# Back Titration

Preparation for Lab – A Guided Back Titration Calculation

Learning Objective:

Calculate analyte concentrations given experimental data from a back titration.

You are asked to determine the mass of calcium carbonate in a 0.125 g stick of chalk. You begin by dissolving the chalk in 50.00 mL of 0.200 M HCl. Then, you titrate any excess HCl with a standardized solution of KOH. You need 32.12 mL of 0.250 M KOH to reach the endpoint. What is the percent by mass of calcium carbonate in the chalk?

Let’s break it down!

1. Draw a rough sketch of the steps in the experiment.

1. Write chemical equations for…
1. the complete reaction of calcium carbonate with HCl

2. the reaction of HCl with KOH.

1. How many moles of KOH did you use in the back titration?

1. How many moles of HCl were present in solution after the chalk dissolved?

1. How many moles of HCl did you add to the solution?

1. How many moles of HCl reacted with the calcium carbonate?

1. How many moles of calcium carbonate were present?

1. What is the wt% of calcium carbonate in the 0.125 g stick of chalk?