College Chemistry
posted by Tabatha on .
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? Express your answer in g/ml

What is the molality of the solution in the phorphoric acid solution

Take the M of the solution times .85 to find the M of the phosphoric acid:
15 M x .85 = 12.75 M
If you assume 1L solution, that would mean there are 12.75 mol H3PO4 (Since M = mol solute/L solvent).
12.75 mol H3PO4 time the molecular weight of H3PO4 (98g/mol) gives you 1249.5 grams in 1 Liter of solution
So 1.2495g in 1mL of solution. Round the significant figures and the answer is 1.2 g/ml. 
15 M = 15 mol / L. In 1 L of solution there are 15 mol of H3PO4. molar mass of H3PO4 = 97.995 g / mol.
Then 15 mol x 97.995 g / mol = 1,469.9 g of H3PO4 which is the 85% of total mass of solution.
Total mass = 1,469.9 g / 0.85 = 1,729 g of solution in 1 L
Density solution = 1,469.9 g/ 1,000.0 mL = 1,729 g / mL
This is slightly high for a 85% H3PO4. The real value is 1.685 g / mL at 25 oC. This is due to the fact that the Molarity of 85% H3PO4 is 14.7 M not 15 M.