1. Analysis of Alum Objectives
To determine the percentage of water in alum hydrate and an unknown hydrate
To calculate the water of crystallization of an unknown hydrate
To develop the lab skills for analyzing a hydrate

2. Empirical Formulas of Hydrates
Last time you figured at the composition of magnesium oxide and copper sulfide
This time you will figure out the composition of a hydrate
A hydrate is an ionic solid AX that has some water trapped in it
AX.yH2O
AX is some known ionic compound like say Na2SO4 or BaCl2 etc
In the experiment we know AX but not y we will be finding the y

3. These are either K+ or Al3+
Hydrates
A solid compound having a fixed number of water molecules H2O in its composition
Water of crystallization
BaCl2.2H2O
2 waters of crystallization
KAl(SO4)2.12H2O (Alum hydrate or potassium aluminum sulfate dodecahydrate)
12 waters of crystallization
These are the H2O
These are either K+ or Al3+
These are the SO42-

4. How much water in my hydrate?
We can measure the mass percent of water in a hydrate quite easily
The mass percent of water in a hydrate is defined as
If we already know the chemical formula for the hydrate we can calculate it theoretically

5. Calculate the theoretical % of H2O in alum hydrate (KAl(SO4)2.12H2O)
In this formula we have
1 K, 1Al, 2 (SO4), and 12H2O or
1K, 1Al, 2S, ( = 20 O), and 24H
The masses are obtained from the periodic table (this is the mass of 6.023x1023 atoms of a given element)
1 x g = g
1 x g = g
2 x g = g
20 x g = g
24 x g = g g

6. Experimental mass % H2O
To get the experimental value we measure the mass before heating mbefore
Heat the crystal to boil off the water
Reweigh mafter

7. 1.150g Alum hydrate loses 0.523g of H2O when heated, what is the mass % water?
Given
mwater = g
mhydrate = g
Use

8. Experimental Technique
The proficiency of your technique can be inferred by comparing the experimental result (45.5%) with the theoretical value (45.58%)
If your value is too small you haven’t removed all the water so heat some more
If your value is too large you heated it too strongly and as the water came out of the crystal it blew some of the crystal out too! So next time heat it more gently

9. Getting the number of waters of hydration y
We have just calculated % H2O, what if we also knew what the ionic solid was?
Let’s say we knew we had Na2SO4 but we didn’t know how many waters were in our hydrate
In other words we want to find y in the formula
Na2SO4.yH2O
How could we do that?

10. Getting the number of waters of hydration y
To get y we use the theoretical formula connecting to the mass % water ie
For Na2SO4.yH2O this would be
𝑚𝑎𝑠𝑠 % 𝑤𝑎𝑡𝑒𝑟= 18.0𝑔×𝑦 𝑔+18.0𝑔×𝑦 ×100%
(Here the mass of one mole of Na2SO4 = g)
We can solve this equation for y
𝑦= 𝑥×142.04𝑔 18𝑔×(1−𝑥) 𝑤ℎ𝑒𝑟𝑒 𝑥= 𝑚𝑎𝑠𝑠 % 𝑤𝑎𝑡𝑒𝑟 100%

11. Getting the number of waters of hydration y
𝑦= 𝑥×142.04𝑔 18𝑔×(1−𝑥) 𝑤ℎ𝑒𝑟𝑒 𝑥= 𝑚𝑎𝑠𝑠 % 𝑤𝑎𝑡𝑒𝑟 100%
OK let’s say we measure the mass percent of water in our hydrate of sodium sulfate and we find it is 55.90%, then
𝑥= 𝑚𝑎𝑠𝑠 % 𝑤𝑎𝑡𝑒𝑟 100% = % 100% =
Then
𝑦= 𝑥×142.04𝑔 18𝑔×(1−𝑥) = ×142.04𝑔 18𝑔×(1−0.5590) =10.06≅10
So our hydrate has the formula
Na2SO4.10H2O

12. Getting the number of waters of hydration y
You will be given a mystery compound but we will tell the molar mass of the part of the compound that doesn’t have the water. Since this is the formula of what is left one you heat it and remove the water we call the part without the water the ANHYDROUS COMPOUND (AC)
Your formula is therefore AC.yH2O and you must find y
We will tell you the molar mass of AC let’s call that mAC
Then following what we did for the sodium sulfate
𝑦= 𝑥×𝑚𝐴𝐶 18𝑔×(1−𝑥) 𝑤ℎ𝑒𝑟𝑒 𝑥= 𝑚𝑎𝑠𝑠 % 𝑤𝑎𝑡𝑒𝑟 100%

13. Procedure for the Alum
Weigh a clean dry 150-mL beaker covered with a watch glass (report to 4 decimal places) m1 = mbeaker + mwatchglass
Add between g of alum hydrate to the beaker and reweigh (with the watch glass on) – again report to 4 sig figs m2 = m1 + mhydrate
Support beaker and watchglass on a ring stand with a wire gauze.
Heat gently (burner 3 inches below beaker) – you should observe moisture of the inside of the beaker
Keep heating till the moisture has gone (Crystal should turn to powder at this point)
If the watchglass is not dry – pick it up with tongs and gently heat it in the bunsen flame till dry
Turn off burner, replace watch glass on beaker, let it cool for 10 mins then reweigh, the beaker, its contents, and the watchglass together m3 = m2 – mwater
Discard powder in trash, clean beaker and repeat a second trial
Calculate the mass % water for each trial

14. Procedure for the Unknown
Weigh a clean dry 150-mL beaker covered with a watch glass (report to 4 decimal places) m1 = mbeaker + mwatchglass
Add between g of your unknown to the beaker and reweigh (with the watch glass on) – again report to 4 sig figs m2 = m1 + mhydrate
Support beaker and watchglass on a ring stand with a wire gauze.
Heat gently (burner 3 inches below beaker) – you should observe moisture of the inside of the beaker
Keep heating till the moisture has gone (Crystal should turn to powder at this point) (You may need to break up the drying crystal with a stirring rod)
If the watchglass is not dry – pick it up with tongs and gently heat it in the bunsen flame till dry
Turn off burner, replace watch glass on beaker, let it cool for 10 mins then reweigh, the beaker, its contents, and the watchglass together m3 = m2 – mwater
Discard powder in trash, clean beaker and repeat a second trial
Calculate the mass % water for each trial