To calculate the mass of the cooled dehydrated salt, we need to determine the amount of water lost during heating. Magnesium sulfate crystals typically contain seven water molecules per formula unit. The formula for magnesium sulfate is MgSO4, and its molar mass is 120.37 g/mol.

Step 1: Calculate the number of moles of magnesium sulfate in the 2.743 g sample.

Moles of MgSO4 = Mass / Molar mass

= 2.743 g / 120.37 g/mol

= 0.0228 mol

Step 2: Determine the number of moles of water molecules in the hydrated magnesium sulfate.

For each mole of MgSO4, there are 7 moles of water molecules.

Moles of water = 7 moles of water / 1 mole of MgSO4

= 7 x 0.0228 mol

= 0.160 mol

Step 3: Calculate the mass of water lost during heating.

Molar mass of water = 18.02 g/mol

Mass of water lost = Moles of water x Molar mass of water

= 0.160 mol x 18.02 g/mol

= 2.883 g

Step 4: Calculate the mass of the cooled dehydrated salt.

Mass of cooled dehydrated salt = Mass of original sample - Mass of water lost

= 2.743 g - 2.883 g

= -0.140 g

The negative value indicates that the mass of the cooled dehydrated salt would be less than the original mass of the sample due to the loss of water molecules during heating.