arrows you can use

Code: [Select]

[tex]\leftrightharpoons[/tex]Which gives you

Code: [Select]

[tex]Fe^{3+}_{(aq)} + e^{-} \leftrightharpoons Fe^{2+}_{(aq)}[/tex]
(that's not entirely perfect if you want to be picky with formatting I guess)

That probably covers a lot of things and once you get your head around it it might save some time when it comes to making posts. There'd be a few things like oxidation numbers which I guess you could do using subscripts and other tricks you can use here and there.

Hey, thanks for that tip. I promised myself that I'd read your LaTex guide, I will do it this week though. I'll make sure my next question is asked using LaTex.