riah kim - determine the formula of hydrated copper ii sulfate 1

Riah Kim Ms. R ebancos ebancos DP Chemistry 11 31 October 2017 Determining the Formula of Hydrated of Hydrated Copper(II) Sulfate Aim: The aim of this of this lab is to determine the formula of hydrated copper(II) sulfate by sulfate by determining the number of moles of water of crystallization of crystallization in crystals of a of a hydrated salt CuSO4·xH2O by CuSO4·xH2O by heating it to constant mass in a crucible.

Research Question: What is the effect of the mass of hydrated copper(II) sulfate on the mass of water lost during the decomposition process decomposition process of hydrated of hydrated copper(II) sulfate measured in grams( ± 0.01 g) when the

heating temperature(same burner, same heating power), the crucible are kept constant and for each trial the crystal had enough time to be fully decomposed?

Hypothesis: If the If the mass of hydrated copper(II) sulfate increases, then the mass of water of water lost during the decomposition process decomposition process would increase as well, due to the law of definite proportions. definite proportions.

Background: A hydrate is a compound that contains H2O. It is usually in the form of a crystal that can be heated, and this means that the crystals contain water molecules within their structure in definite proportions. definite proportions. And the water here, also known as water of crystallization, can be lost by lost by turning into steam. This usually causes the hydrate to lose its crystalline structure. The substance that is left over after the after the hydrate has lost its water is called an anhydrate. By measuring the compound before heating and after, the amount of water of water in the original hydrate can be can be determined and the formula can be discovered. The formula for the for the crystal shows the number of water molecules present molecules present per formula per formula unit of crystal, of crystal, and a dot is put is put before the water.

Substances that absorb water from the air to form hydrates are called deliquescent. Hydrates that lose water of crystallization to form the anhydrous substances are called efflorescent.(“Hydrate | Chemical Compound”) Usually the uptake and loss of water of water are reversible processes, and sometimes these processes result in changes in colour. For example, copper sulfate pentahydrate sulfate pentahydrate (CuSO4·5H2O) is blue is blue and anhydrous copper sulfate (CuSO4) is white.

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As mentioned in the previous paragraph, the reaction between anhydrous copper sulfate and water is reversible: water is driven off from off from hydrated copper sulfate when heated, so the forward reaction is endothermic; energy is transferred from the surroundings. The backward The backward reaction is called exothermic; energy is transferred to the surroundings.(“BBC surroundings.(“BBC - GCSE Bitesize: Reversible Reactions”)

Variables: Variable

Units & Uncertainties

How are they manipulated?

Independent

mass of hydrated of hydrated copper(II) sulfate

g ( ± 0.01 g)

I will put in different amount of hydrated copper(II) sulfate for each for each sample.

Dependent

mass of water of water lost

g ( ± 0 .01 g)

Not manipulated - depends on the mass of hydrated of hydrated copper(II) sulfate.

Controls

Heating temperature

°C

I will use the same Bunsen burner and the heating power(strength).

Crucible

No unit

I will use the same crucible with the same mass and thickness - so the thickness of the crucible doesn’t affect the heating time.

Materials: Item

Quantity Used

Size

Unit of measurement of measurement & Uncertainty

Crucible

1

15 mL

No unit

Hydrated copper(II) sulfate

Between 2.00 g and 3.00 g

No size

g, ± 0.01 g

Balance(Scale)

1

No size

g, ± 0.01 g





Ring clamp

1

No size

No unit

Matches

5

No size

No unit

Crucible Tongs

1

No size

No unit

Scoopula

1

No size

No unit

Procedure:

1. The empty crucible was weighed, and then between 2.00 g and 3.00 g of hydrated of hydrated copper(II) sulfate was weighed and added. All masses were recorded accurate to the nearest 0.01 g. 2. The crucible was placed on the pipe-clay the pipe-clay triangle on the ring clamp fixed to the ring stand, over the over the Bunsen burner.

The crucible and contents were heated over a over a medium Bunsen flame so that the water of crystallisation is driven off steadily. off steadily. The blue The blue colour of the hydrated compound was gradually faded to the greyish-white of anhydrous copper(II) sulfate. Overheating should have been avoided since it might have caused further decomposition, and the heating should have stopped immediately when the colour starts to blacken. If overheated, toxic or corrosive fumes might have been evolved. 4. The crucible and contents were cooled. The crucible and contents were re-weighed once cold. 5. Steps 3 and 4 were repeated until it was heated to constant mass and consistent readings were being were being gathered. 3.





could have led to other possible accidents. A person could have fallen, and if the if the person person was holding a crucible, that crucible would have been have been broken and that could have hurt someone. But this risk was risk was minimized by minimized by being being more careful when turning on the gas. Instead of turning the lever quickly, lever quickly, we tried to turn it slowly while looking at the flame so we could stop turning the lever when lever when the size of the flame was adequate. One toxic or potential harm for the for the environment was a possible a possible toxic fumes that might have been evolved from overheating hydrated copper(II) sulfate. In order to order to prevent this, I made sure that I was not overheating the compound.

Raw Data Table: Mass (grams) (±0.01 g)

Mass of crucible of crucible + lid

38.04

Mass of crucible of crucible + lid + CuSO4 · xH2O before heating

40.36 Trial 1

Mass of crucible of crucible + lid + CuSO4(anhydrous) after heating

Trial 2

Observations

It was solid; blue crystal

39.51 39.51

It was greyish white, and some blueish white were also visible.

Uncertainty Calculation:

- 0.01 ÷ 2.32 = 0.004 g Processed Data Table:

Mass (g, ± 0.01 g)

Uncertainty (g)

CuSO4·xH2O

2.32

0.004

CuSO4

1.47

0.007

H2O lost after heating after heating

0.85

0.012





Calculations:

- Mass of CuSO of CuSO4·XH2O - 40.36 − 38.04 = 2.32 g - Mass of CuSO of CuSO4 - 39.51 − 28.04 = 1.47 g - Mass of water of water lost after heating after heating - 40.36 − 39.51 = 0.85 g 1. Cal Calcul culate ate the the molar masses of H2O and CuSO4 (Relative atomic masses: H=1.01, O=16.00, S=32.06, Cu=63.55). a. H2O: 2(1.01) + 16.00 = 18.02 (g mol-1) b. CuSO4: 63.55 + 32.06 + 4(16.00) = 159.61 (g mol-1) 2. Cal Calcul culate ate the the mass of water driven off, and the mass of anhydrous of anhydrous copper(II) sulfate formed in your experiment. a. Mass of water water driven off: 0.85 g b. Mass of anhydrous of anhydrous copper(II) sulfate: 1.47 g 3. Cal Calcul culate ate the the number of moles of anhydrous of anhydrous copper(II) sulfate formed a. 1.47 / 159.61 = 0.00921 mol 4. Cal Calcul culate ate the the number of moles of water of water driven off. a. 0.85 / 18.02 = 0.0472 mol 5. Cal Calcul culate ate how how many moles of water would have been have been driven off if off if one one mole of anhydrous copper(II) sulfate had been formed. a.

0.00921 : 0 .0472 = 1 : x 0.0472 = 0.00921 · x 0.0472 ÷ 0.00921 = x x = 5.12 5.12 moles of water of water would have been driven off.

6. Determine the formula for hydrated for hydrated copper(II) sulfate. a. CuSO4 · 5.12H2O ≈ CuSO4 · 5H2O

Percentage error calculation: - 5.12−5 · 100% = 2.4% 5





Conclusion:

My research question was what the effect of the mass of hydrated copper(II) sulfate on the mass of water of water lost during the decomposition process decomposition process of hydrated of hydrated copper(II) sulfate measured in grams( ± 0.01 g) is when the heating temperature(same burner, temperature(same burner, same heating power), the crucible are kept constant and for each for each trial the crystal had enough time to be to be fully decomposed. My hypothesis was that if the if the mass of hydrated of hydrated copper(II) sulfate increases, then the mass of water of water lost during the decomposition process decomposition process would increase as well, due to the law of definite of definite proportions. proportions. My hypothesis was not so supported with the data I currently have, since I have only done one trial. But theoretically, theoretically, my hypothesis to the research question would be would be supported due to the law of definite proportions . The Law of Definite of Definite Proportions is a rule that states that a hydrate always contains the same exact proportion of salt and water by mass(Law of definite of definite proportions proportions | Chemistry). For this experiment, I only did one trial using 2.32 grams of hydrated of hydrated copper(II) sulfate. According to the data that I gathered, hydrated copper(II) sulfate weighed less after the after the decomposition, becoming anhydrous copper(II) sulfate. Before the heating process, heating process, CuSO4·xH2O without the crucible weighed 2.32 g, and after the after the reaction, CuSO4 weighed 1.47 g. This means that 0.86 g of water was “burned off” after the after the decomposition. So the conclusion that can be can be drawn from this experiment would be would be that water was lost after the after the decomposition due to the heat, resulting in the loss of mass. of mass.

Evaluation:

After calculating the number of moles of anhydrous of anhydrous copper(II) sulfate formed and the number of moles of water driven off, I was able to calculate how many moles of water of water would have been driven off if off if one one mole of anhydrous copper(II) sulfate had been formed. The result I got from this would be would be the value of coefficient(x) in CuSO4·xH2O. The number that I got was 5.12, meaning that 5.12 moles of water would have driven off. So my final formula would be would be CuSO4·5.12H2O. If I If I round it off to off to one significant figure, it would be approximately CuSO4·5H2O, which is the correct formula for copper(II) for copper(II) sulfate pentahydrate(Pubchem). pentahydrate(Pubchem). Even though the value is approximately the same as the theoretical value, there still is an error range error range of 0.12 of 0.12 moles. When I calculated my percentage error, I got 2.4%. By looking at this, I would conclude that my data were accurate. The reason why the percentage error was error was not 0% would be would be because some of the crystal might have been have been removed or added when it was stirred, to break to break down small clods, by a wooden stick. Some crystals could have stuck to the stick and therefore







uncertainty values of CuSO of CuSO4·XH2O, CuSO4 and H2O lost after heating after heating were 0.004g, 0.007g, and 0.012g. The values are generally low, which make my data highly credible. The strength of my investigation was that when I measured the mass of the of the crystal twice, I got the same value, 39.51 g, for both, both, meaning the hydrated copper(II) sulfate was fully decomposed and H2O was evaporated fully. Another strength could be that I have a low percentage error of error of 2.4%, which means that this experiment was pretty successful and that the procedures I used are reliable and approvable. One limitation would be would be that the strength of fire was not consistent. The fire kept dying out so we had to light the fire again several times, and even when we lighted successfully, it was hard to adjust the level/strength of the of the fire. When I was turning the valve, it was difficult to adjust it to a proper extent; the fire was either too weak or too or too strong. Due to this, the water of crystallisation of crystallisation might not have been have been driven off steadily. off steadily. Another limitation Another limitation was that I was able to do only one trial, so I couldn’t really find out if the if the mass of hydrated of hydrated copper(II) sulfate has effect on the mass of water lost through decomposition. So one area where I can improve my experiment would be would be the number of trials; of trials; If I If I were to do this experiment again, I would do more than one trial so I can figure out the answer for the for the research question experimentally. experimentally. Another point point where I could improve on is about time. For this For this experiment, I did not measure for how for how long I heated the crucible; instead of timing of timing during the heating process using a stopwatch, I just I just waited until the color of the copper(II) sulfate went greyish white. Even though the procedure the procedure did not say anything about measuring time and it only said “heat until the color of the of the crystal fades to greyish white”, I think it would have been useful to record the time. Involving time could lead to a possible extension to this investigation. If I If I measured time, I could even have calculated how long it takes to fully decompose a hydrated copper(II) sulfate crystal depending on the mass. This relates back to back to having more than one trial; I can do various trials, time each, and find the correlation between correlation between the time taken to become anhydrous copper(II) sulfate and the mass of crystals. And then I could also have create a general formula/equation that shows the correlation.





Works Cited

“BBC - GCSE Bitesize: Reversible Reactions.” N.p., n.d. Web. 29 Oct. 2017. “Hydrate | Chemical Compound.” Encyclopedia Compound.” Encyclopedia Britannica. Britannica. N.p., n.d. Web. 29 Oct. 2017. “Law of Definite of Definite Proportions | Chemistry.” Encyclopedia Chemistry.” Encyclopedia Britannica. Britannica. N.p., n.d. Web. 29 Oct. 2017. Pubchem. “Copper(II) Sulfate Pentahydrate.” N.p., n.d. Web. 30 Oct. 2017.

riah kim - determine the formula of hydrated copper ii sulfate 1

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