Topic-Determining the formula of a hydrate

Question

Purpose

·   To determine the percentage composition of water in a hydrate of Copper (II) Sulfate

·   To determine the chemical formula of CuSO4∙xH2O

Equipment and Materials

·   Safety Googles

·   Large test tube

·   Scoopula

·   Electronic Scale

·   Utility Clamp

·   Bunsen Burner

·   Balance 

·   2 to 4 g of hydrated copper(II) sulfate, CuSO4 xH2O

·   Spark lighter

•lab apron

•Gauze pad

1. 2 strips of cobalt chloride paper

Procedure

1) Put on your safety goggles

2) Record the mass of the test tube

3) Add about 2 to 3 cm of CuSO4∙xH2O to your test tube, determine the mass of the contents of the test tube.

4) Attach a utility clamp to the test tube

5) Light the Bunsen Burner. Put on your safety goggles. Heat the test tube and its contents moderately by moving it from side to side for 1 min in a Bunsen Burner flame. Do not localize the flame at any point on the test tube. Check for evidence of condensation forming near the mouth of the test tube.

 6)Test the condensate with a strip of cobalt chloride paper. Test a stream of tap water with the other strip

7)Heat strongly for 2 to 3 min or until there is no further evidence of condensation. Be careful not to overheat the hydrate. If you do, it may decompose to form a black residue: Copper (II) Oxide

8)Place the test tube on a paper towel and allow it to cool.

9)Determine the mass of the contents of the test tube

Data

Quantitative		Quantitative
Colur of test tube strip	Blue	MAss of test tube	19.38g
after	Lightening in colour condensation is occuring	Mass of CuSO4 x H2o	4.01g
Colour of CuSo4 x H2O before	Blue	Mass of test tube and CuSO4 x H2O	23.39g
During	Compound is light	(got dark at the bottom)	Mass of CuSO4 x h2O and sturing rod and test tube	34.45g
Is there condensation	Yes	Mass of stirring rod	11.99g

Analyze and Evaluate

B) what qualitative evidence suggested that copper(II) sulfate was converted into its anhydrous form. 

C)Determine the mass of the water and mass of anhydrous copper (II) sulfate produced.

D) Determine the percentage of water (by mass) in each of your samples of hydrate.

F) Determine the amount of water of hydration in your sample. And the amount of anhydrous copper(II) sulfate in your sample. 

G) determine the value of x and write the complete chemical formula of the hydrate. 

H) predict the effect of each of the following on the percentage by mass of water in the hydrate and the chemical formula of the hydrate:

I) not heating the hydrate enough

II) overheating the hydrate and producing some copper(II) oxide

Answer 1

What specifically is your question? The provided information is data; the necessary transformations include:

1) calculate the mass of CuSO4 (anhydrous) product

2) calculate the moles of CuSO4 (anhydrous) that this represents (use formula weight CuSO4)

3) calculate mass lost = water mass lost

4) convert water mass lost into moles water lost (use molecular mass H2O)

5) Divide (4) by (2), and express in integer ratio.

(H) I and II require a little thought. Assume conversion to (1/2) hydrate (i.e. CuSO4.(0.5)H2O), and work through the calculations based on that **using the correct ratio you just obtained in (5)** ; for II, assume conversion of 0.1 proportion of the CuSO4 to CuO and work through the calculations similarly. The answers will show you what happens to the mass lost. However, for case I, note that you will arrive experimentally at a hydration number less than the correct formula value, whereas, for the case II, you will arrive at a hydration value greater than that for the correct formula.

Now for the fun part of any experiment, which is figuring out IN ADVANCE what you might do to recover from such errors! (Bt the way, this is valuable strategy for succeeding in the world, in general!)

For case I, if you did not get an integer value for hydration number, what might you do to see if you could get closer to the correct value? And for case II, what could you do (in terms of recovering CuO, and quantitating it!), to eventually provide a path to the correct value?

You might think that these are purely hypothetical problems, and so they might be, for CuSO4.xH2O; however, there are experimental situations in which arriving exactly to a desired endpoint is difficult or impossible, and you MUST go beyond, and then compensate, for example by back-titration. Or, use some other means of observing. Case in point: determining the melting point of certain organic materials, which are starting to decompose already at their melting point! Indeed, the antiviral drug ribavirin is such: at the melting point, as it starts melting, bubbles are trapped in the viscous melt, which make easy observation of the exact melting range impossible by eye. So instead, an instrumental device (differential scanning calorimeter) may be used, which detects the heat absorption associated with the melt process, and then mathematically transforms the data back into a temperature range, using previously measured rates of temperature lag from the sample to the detector thermocouple. In this way, the melting range may be rapidly scanned (minimizing decomposition of the substance), and a precise range obtained.