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

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

Overview:

  • Phosphoric acid (H3PO4) is a clear colourless liquid. A 86% (w/w) concentrated Phosphoric acid can be obtained from different suppliers.
  • 85% (w/w) Phosphoric acid means that 100 grams of Phosphoric acid contains 85 grams of H3PO4.
  • The density of 85% (w/w) Phosphoric acid is 1.685 g/ml at 25°C that means that weight of the 1 ml of Phosphoric acid is 1.685 gram at 25°C.
  • Molarity refers to number of moles of the solute present in 1 litre of solution.
  • In simple words, 1 mole is equal to atomic weight of the substance. For example, 1 mole of H3PO4 is equal to 98 grams of H3PO4 (molecular weight = 98).

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Calculation procedure:

Known values:
Density of Phosphoric acid :                             1.685 g/ml
Molecular weight of H3PO4:                           98 g/mole

Concentration of Phosphoric acid:                85% (% by mass, wt/wt)

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 grams of Phosphoric acid contain 85 grams 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 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 litre of Phosphoric acid contains 14.615 moles or in other words molarity of 85% (w/w) Phosphoric acid is equal to 14.615 M.