What are the limitations of a waters of hydration experiment? When a hydrated salt such as $\ce{CuSO4.5H2O}$ is subjected to heat, an anhydrous salt ($\ce{CuSO4}$) is produced. However the theoretical molar ratio of $\ce{H2O:CuSO4}$ and experimental molar ratio of $\ce{H2O:CuSO4}$ are not the same. Instead there is a percentage error, why does this occur? And how can we minimise this in this experiment?
The hydrated salt was left on hot plate at 70 degrees celsius for 25 mins.
You can mention experimental details and how you achieved 1:4 ratio, which may explained if we know the maximum temperature of heating.  CuSO4.5H2O loses water of hydration in steps as a function of temperature showing that those five water molecules are not equivalent. In this figure from German Wiki, follow the green line (ignore blue). The y-axis is % loss in weight and x-axis is the temperature. It clearly shows that water is lost in steps. The labels read Mass change (loss of crystal water). If your heating temperature is not enough you are left with CuSO4.H2O.