Finding the concentration of a solution

We require the following information to determine the concentration of a solution:

• The balanced equation for the reaction
• The volume of the solution with an unknown concentration in the conical flask
• The volume of standard solution in the burette
• The concentration of the standard solution in the conical flask

Work example

Problem:

An acid-base titration is performed to find the concentration of sodium hydroxide. 50.0 mL of a solution of sodium hydroxide is neutralized by exactly 24.50 mL of hydrochloric acid of concentration 0.250 mol/L.

NaOH + HCl → NaCl + H₂O

Calculate the concentration, in mol/L, of the sodium hydroxide solution.

What we know:

Reaction: NaOH + HCl → NaCl + H₂O

Step 1. Calculating the moles of acid

Moles = concentration (mol/L) x volume of solution (mL) = 0.25 x 24.5/1000 = 0.006125 mol

Step 2. Calculating the moles of base

Use the stoichiometry of the balanced equation to calculate the moles of NaOH. The balanced equation shows that 1 mol of NaOH reacts with every 1 mol of HCl.

Moles of NaOH = moles of HCL = 0.006125 mol

Step 3. Calculating the concentration of base after dilution (10x)

Concentration (mol/L) = number of moles of solute (mol)/ volume of solution = 0.006125 /(50/1000) = 0.123 mol/L

Additional step. Calculating the concentration of base before dilution (if the sample was diluted)

Final concentration (mol/L) = concentration (mol/L) x dilution factor = 0.123 x 10 = 1.23 mol/L

*The answer is given in three significant figures because it’s the smallest number of significant figures provided in the data.