calculating molarity

Question

A solution is made by adding 33.9ml  of concentrated phosphoric acid (85.5wt%, density 170g/ml) to some water in a volumetric flask, and then adding water to the mark to make exactly 2000ml of solution. Calculate the concentration of this solution in molarity.

Answer 1

Using the Phosphoric Acid solution's density, you can find the mass of the solution: 33.9mL x (170g/mL) = 5763 grams H₃PO₄.

The solution has a %weight of 85.5%, which means that 85.5% of the 5763 grams is Phosphoric Acid. Therefore, (0.855)(5763) = 4927.365g of the solution is H₃PO₄ and the rest is water.

Molarity of a solution = mol solute / L solution.

To find the mol of the solute (H₃PO₄), convert grams to moles:

4927.365g H₃PO₄ x (1mol / 98g) = 50.7923...mol H₃PO₄ --> Plug this back into the Molarity equation:

Molarity of a solution = 50.7923...mol H₃PO₄ / 2 L = 25.1396...M = 25.1 M in 3 sig figs