Back Titration Calculation for HSC Chemistry Students

Mastering calculation questions are key to success in HSC Chemistry. Let’s have a look at a back titration question you will see in the HSC exam or your school trial exam.

An indigestion tablet was crushed up and 25 mL of 1.00 M hydrochloric acid was added (in excess). The resulting solution was titrated against 0.5 M NaOH requiring 25.8 mL. Calculate the mass of calcium carbonate in the tablet.

Understanding the question:

What is essentially happening in this example is we are taking a tablet that is primarily calcium carbonate (CaCO3) and reacting it with hydrochloric acid (HCl). That is our first acid-base reaction. However, the CaCO3 is the limiting reagent i.e. it runs out first. So we take the remaining HCl and use that to perform a titration with sodium hydroxide (NaOH). Using the results, we are going to try and work backs to get the original mass of CaCO3

Solving the question:

Step 1: Write an equation.

Reaction 1:

Reaction 2 (Titration):

Step 2: Find the number of moles of NaOH used in Reaction 2 i.e. the titration.

So, using the formula, we get…

Step 3: Find the number of moles of HCl that was used in the titration.

Since the mole ratios of HCl to NaOH is 1:1, the number of moles of HCl used is the same as the number of moles…