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VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: sci-fi on September 16, 2008, 08:48:17 pm

Title: sci-fi's chem qst.s
Post by: sci-fi on September 16, 2008, 08:48:17 pm: 1.00g of Ca(OH)2 dissolved in water to make 400mL solution (assume complete dissociation) ?

i've done this about 4 times but keep on getting the wrong answers.

cheers.!
Title: Re: sci-fi's chem qst.s
Post by: Collin Li on September 16, 2008, 08:52:13 pm: What's the question?
Title: Re: sci-fi's chem qst.s
Post by: sci-fi on September 16, 2008, 08:58:56 pm: oh soz,

calculate the [H3O+], [OH-] and pH.
Title: Re: sci-fi's chem qst.s
Post by: Collin Li on September 16, 2008, 09:06:42 pm: $\mbox{M(Ca(OH)}_2\mbox{)} = 40.1 + 2(16.0 + 1.0) = 74.1\mbox{ gmol}^{-1}$

$\mbox{n(Ca(OH)}_2\mbox{)} = \frac{1.00}{74.1} = 0.0135\mbox{ mol}$

$\mbox{n(OH}^-\mbox{)} = 2\times 0.0135 = 0.0267\mbox{ mol}$

$\mbox{[OH}^-\mbox{]} = \frac{0.0267}{0.400} = 0.0675\mbox{ M}$

Since we know that $\mbox{[H}^+\mbox{]}\mbox{[OH}^-\mbox{]} = 10^{-14}$ :

$\implies \mbox{[H}^+\mbox{]} = \frac{10^{-14}}{\mbox{[OH}^-\mbox{]}} = \frac{10^{-14}}{0.0675} = 1.48\times10^{-13}\mbox{ M}$

$\implies \mbox{pH} = -\log_{10}(1.48\times10^{-13}) = 12.8$
Title: Re: sci-fi's chem qst.s
Post by: sci-fi on September 16, 2008, 09:12:36 pm: thanks but my textbook says its incorrect. (hate it when this happens)

[H+]= 1.5 x 10^-13 M

[OH-]= 6.8 x 10^-2
Title: Re: sci-fi's chem qst.s
Post by: sci-fi on September 16, 2008, 09:14:07 pm: when do we round the sig figs?

edit:

i've read the thread with Mao (i think) explaining sig figs. but can not grasp his concept. lol

could someone please explain in short?

thanks
Title: Re: sci-fi's chem qst.s
Post by: Collin Li on September 16, 2008, 09:27:25 pm: My answers are consistent with those answers, if you round to 2 significant figures at the end. However, I believe there should be 3 in this example.

You round the significant figures at the end of your calculation, and never before!

We're given "1.00" (3 significant figures) and "400" (3 significant figures), and also "74.1" (molar mass from the Data Sheet is 3 significant figures)

Note that although I used "1.0" as part of my calculation for the molar mass (hydrogen atom), that should not limit the significant figures to 2. What you do is you add up everything, and then consider your total molar mass' significant figures.

How do you know when? Essentially, significant figures are important at the division and multiplication steps. I use the molar mass in division, but I don't use the "1.0" in division. I use that in subtraction and addition.

For example, when we talk about 25 degrees Celcius, that is not 2 significant figures, because when we calculate temperature, we use Kelvin, which is $25+273 = 298$ Kelvin. That would be 3 significant figures, when we apply it to $pV=nRT$ , for example.

Also, note that textbooks don't necessarily pay great attention to significant figures themselves. They're quite lazy about it, so don't be fazed if your answer is only wrong by significant figures.
Title: Re: sci-fi's chem qst.s
Post by: sci-fi on September 16, 2008, 09:32:16 pm: thank you stacks.
(p.s: how come i cant give KARMA? , is karma phased out during this hour? )

edit: my friend on msn told me that at same time,(need 50 posts+) lol.
Title: Re: sci-fi's chem qst.s
Post by: Collin Li on September 16, 2008, 09:33:06 pm: You need to have 50 posts to give Karma :P (it's to prevent multiple account abuse of Karma)
Title: Re: sci-fi's chem qst.s
Post by: sci-fi on September 16, 2008, 09:38:32 pm: how revelant is unit 1 and 2 to retain for 3/4 chem?
Title: Re: sci-fi's chem qst.s
Post by: Collin Li on September 16, 2008, 09:40:39 pm: Stoichometry (stuff like this) is very relevant. You should try to get a good grasp of it (understand what you're doing) before Units 3&4.
Title: Re: sci-fi's chem qst.s
Post by: sci-fi on September 16, 2008, 09:42:45 pm: so all the oxugen, nitrogen, acid rain cycles etc[greenchemistry] are not important?

redox?
Title: Re: sci-fi's chem qst.s
Post by: Collin Li on September 16, 2008, 09:46:18 pm: Yeah, green chemistry is bullshit.

Redox is fairly important, but don't worry too much about trying to understand electrochemistry a lot -- it will come again (as a rather significant topic), but it won't be assumed knowledge. It will be re-taught again basically.
Title: Re: sci-fi's chem qst.s
Post by: sci-fi on September 16, 2008, 09:48:17 pm: Quote from: coblin on September 16, 2008, 09:46:18 pm
Yeah, green chemistry is bullshit.

lol. i love green chemistry. {NB: not a hippy}
its just easy, watching diagrams flow.
Title: Re: sci-fi's chem qst.s
Post by: Collin Li on September 16, 2008, 09:50:10 pm: Haha, they should save it for Environmental Science.
Title: Re: sci-fi's chem qst.s
Post by: Mao on September 16, 2008, 10:24:22 pm: understanding hazards, dangers and environmental impacts are a small part of both unit 3 and 4

but with that said, it's only a small part. if you pay attention to safety in pracs, and understand the very basics of acid rain (when you eventually learn about sulfuric acid), you don't need to spend much more effort on top of that.
Title: Re: sci-fi's chem qst.s
Post by: sci-fi on September 17, 2008, 08:25:58 am: Find the [H+] given that [OH-]=2.8 x 10-5

not sure of the [H+] in this qst. (is it 3.57 x 10-10M)

cheers.
Title: Re: sci-fi's chem qst.s
Post by: Mao on September 17, 2008, 02:11:25 pm: [H⁺]*[OH^-]=10^-14 M²
Title: Re: sci-fi's chem qst.s
Post by: sci-fi on September 17, 2008, 05:01:03 pm: 6) A swimming pool has a volume of one million litres. How many grams of HCl would need to be added to that swimming pool to bring the pH down from 7 to 4? (assume the volume of the HCl is negligible)

edit:

5) Find the pH and pOH of a solution with a volume of 5.4L that contains 15grams of HCl and 25grams of HNO3 (nitric acid)?
Title: Re: sci-fi's chem qst.s
Post by: Collin Li on September 17, 2008, 05:05:06 pm: Quote from: sci-fi on September 17, 2008, 08:25:58 am
Find the [H+] given that [OH-]=2.8 x 10-5

not sure of the [H+] in this qst. (is it 3.57 x 10-10M)

Yes, you're right, although you should have 2 significant figures.

$3.6 \times 10^{-10}\mbox{ M}$

Quote from: sci-fi on September 17, 2008, 05:01:03 pm
6) A swimming pool has a volume of one million litres. How many grams of HCl would need to be added to that swimming pool to bring the pH down from 7 to 4? (assume the volume of the HCl is negligible)

$\mbox{pH} = 4 \implies \mbox{[H}^+\mbox{]} = 10^{-4}\mbox{ M}$

$\mbox{n(H}^+\mbox{)} = 10^{-4}\mbox{ M} \times 10^6\mbox{ L} = 10^2\mbox{ mol}$

$\mbox{m(HCl)} = 10^2 \times (1.0 + 35.5) = 3650\mbox{ g}$
Title: Re: sci-fi's chem qst.s
Post by: sci-fi on September 17, 2008, 05:06:25 pm: thanks coblin, champ.
Title: Re: sci-fi's chem qst.s
Post by: Collin Li on September 17, 2008, 05:11:31 pm: Quote from: sci-fi on September 17, 2008, 05:01:03 pm
5) Find the pH and pOH of a solution with a volume of 5.4L that contains 15grams of HCl and 25grams of HNO3 (nitric acid)?

Steps

1. Calculate the total amount of $\mbox{H}^+$ present by calculating the amount of $\mbox{HCl}$ and $\mbox{HNO}_3$ , and adding up all the protons released by them.

$\mbox{n(H}^+\mbox{)}_{\mbox{total}} = \mbox{n(HCl)} + \mbox{n(HNO}_3\mbox{)} = 0.81\mbox{ mol}$

2. Now find the concentration of $\mbox{H}^+$ (divide by 5.4 L), and hence find the concentration of $\mbox{OH}^-$

Use these rules:

$\mbox{pH} = -\log_{10}\mbox{[H}^+\mbox{]} = 0.83$

$\mbox{pOH} = -\log_{10}\mbox{[OH}^-\mbox{]} = 13$
Title: Re: sci-fi's chem qst.s
Post by: sci-fi on September 17, 2008, 05:15:03 pm: cheers, i dont understand pOH because its not in our textbook.

what exactly is pOH?

edit: power of hydroxide?
Title: Re: sci-fi's chem qst.s
Post by: Collin Li on September 17, 2008, 05:26:59 pm: You just need to use these formulas:

$m = nM$

$n = cV$

$\mbox{pH} = -\log_{10}\mbox{[H}^+\mbox{]}$

$\mbox{pOH} = -\log_{10}\mbox{[OH}^-\mbox{]}$

$\mbox{[H}^+\mbox{]} \cdot \mbox{[OH}^-\mbox{]} = 10^{-14}$

The pOH is the exact same mathematical function, except applied to the concentration of hydroxide ions, rather than protons. I guess you could call it that (power of hydroxide).

Just try to use a bit of commonsense (don't be afraid to guess), when you're faced with some unknowns that look similar. One day, if you do Chemistry in first year university, you'll see $\mbox{pK}_a$ . No rewards for guessing the formula for that!
Title: Re: sci-fi's chem qst.s
Post by: sci-fi on September 17, 2008, 05:31:29 pm: ok. thanks coblin.

edit: just removed some questions because i understand the pOH now. :police: (fuuny emoticon; so i'll jst use it) :police: :police: :police:
Title: Re: sci-fi's chem qst.s
Post by: sci-fi on September 20, 2008, 02:02:55 pm: I understand that Chemistry Units 1/2 are the foundations to 3/4.. (derr..anyone can figure that out) but are the topics/area of studies in Chemistry 1/2 examinable in the 3/4 exams? Because for methods and spesh (i think), the units 1/2 or gen. math adv. are included in the 3/4's.

edit: for example. will i have to memorise and revise the entire book of units 1/2,,, the history. periodic arrangment of the subshells etc etc
Title: Re: sci-fi's chem qst.s
Post by: Collin Li on September 20, 2008, 02:56:06 pm: Hmm... Don't worry about the history, but you should try to know the subshell arrangements (but honestly, you don't really need to know it for Units 3&4, but it pains me to tell someone not to bother with the more fundamental details of Chemistry).

You will find a lot of stoichiometry in Units 3&4. Understand redox and acid-base processes well.
Title: Re: sci-fi's chem qst.s
Post by: sci-fi on September 20, 2008, 03:25:50 pm: thanks mate!! (yay no history)& subshell config