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% = mass solutemass solution 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 mass%100 = 1000 x 1.325 x 0.300 = 397.5 g
 

Step 2: determine the molarity of the solution:

nNaOH = mNaOHMNaOH    and    nNaOH = Msol x Vsol

⇒ Msol x Vsol = mNaOHMNaOH

⇒ Msol (in mol.L-1) = mNaOH (in g)MNaOH (in g.mol-1) x Vsol (in L) = 397.540.0 × 1.00 = 9.94 mol.L-1