sci-fi's chem qst.s

[sci-fi]

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.!

[Collin Li]

What's the question?

[sci-fi]

oh soz,

calculate the [H3O+], [OH-] and pH.

[Collin Li]

Since we know that :

[sci-fi]

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

[sci-fi]

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

[Collin Li]

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 Kelvin. That would be 3 significant figures, when we apply it to , 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.

[sci-fi]

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.

[Collin Li]

You need to have 50 posts to give Karma (it's to prevent multiple account abuse of Karma)

[sci-fi]

how revelant is unit 1 and 2 to retain for 3/4 chem?

[Collin Li]

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.

[sci-fi]

so all the oxugen, nitrogen, acid rain cycles etc[greenchemistry] are not important?

redox?

[Collin Li]

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.

[sci-fi]

Yeah, green chemistry is bullshit.

lol. i love green chemistry. {NB: not a hippy}
its just easy, watching diagrams flow.

[Collin Li]

Haha, they should save it for Environmental Science.