[browniee]

This is a question from one of my practice papers.
6.853g of sodium cyanide was dissolved in water and brought to a volume in a 100ml volumetric flask. The final mass of the solution was found to be 107.166g.

Calculate the composition of the solution expressed as :
1) Moles, 2) Molarity, 3) Normality, 4) weight/volume %, 5) weight/weight %

For (1) -> Mole composition = moles of NaCN / moles of total mixture
Moles of 6.853g of NaCN = 6.853/49.008 = 0.13983 moles
Mass of remaining water = 107.166 - 6.853 = 100.313g
Moles of this water = 100.313/18.0148= 5.56837 moles
Total moles = 0.13983 + 5.56837 = 5.7082 moles
mole composition = 0.13983/5.7082 = 0.0245

For (2) -> using the same method, i tried to do it but struggled.
Molarity composition = molarity of NaCN / Molarity of solution

Molarity of 6.853g of NaCN that went to the water = 0.13983 /  0.1 = 1.3983 M
Then i got stucked here, arent molarity of 6.853g of NaCn that went into the water same as molarity of the total solution ? Nevertheless i still did it.
Molarity of total solution = 5.7082 / 0.1 = 57.082M
Molarity composition = 1.3983/57.082

For (3) ->
i only know how to calculate the normality of 6.853g of NaCN -> 1.3983 M = 1.3983 x 1 = 1.3983 N
but i got stuck at the normality of total solution

For (4) -> weight/volume % = mass of solute / volume of total solution x 100% .
i just substituted in the relevant numbers.

For (5) -> weight/weight % = mass of solute / mass of total solution x 100%

i believe i am wrong some where. especially from (1)-(3) as when i did it, i did not think it was right. am i correct on my concept? or which part should i make changes? Thanks !

[Borek]

I feel like you are overcomplicating things. Molarity is just that - molarity. I have never heard about "molarity composition", defined the way you listed.

[browniee]

Then is by part 1 correct ? it ask for composition of the solution. Does this mean i use the 107.166g ?

[mjc123]

Parts 1, 4 and 5 are correct.

Molarity composition = molarity of NaCN / Molarity of solution

There is no such thing as "molarity composition" (and if there was, as you define it, it would be the same as the "mole composition" you calculated in part 1). The "composition of the solution expressed as molarity" is 1.3983 M NaCN. That's it. Likewise for the normality.

[browniee]

what is the difference between these 2 questions then ? ( just to clear off my doubts)

1. Calculate the composition of the solution expressed as moles

2. Amount of NaCN in the solution or in the terms i use - number of moles of NaCN in the solution.



For part 2, does it mean 6.853/49.008 ? or is it the same as part 1 like what i did ?

[mjc123]

"Composition of the solution expressed as moles" is an unusual expression; I would take it to mean specifying the number of moles of each component present; or alternatively, the mole fractions. Thus a solution comprising 1 mole A and 9 moles B would have mole fractions of 0.1 A, 0.9 B.
"Amount" generally means number of moles (it is sometimes used more loosely, to mean mass, volume etc.). So the amount of NaCN in your solution is 0.13983 moles.
The difference between the 2 questions is that no. 2 asks for the number of moles of NaCN present; no. 1 (as I understand it) asks for the number of moles of each component. But it is a poorly expressed question.