A 30.00% (by mass) solution of HNO_3 in water has a density of 1.18 g/cm^3 at 20°C. What is the molarity of HNO_3 in the solution?

1 Answer
anor277
Apr 22, 2017

We are given that "Mass of nitric acid"/"Mass of solution"xx100% = 30.00%.

Explanation:

Now "Concentration"="Moles of solute"/"Volume of solution".

How do we turn one t'other? Well, we need the density of the solution, which you have supplied, rho=1.18*g*cm^-3, and some tedious 'rithmetic................

So we work with a 1*L volume of solution, i.e. 10^3*cm^3.

"Moles of solute"=(30.00%xx10^3*cancel(cm^3)xx1.180*cancelg*cancel(cm^-3))/(63.01*cancelg*mol^-1)

=5.62*mol

But this molar quantity was dissolved in a 1L volume of solvent.....

"Concentration"=(5.62*mol)/(1*L)=5.62*mol*L^-1.

Now I happen to know that conc. nitric acid, which is commercially supplied as a 68%(w/w) solution, has a molar concentration of approx. 15*mol*L^-1, so the value we calculated is reasonably consistent. You should keep doing these types of problems; it's all too easy to get bewildered and make an error.

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