Final lab- Formula of Hydrated salt PDF

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Summary

The purpose of this lab is to determine the number of water molecules attached to a specific ionic compound....

Description

Name: Title:

Formula of a Hydrated Salt

Objective:

The purpose of this lab is to determine the number of water molecules attached to a specific ionic compound.

Physical Properties of Chemicals: Table 4.1: Several hydrate salts Compound Molar Mass

Density

Barium chloride

208.23 g/mol

Cobalt (II) sulfate

Anhydrous

Hydrated

3.856 g/cm3 962 °C

BaCl2

BaCl2.2(H2O)

154.996 g/mol

3.71 g/cm3

735 °C

CoSO4

CoSO4.7H2O

Copper (II) sulfate

159.609 g/mol

3.60 g/cm3

110 °C

CuSO4

CuSO4·5H2O

Nickel (II) sulfate

154.75 g/mol

4.01 g/cm3

> 100 °C

NiSO4

NiSO4·6H2O

Sodium carbonate

105.9888 g/mo l 161.47 g/mol

2.54 g/cm3

851 °C

Na2CO3

Na2CO3·10H2O

3.54 g/cm3

680 °C

ZnSO4

ZnSO4·7H2O

Zinc sulfate

Melting Point

Summary of Procedure: 1. Before the lab, calculate and record the molar mass for each anhydrous salt in table 4.1 2. Obtain an unknown hydrate salt and record its number on the data sheet (Line A) 3. Obtain the name of your anhydrates salt from your instructor and find its formula in a Table 4.1. Record the anhydrous formula on the data sheet (line K). 4. Set up a hot plate, 100 mL beaker and watch glass cover as shown by your instructor (Fig. 4.1). 5. Heat the beaker (with the cover) over strong heat or 5 minutes. Use tongs to carefully remove the beaker and cover from the hot plate and place them on a piece of wire gauze to cool or about 5 minutes. Do not place the hot beaker directly on the cool benchtop! Anything you weigh must be at room temperature in order to obtain a stable reading. 6. Use tongs to transfer the beaker (with the cover) to the balance and record its exact mass on the data sheet (line B). 7. Place 0.6 to 1.0g of the hydrated salt into the beaker and reweigh it accurately with the cover. Record the exact mass on the data sheet (line C) You must record the actual mass, including all of the digits shown on the balance. Do not tare the beaker! 8. Use tongs or gloves (to avoid fingerprints!) to place the beaker and cover back onto the hot late.

9. Heat the beaker for about 5 minutes. 10. Let the beaker cool for about 5 minutes on a piece of wire gauze. Do not place the hot beaker directly on the cool benchtop! Use tongs to weigh the cooled dish and record its exact mass on the data sheet (line D). 11. Transfer the beaker back to the hot plate and reheat it for another 5 minutes, cool it again and record its mass on the data sheet (line D). The two masses from steps 10 and 11 should match (within 0.0090 g). If they do not match, reheat the beaker, cool it, and weigh it again. 12. Perform your calculations on the data sheet (Line E through N) and provide your conclusion (formula and name of your unknown; lines O and P). Stop at line H if you have not yet covered the topics of “the mole” in class. A. Number and/or color of unknown hydrated salt B. Mass of beaker and cover C. Mass of beaker, cover, and hydrated salt before heating D. Mass of beaker, cover, and hydrated salt after heating After second 5 minutes of heating After third 5 minutes of heating (if necessary) After fourth 5 minutes of heating (if necessary) E. Mass of hydrated salt (line C-B) F. Mass of anhydrous salt (last line D-B) G. Mass of water in hydrated salt (line E-F) H. Percent by mass of water in hydrated salt ((line G/E) *100) I. Molar mass of water J. Number of moles of water in hydrated salt (line G/I) K. Anhydrous formula of unknown (from instructor) L. Molar mass of anhydrous salt from Table 4.1 M. Number of moles of anhydrous salt (line F/L) N. Relative molar ratio of water to salt (line J/M) O. The formula of the hydrated salt is P. The name of the hydrated salt is

25.3678 g 26.1503 g 25.8747 g 25.8725 g g g 0.7825 g 0.5047 g 0.2778 g 35.502 % 18.0153 g/mol 0.01542 mol CuSO4 159.609 g/mol 0.00316 mol 4.88 ≈ 5 CuSO4·5H2O Copper (II) sulfate

Percent error: Percent error or percentage error expresses as a percentage the difference between an approximate or measured value and an exact or known value. In this lab the we found the water molecule is 4.88. The experimental ratio between Copper (II) sulfate and water was found 1: 4.88. And, the accepted ratio between copper sulfate and water was: 1:5 Now, when determining the formula mass for a hydrate, the waters of hydration must be included. (1 Cu) (63.55 g/mol) + (1 S) (32.07 g/mol) + (4 O) (16.00 g/mol) = 159.62 g/mol Formula mass = 159.62 g/mol + (5 H20) (18.0153 g /mol) = 249.696 g/mol Now, divide the mass of water in one mole of the hydrate by the molar mass of the hydrate and multiply this fraction by 100.

The mass of water in one mole of the hydrate is, 5X 18.0153= 90.0765 So, the percent of hydration = (90.0765 g /249.696 g) (100) = 36.07% The percent error within this measurement is, PE = (|Theoretical value – experimental value| / Theoretical value) x 100% = (|36.07 – 35.502| / 36.07) x 100% = 1.57% We did have a 1.57% error, however, meaning there was slightly more mass from water than the integer we expected. This is probably due to the fact that we did not have enough time to fully heat and dry the compound until there was no mass change, and nearly all of the water was gone. This would mean that our observed percent of water to CuSO4 was actually less than it should be. Sources of error: 1. If not all the water is driven out of the solid salt before it is weighed than the salt will seem to have a greater mass than what it should actually be. This will make the results for the % of salt to be higher than it actually is. 2. If some salt pops out of the beaker when it is drying it would cause the % of salt to go down. This is because when it is weighed some of the salt from the original mixture is not present, making it. So, the weight of salt is less which lowers the % of salt calculated from the original mixture. 3. When starting the experiment, it is important to make sure that the beaker and watch glass are completely dry with no water drops on it, and to avoid fingerprints. Fingerprints and other greases from our hands would add mass and it would not give us the accurate measurement. Conclusion: In this experiment, we get to know that, depending on the relative molar ratio of water to anhydrous salt in the hydrated salt, will determine the number of water molecules in the formula of a hydrated salt. After calculations, we find the name of the hydrated salt was CuSO4·5H2O. And the relative molar ratio of water to salt was 4.88. Although the experiment was done with the right steps, there was still a 1.57% error, that can contribute to the humidity in the room or simply not heating the salt enough. The molar ratio of water to salt was 4.88, but the correct number was 5. The formula of the hydrated salt is determined by rounding the result of the last equation to the nearest whole number. Overall, the important lesson that was learned from this experiment was that errors occur all the time in chemistry experiments, it’s the way that the errors are interpreted that make the experiment successful. Pre-lab questions/ Problems:

1. A student measured a 6.790 g sample of a hydrated salt. After heating, the mass of anhydrous salt was found to be 4.501 g. Use these data to calculate: a) The mass of water in the hydrated salt.

6.790g- 4.501g = 2.289g b) The percent by mass of water in the hydrated salt. (2.289g/6.790g) × 100% = 33.71% 2. If you have covered the topic of “the mole” in class, answer this question. Hint: Re-read the example in the introduction. 4.64 g of Na2SO4.xH2O were heated to yield 2.84 g of Na2SO4. What is the value of x? Mass of water = 4.64g-2.84g = 1.8g Molar mass of water = 18.0153g/mol Number of moles of water = 1.8g/(18.0153g/mol) = 0.1mol Mass of Na2SO4 = 2.84g Molar mass of Na2SO4 = 142.04g/mol Number of moles of Na2SO4 = 2.84g/(142.04g/mol) = 0.02mol Value of x = 0.1mol/0.02mol = 5...