Question

An aqueous solution of concentrated hydrobromic acid contains 48% HBr by mass. If the density of the solution is 1.50g/mL, what is its concentration?
a.) 11.4 mol/L
b.) 8.9 mol/L
c.) 5.9 mol/L
d.) 18.5 mol/L

Answer 1

Hint: We must recall the basic definition of molarity and we must also keep in mind the correlation of volume and density of a solution. Molarity is the number of moles of solute per liter of solution.

Complete step by step answer:
Let us first assume that there is 1000mL of the resulting solution. So, we can write the given density as 1.5g/mL= 1.5 x 1000 = 1500g/L

We know,
The solution contains 48% HBr, which means the mass of HBr in the solution is:
\[\dfrac{{48}}{{100}} \times 1500 = 720\,g\,HBr\]
We can calculate the Molecular weight of HBr from the periodic table itself. It comes out to 80.912g/mol.
With this information we may now calculate the number of moles of HBr in the solution:
\[No.\,of\,moles\,of\,solute\,\,\, = \dfrac{{Weight\,of\,solute\,}}{{Molecular\,weight}} = \dfrac{{720}}{{80.912}} = 8.9\,moles\]
Hence, the correct answer is Option (B) 8.9 mol/L

Additional information:
Molar concentration can be used to convert between the mass or moles of solute and the volume of the solution.
Molarity is a measurement of the moles in the total volume of the solution, whereas Molality is a measurement of the moles in relationship to the mass of the solvent. Hence, the two should never be confused.

Note: We should use Molality instead of Molarity in experiments involving significant temperature changes. This is because as the volume of a solution increases with temperature, heating causes molarity to decrease; however, since molality is based on mass rather than volume, molality remains unchanged.