another chem question: dimensional analysis

[hokie4life]

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10.0 mL of 0.40 M CuSO​

​

4 solution contains what mass
of CuSO4?
A. 0.40 mg
B. 4.0 mg
C. 384 mg
D. 400 mg
E. 638 mg​

E​

This is another question that can be solved using dimensional analysis:​

(​

​

160 g/1 mol CuSO4 ) x (0.40 mol/1 L ) x (.01L) = 0.64 g = 640 mg

Where are they getting the 1L from? shoudln't you use 22.4L cuz that's how many L there are in one mole?​

 

[Anemone]

10.0 mL of 0.40 M CuSO​

​

4 solution contains what mass
of CuSO4?
A. 0.40 mg
B. 4.0 mg
C. 384 mg
D. 400 mg
E. 638 mg​

E​

This is another question that can be solved using dimensional analysis:​

(​

​

160 g/1 mol CuSO4 ) x (0.40 mol/1 L ) x (.01L) = 0.64 g = 640 mg

Where are they getting the 1L from? shoudln't you use 22.4L cuz that's how many L there are in one mole?​

molarity=moles per liter. gases @ STP have a molar volume of 22.4L/mol.

 

[swooth01]

This is how I solved the problem.

Moles = Molarity x V

Molarity = .4M
V = 0.01L this is where they got the 0.01L from that you asked about

Moles = 0.4 x 0.01 = 0.004 moles

0.004 moles x 160g/mol = 0.640g so now you convert the g to mg which is times by 1000

so that gives you 640mg

hope this helps

 

[hokie4life]

This is how I solved the problem.

Moles = Molarity x V

Molarity = .4M
V = 0.01L this is where they got the 0.01L from that you asked about

Moles = 0.4 x 0.01 = 0.004 moles

0.004 moles x 160g/mol = 0.640g so now you convert the g to mg which is times by 1000

so that gives you 640mg

hope this helps

hi! thank you for your responses! okay I totally get this but the 1L I was talking about was not the .01L but the 1L under the .4M. I see how you get .01L but why do they also put 1L in? Molarity is m/liter but don't you use 22.4L for 1 mole?

I understand your explainations but I still can't follow why they put in 1L and not 22.4L in their setup for under .4M?

 

[Anemone]

hi! thank you for your responses! okay I totally get this but the 1L I was talking about was not the .01L but the 1L under the .4M. I see how you get .01L but why do they also put 1L in? Molarity is m/liter but don't you use 22.4L for 1 mole?

I understand your explainations but I still can't follow why they put in 1L and not 22.4L in their setup for under .4M?

see my post!

 

[bxs023100]

if they tell you .40 M of CuSO4, it means that .40M is the same as .4 mol/ 1 Liter. you dont use 22.4 in this problem at all bc they dont say anything about STP.

 

[thefifthbeatle]

hi! thank you for your responses! okay I totally get this but the 1L I was talking about was not the .01L but the 1L under the .4M. I see how you get .01L but why do they also put 1L in? Molarity is m/liter but don't you use 22.4L for 1 mole?

I understand your explainations but I still can't follow why they put in 1L and not 22.4L in their setup for under .4M?

Like the post said above, 22.4 is for GASES at STP

 

[swooth01]

hey hokie,

as everyone else has mentioned you don't use V = 22.4L unless it is under STP (273K and 1atm).

and remember that Molarity = Moles / L

so if something is 0.5M it's L =1

ie. 0.6M = 0.6 mol / 1 L
1.7M = 1.7 mol / 1 AL

 

[hokie4life]

ohhh okay yes all this totally makes sense now THANK YOU SO MUCH! I think I got messed up thinking .40 M meant moles and not Molarity even though I knew what molarity means I thoguht of the M to be moles and not molarity. Here's a spin off question just to check my recognition of variables then:

n = moles
M= Molarity
M or m= Molality as well? do I just have to be aware of what the problem is and what constants and vairalbes are used in it to be able to remember if they use molality (and not molarity or moles as the m) in freezing pt depression or that we use molarity in osmotic pressure, etc?

 

[Anemone]

ohhh okay yes all this totally makes sense now THANK YOU SO MUCH! I think I got messed up thinking .40 M meant moles and not Molarity even though I knew what molarity means I thoguht of the M to be moles and not molarity. Here's a spin off question just to check my recognition of variables then:

n = moles
M= Molarity
M or m= Molality as well? do I just have to be aware of what the problem is and what constants and vairalbes are used in it to be able to remember if they use molality (and not molarity or moles as the m) in freezing pt depression or that we use molarity in osmotic pressure, etc?

m is usually molality, and M is moles

for the colligative properties, you use molality.

 

[hokie4life]

thank you! 🙂