Hydrated calcium sulfate - problem determining mass of H2O

In summary, the hemihydrate is made by heating a solution of calcium sulfate and water. The temperature and moisture are controlled, and the pressure is adjusted.
  • #1
Mitchtwitchita
190
0
I just finished a lab and I was wondering if anyone could point out where I'm going wrong.

Here are my findings for CaSO4:

After First Heating After Second heating

Wt. of crucible, lid, and hydrate 35.0537g 34.9779g
Wt. of dry crucible and lid 34.0226g 34.0226g
Wt. of hydrate 1.0331g 0.9553g

Wt. of crucible, lid, and anhydrate 34.9779g 34.9781g
Wt. of crucible and lid 34.0226g 34.0226g
Wt. of anhydrate 0.9553g 0.9555g


Questions:

1. Calculate the % by weight of water in your unknown hydrate. Report the % to four digits.

1.0331g CaSO4*H2O - 0.9555g CaSO4 = 0.0756 g H2O
0.0756g/1.0311g x 100% = 7.332%

2. Using the % water from Part 1 above, calculate the moles of water. (Hint: Assume a 100g sample of hydrate.) Report to four digits.

7.332g H2O x 1mol. H20/18.02 g H2O = 0.6806 mol H2O

3. Using the & anhydrate, calculate the moles of anhydrate. Report four digits.

100% - 7.332% = 92.668%
92.668g CaSO4 x 1 mol CaSO4/136.15g CaSO4 = 0.6806 mol CaSO4

4. Using the anhydrate formula (CaSO4) given to you in lab, and the values from Parts 2 and 3 above, give the formula of your unknown hydrate. Show your calculation. Round the water to the nearest 1/2 or whole number of moles.

H2O = 0.4069 mol/0.4609 mol =1
CaSO4 = 0.6806 mol/0.4609 mol = 1.673
x 3 = 5CaSO4*3H2O

These calculations don't seem to be right, can anybody please help me understand?
 
Physics news on Phys.org
  • #2
first, 1.0331 - 0.9555 = 0.0776g of water in 1.0331g of the hydrated salt. The % by weight of H2O is 7.511%

i find this method weird. this is what i;d have done:

weigh crucible + lid = 34.0226 g
weigh crucible + lid + hydrated salt = 35.0537 g
mass of hydrated salt = 1.0311 g

heat first time:

weigh crucible + lid + salt = 34.9779 g

heat second time:

weigh crucible + lid + salt = 34.9781 g

therefore, mass of water of crystallisation evaporated = 35.0537 - 34.9781 = 0.0756 g
and mass of anhydrous salt = 34.9781 - 34.0226 = 0.9555 g

this is all i will be needing.

i then calculate the number of moles of water of crystallisation = 0.0756/18 = 4.2*10^-3 mol

this amount is present in 1.0311 g of the hydrated salt.

i know the formula of the salt is CaSO4.xH2O

1 mol CaSO4.xH2O contains 1 mol CaSO4

i know the mass of CaSO4 which is 0.9555 g, and the molecular mass is 136
the number of moles of CaSO4 thus present is 7.026*10^-3.

this implies that 7.026*10^-3 mol CaSO4.xH20 is present.

now coming back to the water,

7.026*10^-3 mol CaSO4.xH20 contains 4.2*10^-3 mol water of crystallisation
1 mol CaSO4.xH2O contains 0.59778 mol water

you can approximate this value to 0.5 and you will get bassanite, CaSO4.(0.5)H2O
 
  • #3
i think i found the error, in part 2.

(1) first the % water is wrong, it is 0.0776/1.0311 x 100% = 7.526 %

(2) you are saying 100g of hydrate contains 7.526 g water, which is 0.418 mol
(3) then 100g hydrate contains 92.474 g anhydrate, which is 0.680 mol

(4) but then, CaSO4.xH2O contains 1 mol CaSO4.
this implies that 100g of hydrate contains 0.680 mol of hydrate.

you know that 0.680 mol hydrate contains 0.418 mol water
therefore, 1 mol hydrate contains approximately 0.5 mol water (it is not the exact value obtained)

same as above ;)
 
Last edited:
  • #4


I want to know about chemistry process to produce gypsum hemihydrate ( CaSO4. 1/2 H2O )
and what should be controlled in the process to get stable CaSO4.1/2H2O:
a. Temperature
b. Moisture
c. Pressure
d. Air/powder ratio
e. Any others parameter that needed
 
  • #5


Please do not hijack the threads.

Do you know how the hemihydrate is made?
 

1. What is hydrated calcium sulfate?

Hydrated calcium sulfate, also known as gypsum, is a white or colorless mineral composed of calcium, sulfur, and oxygen. It is commonly used in construction materials, such as plaster and drywall, and in fertilizer and food production.

2. How is hydrated calcium sulfate formed?

Hydrated calcium sulfate is formed through the reaction of calcium carbonate with sulfuric acid. It can also be formed through the evaporation of water from calcium sulfate solutions, creating crystals of different shapes and sizes.

3. Why is it important to determine the mass of H2O in hydrated calcium sulfate?

The mass of H2O in hydrated calcium sulfate is important because it affects the properties and uses of the mineral. For example, the amount of water present can determine the strength and drying time of plaster, or the solubility and nutrient availability in fertilizers.

4. What challenges may arise in determining the mass of H2O in hydrated calcium sulfate?

One challenge in determining the mass of H2O in hydrated calcium sulfate is that it can vary depending on the source and production process of the mineral. Other factors, such as temperature and humidity, can also affect the water content. Additionally, the presence of impurities or other elements in the mineral can complicate the analysis.

5. How can the mass of H2O in hydrated calcium sulfate be accurately determined?

The mass of H2O in hydrated calcium sulfate can be accurately determined through various analytical techniques, such as thermogravimetric analysis, Karl Fischer titration, or infrared spectroscopy. These methods involve heating or chemically reacting the mineral to separate the water and measure its mass. It is important to use standardized and validated methods to ensure accurate results.

Similar threads

  • Biology and Chemistry Homework Help
Replies
2
Views
1K
  • Biology and Chemistry Homework Help
Replies
3
Views
4K
  • Biology and Chemistry Homework Help
Replies
4
Views
6K
  • Biology and Chemistry Homework Help
Replies
2
Views
2K
Replies
1
Views
2K
  • Biology and Chemistry Homework Help
Replies
16
Views
2K
  • Biology and Chemistry Homework Help
Replies
5
Views
2K
  • Biology and Chemistry Homework Help
Replies
2
Views
107K
  • Biology and Chemistry Homework Help
Replies
2
Views
4K
  • Biology and Chemistry Homework Help
Replies
2
Views
3K
Back
Top