# general chemistry II

Phosphoric Acid is usually obtained at an 85% phosphoric acid solution, by mass. If this solution is 15 M, what is the density of the solution? What is its MOLALITY?

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1. Assume 1000 grams of solution.
then you have 850 grams phosphoric acid, and 150 grams of water.

15M=850/molmassH3PO4 / volumeinL

solve for volume, = 850/(molmass*15), figure molmass, then calculate volume.
finally, density= masssoluton/volumeL=
= 1000g/volumeL figured above
you may want to change that to g/ml

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2. density x 1000 mL x %H3PO4 x (1/molar mass) = 15M
Solve for density = approximately 1.7 but you need to do it more accurately.

mass of 1000 mL = 1.73 x 1000 mL = 1730 g
mass H3PO4 = 1730 x 0.85 = 1470g
g solvent = 1730g-1470 = 260g
m = mols solute/kg solvent/kg solvent = 15/0.260 = about 57m.

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