Question

A 20.0 % by mass solution of phosphoric acid (H3PO4) in water has a density of 1.114 g/mL at 20°C. What is the molarity of this solution?

Answer 1

Answer:

Molarity is 2.272M

Explanation:

The mass % of phosphoric acid is 20.0%

It means in 100 g of solution there is 20g of phosphoric acid in it.

Molarity = moles of solute dissolved per litre of solution.

So we need volume of solution

Volume can be calculated from mass and density as

Volume = mass / Density = 100/1.114 = 89.77 mL = 0.08977 L

Moles of solute = mass of solute / molar mass of solute

moles of solute = 20g / 98 = 0.204

molarity = $\frac{moles}{volume}=\frac{0.204}{0.08977} =2.272M$

Answer 2

Assume you have 1 L of the material (you can assume any volume you want, you'll get the same answer). 

Using the density, the one L has a mass of 1114 g 

20% by mass of phosphoric acid means the mass of phosphoric acid is 0.20 * 1114 g 

Calculate the moles of phosphoric acid 

Molarity, M, mole / L = moles of acid / volume or 1 L