How to Calculate Acid Concentration in Chemistry Problems

Concentration quantifies the amount of a dissolved compound in the solution. Molarity is the most common metrics for concentration and represents the number of moles of the dissolved substance in one liter of the solution. The common chemistry problem on the acid concentration is to calculate it from the titration. Titration is a technique that allows to determine quantity of the acid based on its chemical reaction with the solution of a base. As an example, calculate the concentration of the hydrochloric acid (HCl) solution if its 20 ml aliquot can be completely titrated out with 15.6 ml of the 0.25 molar solution of sodium hydroxide (NaOH).

Things You'll Need

• Calculator

Instructions

1. Write down the chemical reaction equation the titration is based on. In our example, it is the reaction of the neutralization expressed as HCl + NaOH = NaCl + H2O.

2. Multiply the volume of the titrant solution (in liters) by its concentration to calculate the number of moles of NaOH used for the acid titration. In our example, the volume of the NaOH solution used is 15.6 ml or 0.0156 L. Hence, the number of moles (NaOH) = 0.0156 L x 0.25 mole/L = 0.0039 moles.

3. Determine the number of moles of the acid using the equation in Step 1. In our example, one molecule of HCl reacts with one molecule of NaOH. Thus, 0.0039 moles of NaOH (Step 2) will interact with 0.0039 moles of HCl.

4. Divide the acid number of moles by the aliquot volume (in liters) to calculate the acid concentration. In the example, acid concentration is 0.0039 moles/0.02 L = 0.195 mole/L.

References

• Chemistry; K.W. Whitten, R.E. Davis, L. Peck, and G.G. Stanley; 2009.

Resources

• Acid-base reaction