Post by: epinephrine on July 23, 2011, 08:39:42 pm
Decided to make my own thread for my year 11 chemistry questions so any help would be much appreciated!
Anyways first question:
1. 10.0g of NaOH was completely dissolved in water so that the final volume of solution was 200 ml. What is the concentration of this solution?
Post by: REBORN on July 23, 2011, 08:47:43 pm
Post by: ariawuu on July 23, 2011, 08:51:15 pm
We know the volume (0.2L)
We need to find the moles of NaOH (n)
the equation for the n(NaOH)=m/M, whereby mass is in grams.
Sub in what we know; m= 10g, M= mass number of Na plus mass number of O plus mass number of H,refer to periodic table.
Work what n is then divide by 0.2 will give you your concentration.
Hope that was helpful. :)
Post by: ariawuu on July 23, 2011, 08:53:38 pm
Is it 1.25M?yerp.
Post by: epinephrine on July 23, 2011, 08:56:26 pm
Post by: ariawuu on July 23, 2011, 09:00:21 pm
eg find the concentration(m/v%) of this then you would use mass of solute/volume ^^
whats the answer at the back of your book?
EDIT: that was a good question, but if it's not asking you specifically for m/v% then you would assume its n/v
Post by: tony3272 on July 23, 2011, 09:00:48 pm
Haha thank you but why do we use C = n x V , rather than using concentration = mass of solute / volume ?The formula for concentration is c=n/V rather than c=n x V. Using n rather than m is just so that it's units are in
Post by: REBORN on July 23, 2011, 09:01:36 pm
Haha thank you but why do we use C = n x V , rather than using concentration = mass of solute / volume ?it's c = n DIVIDED BY V.
You can find it your way but you'll get it in g/L...the king of concentration is M, molarity, n/L! :)
EDIT: Beaten.
Post by: Lasercookie on July 23, 2011, 09:01:58 pm
Post by: ariawuu on July 23, 2011, 09:05:22 pm
I think the reason is because n = c x V gives you the answer in Molars (1M = 1 mol/L) which is a nicer unit.
Haha thank you but why do we use C = n x V , rather than using concentration = mass of solute / volume ?it's c = n DIVIDED BY V.
You can find it your way but you'll get it in g/L...the king of concentration is M, molarity, n/L! :)
EDIT: Beaten.
Haha thank you but why do we use C = n x V , rather than using concentration = mass of solute / volume ?The formula for concentration is c=n/V rather than c=n x V. Using n rather than m is just so that it's units are in)
probably was a typo....
Post by: epinephrine on July 23, 2011, 09:07:52 pm
Haha but thanks anyways guys !
Post by: ariawuu on July 23, 2011, 09:09:36 pm
and well done for creating your own chem thread :)
Post by: epinephrine on July 24, 2011, 12:25:00 pm
2. calculate the molar concentration present in 1.55L of solution, which contains 1.223g of CaCl2
Ok so I got 7.11 x 10-3 but apparently thats wrong?
Post by: REBORN on July 24, 2011, 12:29:09 pm
Post by: epinephrine on July 24, 2011, 12:31:28 pm
Post by: REBORN on July 24, 2011, 12:33:00 pm
Post by: Lasercookie on July 24, 2011, 12:34:23 pm
Chapter 11, 12a? I'll check what the Heinemann worked solutions did.
Edit:
This is what Heinemann did:
Quote
n(CaCl2) = m/M = 1.223/75.5 = 0.0162 mole
c(CaCl2) = n/V = 0.0162/1.55 = 0.0105 M
I'm positive they calculated that Molar Mass of CaCl2 wrong.
Post by: epinephrine on July 24, 2011, 12:34:42 pm
Post by: pi on July 24, 2011, 12:34:57 pm
I got the same answer...who said it was wrong?
Units? :P
Nah, your figure is correct, answers must be wrong :)
Post by: tony3272 on July 24, 2011, 12:35:15 pm
Post by: REBORN on July 24, 2011, 12:37:41 pm
Check the back of the book...they give it right as 0.00711M.
(my v. is ENHANCED, maybe the old edition has it wrong on the back of the book too...)
Post by: epinephrine on July 24, 2011, 12:39:13 pm
a. 4M
b. 0.075 M
c. .475 M
d. 2.76 g/L
Post by: Lasercookie on July 24, 2011, 12:39:22 pm
Oh man don't ever use the worked sols. They are absolutely shocking...wait till you get to pH questions (c14).Yeah old edition has the wrong answer at the back of the book.
Check the back of the book...they give it right as 0.00711M.
(my v. is ENHANCED, maybe the old edition has it wrong on the back of the book too...)
Post by: tony3272 on July 24, 2011, 12:43:20 pm
ok cool man there are so many incorrect solutions in that book, anyways what about question 13 same page could you guys confirm if my answers are correct?
a. 4M
b. 0.075 M
c. .475 M
d. 2.76 g/L
c. should be 0.025.
the solution is diluted to 100mL, so new concentration is
Post by: REBORN on July 24, 2011, 12:43:32 pm
ok cool man there are so many incorrect solutions in that book, anyways what about question 13 same page could you guys confirm if my answers are correct?C is wrong. And (d) is automatically wrong 'cause it's not in M (n/L)
a. 4M
b. 0.075 M
c. .475 M
d. 2.76 g/L
EDIT: BEATEN!! DAMN
Post by: epinephrine on July 24, 2011, 12:48:14 pm
Post by: REBORN on July 24, 2011, 12:49:40 pm
your answer to (d) is not even answering the question...do you get what I mean? Molar conc = M = n/L
I uh don't know how to do latex so I'll leave someone else to do the working *cough, lazy*
Post by: epinephrine on July 24, 2011, 12:51:20 pm
Post by: REBORN on July 24, 2011, 12:52:23 pm
haha, ignore me -.-