Molarity of 85% (w/w) Phosphoric acid (H3PO4)

• Phosphoric acid (H3PO4) is a clear colorless liquid. A 85% (w/w) concentrated Phosphoric acid can be obtained from different suppliers.
• A 85% (w/w) Phosphoric acid means that 100 g of Phosphoric acid solution contains 85 g of H3PO4.
• The density of 85% (w/w) Phosphoric acid is 1.685 g/ml at 25°C which means that the weight of the 1 ml of Phosphoric acid is 1.685 gram at 25°C.
• Molarity refers to the number of moles of the solute present in 1 liter of solution.
• In simple words, 1 mole is equal to the atomic weight of the substance. For example, 1 mole of H3PO4 is equal to 98 g of H3PO4 (molecular weight = 98).

Step 1: Calculate the volume of 100 grams of Phosphoric acid.
Formula:
Density =  weight / volume or
Volume =  weight / density or
Volume of 100 gram of Phosphoric acid : 100/1.685 =  59.347 ml

Note: 85% (w/w) Phosphoric acid means that 100 g of Phosphoric acid solution contain 85 g of H3PO4.

The volume of 100 grams of Phosphoric acid is 59.347 ml. That means 85 grams of H3PO4 are present in 59.347 ml of Phosphoric acid.

Step 2: Calculate how many grams of H3PO4 is present in 1000 ml of Phosphoric acid.
59.347 ml of Phosphoric acid contain    = 85 grams of H3PO4
1 ml of Phosphoric acid will contain    = 85/59.347 grams of H3PO4
1000 ml of Phosphoric acid will contain  = 1000 x 85/59.347 = 1432.254 grams of H3PO4
1000 ml of Phosphoric acid will contain 1432.254 grams of H3PO4

Step 3: Calculate the number of moles of H3PO4 present in 1432.254 grams of H3PO4.
98 grams of H3PO4 is equal to 1 mole.
1 gram of H3PO4 will be equal to 1/98 moles.
1432.254 grams will be equal to = 1432.254 x 1/98 = 14.615 moles
Therefore, we can say that 1 liter of Phosphoric acid contains 14.615 moles or in other words molarity of 85% (w/w) Phosphoric acid is equal to 14.615 M.

Use Calculator to calculate the molarity of concentrated Phosphoric acid (H3PO4) when concentration is given in % by mass (w/w)

Molecular weight of Phosphoric acid (H3PO4): 98.00 g/mole