First, we calculate the amount of HCl remaining from the NaOH titration. Then we calculate the amount of iron oxide from the amount of HCl that reacted. Finally, the amount of water is the difference between the mass of the sample and the mass of the iron oxide present in the sample.
NaOH Titration of residual HCl
The balanced reaction for the iron oxide – HCl reaction is:
Molecular weight of
1.89g sample – 1.129g iron oxide = 0.761g water.
0.791g water is
Converting this to molecules with Avogadro's Number:
The amount of
This is an example of a back titration, in which you add excess
Step 1. Calculate the moles of
Step 2. Calculate the moles of
Step 3. Calculate the excess moles of
The equation for the titration is
Step 4. Calculate the moles of
Step 5. Calculate the moles of rust reacted
The equation for the reaction is
Step 6. Calculate the molar mass of the rust
Step 7. Calculate the value of
Thus, there are six molecules of water of hydration per formula unit of the rust.