Chemistry

PERFORMANCE TASK #2

You will work on the following activities alone without consulting classmates or the instructor. You must email in all these sheets, any data sheets, and your graph before you leave today.

This activity has a point value of 50. Part I: Hydrates

Hydrates are crystalline compounds in which one or more molecules of water are combined with each formula unit of a salt. The water is not bound tightly and can usually be driven off by subjecting the crystals to the hottest flame of a Bunsen burner for 5 to 10 minutes. If the hydrate is colored, a color change usually results upon heating as the anhydrous salt forms.

For example the following change occurs upon heating:

CuSO4Ÿ5H2O (s) ® CuSO4 (s)  + 5 H2O (g)

bright blue white

A generalized form of this reaction is:

CuSO4ŸxH2O (s) ® CuSO4 (s) + x H2O (g)

Recall that the percentage of a component in a compound is calculated by:

(Part/Whole) x 100 = %

Preliminary Questions (5 pts)

Describe how you could find the % by mass of water in the hydrate CuSO4Ÿ5H2O assuming that you DID NOT already know the formula. Be specific and list the laboratory procedural steps you would take in the order that you would perform them.

Once you know the amount of water in grams in a particular sample of a hydrate, how can you find how many moles of water were in the hydrate?

How can you find out how many moles of the anhydrous salt (CuSO4 in this example) were produced?

How can find the relative number of moles of salt and water in the hydrate?

How will you know the coefficient to place in front of the H2O in the formula for the hydrate?

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You will now perform the required laboratory activity to answer the following question:

Based on the DATA IN THE VIDEO, what is the complete formula (Salt · x H2O) for the hydrate discussed in the video?

(10 pts)

General Procedural guidelines for this lab:

1. From the very beginning of this experiment, only handle the crucible or evaporating dish with crucible tongs. SAFETY: The crucible will be very hot! Never touch the crucible while it could be hot. Use crucible tongs.

2. Use the same scale throughout this lab.

3. Do all of your calculations with proper sig figs.

A. For additions and subtractions, keep the fewest number of decimal places.

B. For multiplication and division keep the fewest number of significant figures.

4. While things are heating and cooling, work on Part II of this task.

Procedure: Watch this video, read along with the following procedure, and take notes: https://www.youtube.com/watch?v=OuF4hjTFdsg

1. Wash out a crucible and flame dry. Heat the crucible with a Bunsen burner for 5 minutes on high and let it cool for 10 minutes. 

2. Zero the balance.



 

3. Mass the crucible and record the mass (with units). __________

4. Fill the crucible about ¾ full with the hydrate.

5. Mass the crucible with the hydrate and record this mass. __________

 

Record the color of the hydrate before heating. __________

6. Heat the crucible with the hydrate until all water is driven off.

 

Record the color of the hydrate after heating. __________

7. Let the crucible cool for a few minutes (at least 10). Then mass the anhydrous sample and crucible and

 

record this mass. __________

8. Complete the calculations to determine the (i) % mass of water in the hydrate and (ii) the value for x for your hydrate. Show all of your calculations. The calculations for the % mass of water in the hydrate are outlined in the video. You will have to do the calculations for x by yourself.

USE THIS AREA TO PRESENT ALL YOUR CALCULATIONS:

CHM 1010

Part II: Graphing

(20 pts)

Using this set of data, graph the data by hand and answer the questions below using a graphing calculator.

1. Use a piece of graph paper and plot the following data by hand. Be sure it is in the correct format (dependent and independent variables, axes labels and scales, appropriate use of graph space, etc.) As an experiment to measure the radioactivity of a sample of Strontium-90, a student collected data on the intensity of radiation with respect to distance from the sample. She measured the intensity of the radiation, R, in counts per minute (cpm) at different distances, D, from the sample:

Distance from sample, 

mm

Radiation intensity, 

cpm

79.8

814.0

70.0

1058

59.9

1644

50.1

2681

40.3

4674

29.8

9017

2. The relationship between the variables can be described as: 

(a) A linear relationship between the variables, or

(b) A power relationship between the variables

Your answer: _______________________

3. Check your answer to question 2 by entering the data as L1 and L2 into your graphing calculator and performing the appropriate regression analysis.

Write the equation for the relationship between the intensity of the radiation (R) and the distance (D) in terms of these variables:

___________________________________

Additional Stoichiometry Question (15 pts)

One hydrate of sodium sulfate has the formula Na2SO4·xH2O. While doing an experiment to determine the value of x, you collect the following data:

Mass of crucible: 25.67g

Mass of crucible + unknown hydrate: 26.92 g

The crucible with unknown hydrate in it is then heated on a Bunsen burner for 10 minutes.

We let the crucible + contents cool down before weighing.

Then: Mass of crucible + anhydrous form of hydrate: 26.33 g

What is the value of x? Show ALL your calculations!