Possible sources of error in hydrate lab

Handout Investigation: How Much Water is in the Hydrate?

Name: ________________________________ Group #: ___ Lab Date/Time: ____________

Directions: This worksheet will outline the information needed to write all of the sections in your

first group report. As you work on the investigation, write data/results and/or observations to

help with calculations in your notebook. Information included in your notebook will be used in

the final exam.

*The majority of the report is written in 3 rd

person, past tense, passive voice, with no use of first

person.

*The report is to be double-spaced, written with proper grammar and spelling. Label each section.

*Attach Team Contribution Forms from each group member to your report (this is critical to ensure

fair grades).

*Attach individual section(s).This investigation requires you to complete the procedure section

of the report, individually. The rest of the report is a worksheet completed as a group. The

group worksheet and individual sections are due 2 days after your lab! Due date _________time

_______where ______________________________________________________.

Introduction (1 point): When chemical compounds incorporate water into their crystalline

structure, they are called hydrates. Two salts were studied in the course of this investigation. First, a

hydrate with a known molar ratio of water to salt was studied: copper sulfate pentahydrate, CuSO4

•5H2O. By heating the hydrate, the water was driven off. After dehydrating the sample, the molar

ratio of water to copper sulfate was determined. Next, an unknown hydrate was studied. A

procedure was developed to determine the molar ratio of water to salt for a provided unknown

hydrate, (circle )MgSO4•XH2O /FeSO4•XH2O. The percent by mass of water in the sample and the

empirical formula of the hydrate were determined.

Procedure (2 points):In this section, describe what your group did in both parts of the investigation

in a paragraph format. Remember: **PAST TENSE!** (do not list/number procedures; use

paragraphs).

-When recording mass all numbers should be recorded and never rounded.

-Include equipment, materials used, and concentration of any chemicals

-Explain the details about the procedure for the known hydrate (Part 1) and designing the procedure

used for the unknown hydrate (Part 2). Be concise and descriptive.

2

-Describe any safety precautions taken within the procedure you followed.

-Describe any other procedures you also tried (if applicable).

-This section should be detailed enough that another group could duplicate your work, but do not

include descriptions or analysis of the observed or calculated results so as to not overlap with the

results and discussion sections.

Results (1.5 points):

For Part 1, copper sulfate pentahydrate was studied to confirm the validity of the technique. Upon

heating the sample of the hydrate, the percent by mass of water in the hydrate was determined, as

can be seen in Table 1

Write a descriptive title for this table.

Table 1: _______________________________________________________________________

CuSO4 •5H2O

Mass of

Hydrated

Salt

(g)

Mass of

Anhydrous

Salt

(g)

Mass of

Water

Lost

(g)

Experimental

Mass %

Water

Average

Experimental

Mass %

Water

Theoretical

Mass %

Water

% Error

Trial 1

Trial 2

If ___________% of the mass of the hydrate is water, _________% of the initial mass is due to the

copper sulfate.

For Table 2, the masses of hydrated salt and water were divided by their molecular weights to

determine the moles of anhydrous salt and moles of water present. The ratio of moles of water to

anhydrous salt gave a molar ratio used to propose an empirical formula for the hydrated salt.

3

Write a descriptive title for this table.

Table 2: ________________________________________________________________________

Trial 1 Trial 2

CuSO4 H2O CuSO4 H2O

Initial moles

Mole Ratio

(exact calculation)

Empirical Formula

(rounded mole ratio) CuSO4 •____H2O CuSO4 •____H2O

For Part 2, an unknown hydrate was analyzed. Table 3 shows the mass percent water of hydration

for the salt.

Write a descriptive title for this table.

Table 3: _______________________________________________________________________

____SO4 •XH2O

Mass of

Hydrated

Salt

(g)

Mass of

Anhydrous

Salt

(g)

Mass of

Water

Lost

(g)

Experimental

Mass %

Water

Average

Experimental

Mass %

Water

Trial 1

Trial 2

For Table 4 the masses of hydrated salt and water were divided by their molecular weights to

determine the moles of anhydrous salt and moles of water present. The ratio of moles of water to

anhydrous salt gave a molar ratio used to propose an empirical formula for the hydrated salt.

4

Write a descriptive title for this table.

Table 4: ________________________________________________________________________

Trial 1 Trial 2

_________ H2O _________ H2O

Initial moles

Mole Ratio

(exact calculation)

Empirical Formula

(rounded mole ratio) ____SO4 •____H2O ____SO4 •____H2O

Calculations (1 point):

The following sample calculation was used to determine the mass percentage water in CuSO4 •5H2O

using experimental data from Table 1.

Sample Calculation 1:

Because the formula of CuSO4 •5H2O is known, the theoretical mass percentage of water in the hydrate

from Table 1 was calculated (Sample Calculation 2). The % error between the experimental and

theoretical mass percentages were also calculated (Sample Calculation 3)using data from Table 1:

Sample Calculation 2: Sample Calculation 3:

5

The following sample calculation was used to determine the moles of water and anhydrous salt from

the masses using the molecular weights. Data from Table _______ was used.

Sample Calculation 4:

Discussion (1.5 points):

Using data from Part 1 of the experiment, the known hydrate, copper sulfate pentahydrate, was

analyzed. As seen in Table 1, its percent by mass was determined experimentally as proof of concept

with a __________% error.

Briefly describe what this means about the potential validity of your results for analysis of an

unknown hydrate.

_________________________________________________________________________________

_________________________________________________________________________________

_________________________________________________________________________________

_________________________________________________________________________________

________________________________________________________________________________

As seen in Table 2, the masses of anhydrous salt can be calculated once the mass of water lost is

determined. When converted to a molar ratio, the results should be consistent with the true formula

of CuSO4 •5H2O with a ratio of 5:1, water to anhydrous salt.

6

Briefly describe how your results compared to the 5:1ratio. State if your ratio was low or high,

and explain why.

_________________________________________________________________________________

_________________________________________________________________________________

_________________________________________________________________________________

_________________________________________________________________________________

________________________________________________________________________________

The same procedure was followed for the determination of % mass of hydration and empirical

formula mole ratio for the unknown hydrate, (circle) MgSO4•XH2O /FeSO4•XH2O. The data in

Tables 3, and 4 show the results of this analysis. It was determined that (circle)

MgSO4•XH2O/FeSO4•XH2O has a %mass of hydration of ____________________, and an

empirical formula of __________________________________.

There are several possible sources of error and corrections that may have prevented each error.

Fill in the table below, noting any experimental details or incongruences that may have led to

errors in the results obtained. (“Human error” is not an acceptable response, be more specific.)

Error Possible Corrections

7

Conclusion (1point):

Summarize the results obtained by this experiment:

_________________________________________________________________________________

_________________________________________________________________________________

_________________________________________________________________________________

The procedure developed during application of this experiment allowed/ did not allow for a

successful analysis of hydrate samples for the following reasons:

_________________________________________________________________________________

_________________________________________________________________________________

_________________________________________________________________________________

_________________________________________________________________________________

Alternate methods or approaches:

_________________________________________________________________________________

_________________________________________________________________________________

_________________________________________________________________________________

_________________________________________________________________________________

Works Cited:

ACS Style. Use at least one reputable source other than your lab manual on the space provided

below.

Bauer, R., Birk, J., Sawyer, D. Laboratory Inquiry in Chemistry, 3rd ed.; Brooks/Cole Cengage

Learning: Belmont, 2009.

_________________________________________________________________________________

_________________________________________________________________________________

_________________________________________________________________________________

_________________________________________________________________________________

8