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AS Hard mole question on solutions and concentrations

Find the mass concentration, in mol dm-3 for the following solutions.

1) 0.042 moles of HNO3 dissolved in 250cm3 of solution.

2) 0.500 moles of HCl dissolved in 4dm3 of solution.

3) 3.56-3 moles of H2S04 dissolved in 25cm3.

I tried and tried but just dont get it by the way this is at AS level.

I have the answers but no clue how to get them.

I think your supposed to use the equation n= c x V (in dm3) for the first and last question, and for question two your supposed to use
n = (V in cm3 / 1000) x c

Thanks
Reply 1
Avatar for Narik
Narik
13
1. You would just divide 0.042 moles by the volume in dm3, which is 0.25dm3 - there's your concentration.
2. Same as above.
3. Same as above, except its already in dm3.
4. Same as above

What I think you're doing wrong is that you're trying to work out number of moles, whereas you're meant to work out the concentration... [well that's what it seems like anyway].
Reply 2
Avatar for Miss Mary
Miss Mary
14
dumbgeek
Find the mass concentration, in mol dm-3 for the following solutions.

1) 0.042 moles of HNO3 dissolved in 250cm3 of solution.


250cm3⟶0.042moles250 cm^3 \longrightarrow 0.042 moles
1000cm3⟶x=0.168moles1000 cm^3 \longrightarrow x=0.168 moles

As 1000cm3=1dm31000 cm^3 = 1 dm^3 , concentration is 0.168molesdm30.168 \frac{moles}{dm^3}.

That's how I'd do it :p:
Reply 3
Avatar for Sundarii
Sundarii
1) convert the cm3 into dm3 1st :biggrin:
conc^ = moles / vol
= 0.042 / 0.250 = 0.168 mol dm-3

2) conc^ = moles / vol
0.500 / 4 = 0.125 mol dm-3

3) 3.56-3 << is that taking away 3? I dont understand :/
0.560 moles??

if thats the case,
0.560 / 0.0250 = 22.4 mol dm-3

Hope I'm right.. If I'm wrong, do correct me :smile:
Reply 4
Avatar for no_one001
no_one001
12
concentration=moles/volume in dm^-3
Reply 5
Avatar for dumbgeek
dumbgeek
OP
1
Miss Mary
250cm3⟶0.042moles250 cm^3 \longrightarrow 0.042 moles
1000cm3⟶x=0.168moles1000 cm^3 \longrightarrow x=0.168 moles

As 1000cm3=1dm31000 cm^3 = 1 dm^3 , concentration is 0.168molesdm30.168 \frac{moles}{dm^3}.

That's how I'd do it :p:


Thats wrong the book says it 10.58 I got that answer at first
Reply 6
Avatar for dumbgeek
dumbgeek
OP
1
Sundarii
1) convert the cm3 into dm3 1st :biggrin:
conc^ = moles / vol
= 0.042 / 0.250 = 0.168 mol dm-3

2) conc^ = moles / vol
0.500 / 4 = 0.125 mol dm-3

3) 3.56-3 << is that taking away 3? I dont understand :/
0.560 moles??

if thats the case,
0.560 / 0.0250 = 22.4 mol dm-3

Hope I'm right.. If I'm wrong, do correct me :smile:


the answer is in g mol dm -3 itS MASS CONCENTRATION not just CONCENTRATION

this had really baffled me lol
dumbgeek
the answer is in g mol dm -3 itS MASS CONCENTRATION not just CONCENTRATION

this had really baffled me lol

To be fair, in the initial question you did say "in mol dm^-3"
Ok the way you do it is this:
Moles= MV/1000
Rearranging this gives:
M(concentration) = moles x 1000/V
= (from the question)
= 0.042 x 1000/250
= 0.168 mol dm-3
then to convert it into gm mol dm-3 as you said u do:
= 0.168 x (Ar of HNO3)
= 0.168 x 63 = 10.58 (2 d.p) gm mol dm-3
Its easy...You have show them as moles/dm3. 1dm3 = 1000cm3.1. 0.042 MOLES IN 250 cm3 250:1000 1:4 Therefore 0.042 multiplied by 4. =0.168 moles/dm3

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