Determining the Chemical Formula of a Hydrate

Laura Trider
April 8th, 2012
Ms MacLellan – Chem 11

Purpose: The prupose of this lab was to identify the unknown hydrate given by calculating the percent by mass of water in the unknown hydrateby heating it, then comparing the mass of the known formulas.

Materials:
-beaker
-tongs
-scoopula
-electronic balance, precise to two decimal places
-glass rod
-hot pad
-3g to 5g of hydrate

Data Table **used someone else’s data
Unknown #
1
Mass of empty beaker (g)
50.16g
Mass of beaker + hydrate (g)
53.76g
Mass of hydrate (g)
3.6g
Mass of beaker + anhydrous compound (g)
53.25g
Mass of beaker + anhydrous compound after second heating (g)
53.21g
Mass of water lost (g)
0.55g
% by mass of water in the hydrate
15.3%
Formula of the hydrate
BaCl2 ∙ 2H2O

Conclusion:
It is concluded that the given hydrate #1 was identified as BaCl2 ∙ 2H2O. When heated up over the two time periods, the unknown hydrate lost 15.3% of its mass due to water loss. This means the mass was made up of 15.3% water. After comparing this with the known formulas, it was determined that BaCl2 ∙ 2H2O was the best match. Sources of error in this lab may include: not heating the hydrate properly, the hydrate may have split or stuck to stir stick, human calculation errors, or forgetting to zero the scale before measurements.

Analysis

1. a) 5.5g/3.6g = 0.1527 x 100 = 15.3%
b) It is expected that the mass percent of water to be similar amongst the other groups which had the same unknown hydrate. This is because each group should have had approximately the same heating period.

2. Based on the observations and calculations, it is determined that the molecular formula of the unknown hydrate is BaCl2 ∙ 2H2O.

3. If a sample of a hydrated ionic compound was heated in a test tube, one would expect to see condensation around mouth