# Exercise 5 | Chemical Calculations for Solutions

A concentrated sodium hydroxide solution is 30.0% NaOH by mass and has a density of 1.325 g.mL-1 at 25°C.

Calculate the molarity of this solution.

mass% =  x 100

⇒ in the solution, there are 30.0 g of NaOH in 100 g of solution

Step 1: determine the mass of NaOH mNaOH in one liter of solution:

mNaOH (in g) = V (in mL) x d (in g.mL-1) x $\frac{\mathrm{mass}%}{100}$ = 1000 x 1.325 x 0.300 = 397.5 g

Step 2: determine the molarity of the solution:

nNaOH = $\frac{{\mathrm{m}}_{\mathrm{NaOH}}}{{\mathrm{M}}_{\mathrm{NaOH}}}$    and    nNaOH = Msol x Vsol

⇒ Msol x Vsol = $\frac{{\mathrm{m}}_{\mathrm{NaOH}}}{{\mathrm{M}}_{\mathrm{NaOH}}}$

⇒ Msol (in mol.L-1) =  =  = 9.94 mol.L-1