Molarity of 25% HNO3?

Question

Molarity of 25% HNO3(nitric acid)?

Answer 1

For 25% by weight:

I find it easiest to imagine the HNO₃ solution is 100 g.

First, let's find how many moles of HNO₃ are in the solution.

100 g 25% HNO₃ (by weight) would have 25 g HNO₃ and 75 g H₂O

HNO₃ molar mass = 63 g/mol

Therefore,

25 g HNO₃*(1 mol HNO₃/63 g HNO₃) = 0.40 mol HNO₃ in the solution

Next, Let's find the volume of the solution.

Density of HNO₃ = 1.51 g/mL

Therefore, 25 g HNO₃*(1 mL HNO₃/1.51 g HNO₃) = 16.6 mL HNO₃

Density of H₂O = 1.00 g/mL

Therefore, 75 g H₂O = 75 mL H₂O

Total volume of solution = volume of HNO₃ + volume of H₂O

Total volume of solution = 16.6 mL HNO₃ + 75 mL H₂O = 91.6 mL of solution

Now convert from mL to L

91.6 mL*(1 L/1000 mL) = 0.0916 L

Now we can calculate the molarity of the solution:

Molarity = (mol of HNO₃)/(L of solution)

Molarity = (0.40 mol HNO₃)/(0.0916 L solution) = 4.4M HNO₃ solution

For 25% by volume:

I find it easiest to imagine the HNO₃ solution is 1000 mL (or 1 L).

Now let's find how many mol of HNO₃ are in the solution:

1 L of 25% HNO₃ (by volume) would have 250 mL HNO₃

First let's convert ml to g:

Density of HNO₃ = 1.51 g/mL

Therefore,

25 mL HNO₃*(1.51 g HNO₃/1 mL HNO₃) = 377.5 g HNO₃

Now let's convert g to mol:

HNO₃ molar mass = 63 g/mol

Therefore,

377.5 g HNO₃*(1 mol HNO₃/63 g HNO₃) = 5.99 mol HNO₃

Now we can calculate the molarity:

Molarity = (mol of HNO₃)/(L of solution)

Molarity = (5.99 mol of HNO₃)/(1 L of solution) = 6.0 M HNO₃ solution

NOTE: The reason the molarity differs between 25% by weight and 25% by volume is HNO₃ density is not 1. (i.e. 1 g HNO₃ is not the same amount of HNO₃ as 1 mL HNO₃)