Question about Gen Chem

[nt4reall]

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What is the molality of 85% phosphoric acid , H3PO4 if the density of the solution is 1.7g/mL? ANybody , help please.

 

[Maygyver]

Wouldn't you just do 1.7g H3PO4/98g=0.01734mol/0.0001kg= 17.34m. I could be wrong though. Someone else will have to verify.

 

[drgreen]

maybe since molality is the number of moles of solute per kilogram of solvent.

# moles of solute:
lets use 1 mL of solution
1 mL solution weighs 1.7g 85% of which is H3PO4 which gives us 1.45g H3PO4. convert to Moles: 1.45/98=.015mol

Kg Solvent:
in 1 mL there is 1.7g solution.
this is .0017 Kg solution.

Molality=mol solute/Kg solvent
=.015mol/.0017 Kg=8.82m

 

[joonkimdds]

ooooops

 

[joonkimdds]

0.017mol over 0.001kg = 17.34molality.
but it's 85% so times 0.85 should give 14.74molality

I don't understand why drgreen didn't apply 85% to kg of solvent.

 

[ADentalHopeful]

i'll try at this
i dont know if this is right
but

(85g/100g)(mole/98g)(1.7g/ml)(10^3ml/L)=14.7 M

hard to show, but everything cancels but Mole/L

and it is the same as if you leave out the 85/100 and later pretend it is like 17.3 M represents 100% so 85% is then 14.7M