i need help

[nathan yansky]

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i am having truoble doing molar calculations
can somone help me with this type of calculation

Calculate the concentration of anions if 300 mL of a 0.4564 M solution of iron(III) sulfate is diluted with water to make 300.0 mL in a volumetric flask. Represent your answer to the correct number of significant figures

if someone could help thta would be great
thanks

[abdooooo!!!]

Originally posted by nathan yansky
Calculate the concentration of anions if 300 mL of a 0.4564 M solution of iron(III) sulfate is diluted with water to make 300.0 mL in a volumetric flask. Represent your answer to the correct number of significant figures

300mL diluted to 300.0mL. maybe i misread the question, but how is 300mL diluted to 300.0mL?

for concentration questions you just use the cV1 = cV2 formula, because concentration times volume will give you the mole, and the amount of moles never changes no matter how much you dilute the solution.

and for significant figures in your answer, it should be the least accurate of the figures given in the question. for example 300mL and 300.0mL, 300mL will be the least accurate, so its 3 sig.fig. because you can not be more accurate than 3 sig.fig if you deriving your answer from 300mL.

hope that helps

[+:: $i[Q]u3 ::+]

hey dude! =)

when ya got molar calculation probs and ur not sure try *not* to use cv1=cv2... if the mole ratios are different then u screw up and u'll not know why...

Whatever u do, start from the basics; and if you have absolutely no idea, calculate the number of moles in whatever you have, then see if u can use relevent formulae:

n = m / Mm
ie number of moles = mass / molar mass

c = n / V
or M = n / V
ie concentration or molarity = number of moles / volume

so.. for a concentration prob.....

you have the volume and concentration of the solution eg 300ml of 0.4564M ferric sulfate
so calculate the number of moles using n =c/V

that's the number of moles of ferric sulfate
but when ferric sulfate dissociates in water,
Fe2(SO4)3 ---> 2 Fe + 3 SO4
the anion is sulfate, and there are 3 times as many sulfate ions as feSO4 moles so then u can calculate the number of sulfate ions... (n times 3)

you have number of moles of the anion! divide it by the volume (whether you dilute it or not) since c = n/V
so you have concentration of whatever... ^^
all done~!!!

good luck ~

[abdooooo!!!]

Originally posted by +:: $i[Q]u3 ::+
when ya got molar calculation probs and ur not sure try *not* to use cv1=cv2... if the mole ratios are different then u screw up and u'll not know why...

why??? if you worked out cV1 = cV2 just times c by 3, since there is 3 SO4^-2 per Fe2SO4. you timed n by 3, which is the same as times c by 3 because n = cV and V remains the same no matter what ratio of moles you got.

[+:: $i[Q]u3 ::+]

why??? if you worked out cV1 = cV2 just times c by 3, since there is 3 SO4^-2 per Fe2SO4. you timed n by 3, which is the same as times c by 3 because n = cV and V remains the same no matter what ratio of moles you got.

yeh it's just a difference in method...

if you use cv1=cv2, then what you do end up with is the concentration of fe2so4
but the question asks for the conc of anions

it's perfectly valid to just multiply the molarity of the solution by the ratio of the number of anions
but the thing i find with this is a lotta ppl forget, or don't understand why they're doing it

i find it easier to start from basic concepts.. it's easier to find errors in working out, and the examiner knows that you understand what it's all about in the first place.

it's just our chem teacher's pov and it's worked dayum well so far but yeh... whatever turns you on ^^

[+:: $i[Q]u3 ::+]

oh dammit

n = cv

sorry -.-;;

[abdooooo!!!]

Originally posted by +:: $i[Q]u3 ::+
it's perfectly valid to just multiply the molarity of the solution by the ratio of the number of anions
but the thing i find with this is a lotta ppl forget, or don't understand why they're doing it

i find it easier to start from basic concepts.. it's easier to find errors in working out, and the examiner knows that you understand what it's all about in the first place.

thats fair enough... i guess its always better to understand what you doing rather than just using formulas.

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