Hydrated and Anhydrated Formula's--Need HELP NOW!!!(please)?

Question

This is about the 50th time I am asking for help, so bear with me. I need to find out the answers to the following questions, so if you could answer any one or all of them, that would be greatly appreciated! Please put the number of the one you solved next to it. Here is the info I have(this was a lab)

mass of empty crucible- 6.31 grams
mass of crucible with MgSO4 (that big black dot goes here) n H2O- 11.33 grams
mass of crucible and anhydrous MgSO4- 8.71
Here are the questions:

1. Calculate the mass and number of moles of anhydrous MgSO4.
2. calculate the percent of water in the hydrated compound.
3. calculate the ration of moles of water to moles of anhydrous MgSO4.
4. Write the value of n and the formula for MgSO4 (big dot) H2O
5. The accepted value for n is 7. Calculate the percent error for the value of n.

That's all! If you could show how you got the an, that's be great. I know that it's a lot, but these questions are making me really comfused. Thanks!! ;)

Thanks, Richard!! You were a great help!

Answer 1

To start, you need the mass of the hydrated salt and anhydrous MgSO4:

Mass of hydrated salt = 11.33 - 6.31 grams = 5.02 grams
Mass of MgSO4 = 8.71 - 6.31 grams = 2.40 grams
Mass of water = 5.02 - 2.40 grams = 2.62 grams

The MM of MgSO4 = 120.36 g/mol
The MM of MgSO4•7H2O = 246.48 g/mol
The MM of H2O = 18.02 g/mol

1. Calculate the mass and number of moles of anhydrous MgSO4.
The mass is 2.40 grams.
The moles = 2.40 g/ 120.36 g/mol = 0.0199 moles

2. Calculate the percent of water in the hydrated compound.
2.62 g / 5.02 g * 100 = 52.2 % water

3. Calculate the ratio of moles of water to moles of anhydrous MgSO4.
2.62 g / 18.02 g/mol = 0.145 mole
0.145 mole / 0.0199 mole = 7.27 (nearest whole number is 7)

4. Write the value of n and the formula for MgSO4 (big dot) H2O.
n= 7.27 or MgSO4•7.27H2O (practicle solution is MgSO4•7H2O)

5. The accepted value for n is 7. Calculate the percent error for the value of n.

7.27 - 7.00 = 0.27
0.27 / 7.00 * 100 = 3.86 %