Question

When asked to give the concentration of a 25% aqueous sodium hydroxide solution and a 5% aqueous sodium hydroxide solution, will the concentrations just be 25 mol/L and 5 mol/L, respectively? Is a 25% aqueous sodium hydroxide solution just 25 M, and a 5% aqueous sodium hydroxide solution just 5 M?

Answer 1

When we say 25% (w/v) aqueous sodium hydroxide solution, it means there are 25 g of NaOH in 100 mL of the solution.

Molar mass of NaOH = 40.0 g/mol

On the other hand molarity is defined as the number of moles of solute ( i.e. sodium hydroxide here) in a litre of solution and expressed in the units of mol/L or M.

Hence, the molarity of a 25% weight by volume solution of sodium hydroxide can be calculated as follows:

Number of moles of NaOH in 25 g in a V=100 mL = 0.100 L solution is

25 g n = mass molar mass 40.0 g/mol =0.625 mol

Hence, the molarity of 25% solution of NaOH can be calculated as

n molarity = = 0.625 mol - = 6.25 mol/L = 6.25 M 0.100 L

Hence, a 25% solution of sodium hydroxide is not 25 mol/L but actually is 6.25 mol/L or 6.25 M.

Similarly, we can calculate the molarity of 5% NaOH as follows:

5 % NaOH solution has 5 g of NaOH in 100 mL of solution.

Hence, the molarity of the 5% solution is

molarity = (5g/40.0 g. mol-) 0.100 L = 1.25 mol/L = 1.25 M

Hence, the answer is

NO, the 25% aqueous sodium hydroxide is not just a 25 M solution and 5% solution is not 5 M.