Hmm, I can see why this answer is puzzling - I'm not sure what the purpose is of including the density in the calculation when it is clearly unnecessary. That's also why it just cancels itself out in the equation... I worked it out though and my final (calculated) answer would be the same as the one given:
First, whenever they give you a percentage in a solution, it is a mass percent so you assume it is out of 100 g solution. So in this case 85% gives you (85g H3PO4/ 100g solution). Since the question asks for molality, you need to ultimately end up with moles of H3PO4 over kilograms of solvent. Dividing by A.W. of H3PO4 (1/98g) gives you moles. Now to get kg of solvent: if 85 out of 100g is the solute, then the 15g leftover is solvent. So for every 100g solution you have 15g solvent (100g/15g). Then you multiply by 1000 to convert grams to kilograms.
(85/100) (1/98) (100/15) (1000) = mol H3PO4/ kg solvent
That's the best I can do. I don't know what the other answer choices were but I would hope if we got this far we would choose the correct answer realizing that the densities cancel out... unless they have what I just wrote as one of the wrong answers - but that would just be cruel. Hope that helps...
