In this experiment you will determine the value of y in the formula for hydrated barium chloride, BaCl₂·yH₂O - CIE - A-Level Chemistry - Question 2 - 2017 - Paper 1

To find the number of moles of barium chloride, use the following formula:

$ext{moles of BaCl}_2 = \frac{\text{mass of BaCl}_2}{\text{molar mass of BaCl}_2}$

Assuming the molar mass of BaCl₂ (Barium Chloride) is approximately 208.3 g/mol, substitute the mass of the residue recorded in your results into this equation.

Use the similar formula for calculating the moles of water lost:

$ext{moles of H}_2O = \frac{\text{mass of water lost}}{\text{molar mass of H}_2O}$

Here, the molar mass of water (H₂O) is approximately 18 g/mol. Insert the mass of water lost from your results into this expression.

To compute the value of y, use the relationship between the moles of BaCl₂ and water:

$y = \frac{\text{moles of H}_2O}{\text{moles of BaCl}_2}$

Insert the values calculated in steps (b)(i) and (b)(ii) to determine y.

If BaCl₂ decomposes during heating, the mass of the residue would be lower than expected. As a result, the calculated moles of BaCl₂ would be reduced. Since the moles of water lost would remain relatively unchanged, this would lead to a higher calculated ratio of moles of water to moles of BaCl₂, potentially resulting in an erroneously high value for y.

To modify the experiment for greater accuracy in measuring the mass of water lost, consider using a desiccator for cooling instead of exposing the crucible to the environment, which may introduce moisture. Additionally, heating the sample more gradually could help prevent any excessive loss of BaCl₂, thus yielding a more consistent measurement of the mass lost.