DAT Achiever Chem Question

[dentalengineer]

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What is the molality of 85% phosphoric acid, H3PO4, if the density of the solution is 1.70 g/ml?

I don't understant their explanation, could anyone explain how to solve the problem without paying attention to the given answers?

Thanks

 

[dlink]

Molality means # of mole of solvent (H3PO4) dissolved in 1 kg of solvent (H2O).

From the density of solution given, let's choose 1 ml of solution which gives you 1.70 g of solution.

Now, an 85% solution means a ratio of 85 g of solute to 15 g of solvent, meaning also that out of the 1.70 g of solution chosen, there're (1.70 x 0.85) g of solute and (1.70 x 0.15) g of solvent. But remember you'll need to express the solvent weight in kg:

(1.70/1000) x 0.15 ---- (1)

Next, let's do the # of mole of solute:

(1.70/M.W.) x 0.85 ---- (2) where M.W. = molecular weight of solute

molility of solution : (2)/(1) = [(1.70/M.W.) x 0.85]/[(1.70/1000) x 0.15] = (1000 x 0.85) / (M.W. x 0.15)

Note: Answer expression in DAT Achiever has not been fully simplified.

Let me know if any of the above need further explanation...

 

[dentalengineer]

Molality means # of mole of solvent (H3PO4) dissolved in 1 kg of solvent (H2O).
Excellent explanation, thanks.

 

[JavadiCavity]

Uhhhh, I actually remember knowing this information. I'm utterly useless now.

 

[americanpierg]

yup this stuff is gonna be just as useful as knowing the reproductive organs of monocious plants and how they function through weird interrelated plant hormones