Joanne M. answered • 01/10/20
Biology, Chemistry & Physical Science
This is going to come down to understanding empirical formulas and mole ratios, but it is simple.
First consider the formula of the anhydrous salt..."Calcium Sulfate" Calcium ions have a 2+ charge and Sulfate ions have a 2- charge... so the formula is CaSO4. How this is a hydrated salt before heating. That means that initially there were water molecules attached in a specific ratio. We will figure out this ratio in the end of the problem, but for now lets represent this using the setup below:
1 CaSO4 • X H2O essentially a 1:X mole ratio. We have to figure out X.
Using the data, we can figure out the mass of water in the hydrate. (Percentages can also be used if you want an extra step). We will use this data to determine the "empirical formula" of this hydrated crystal.
Before heating the mass was 4.89 grams
After heating the mass was 3.87 grams
1.02 grams "left the material" during heating. This was the water evaporating.
CaSO4 : H2O
3.87g : 1.02g <--- we are going to divide these masses by the formula mass to get a mole ratio
3.87g : 1.02g
136g 18 g <--- Formula masses were rounded to nearest whole number. Your instructor may require more specific masses
0.0285 : 0.0567 <----- this is a mole ratio, however we need to try and get these to whole numbers. So we inspect these numbers and find that 0.0285 is the smallest value. We are going to divide both of these numbers by that small value to attempt to get a whole number
0.0285 : 0.0567
0.0285 0.0285
1: 1.989 <---- the value of 1.989 is essentially 2. So the mole ratio between CaSO4 : H2O is a 1:2 ratio. This will make the final formula:
1 CaSO4 • X H2O where X = 2
1 CaSO4 • 2 H2O <--- omit the coefficient of 1 before CaSO4
CaSO4 • 2 H2O <---- final chemical formula for gypsum.... calcium sulfate dihydrate
