Brad Wang, Daniel Kang, Scott Corkum, Sourabh Das
25/01/2014
Determining the Chemical Formula for a Hydrate Purpose: Find the chemical formula of hydrated copper (II) sulfate.
Pre-Lab Answers: 1. When the color of the copper (II) sulfate turns white, it will indicate that it has become dehydrated. This means that the reaction is complete. 2. Assuming that the steps in the procedure are carried out accurately and there are no other external influences during the experiment, the results should be the same regardless of how much hydrated salt is used. As the mass of the hydrated salt is increased, the mass of the water evaporated from the salt will be proportionately more, and the ratio will remain constant. 3. In terms of reaction time, it will take longer for the hydrate to evaporate as the surface area of the bottom of the test tube is much smaller than that of a beaker. Due to the narrow shape, the crystals of copper (II) sulfate will also pile vertically so it will not receive uniform heat as it is being heated by a hotplate. This will result in either some crystals remaining hydrated or some crystals starting to decompose which will result in inaccurate results in either case. Also the test tube may remain colder near its mouth as it isn’t receiving any direct heat from the hotplate. This would lead to the evaporating water condensing near the top of the test tube as water droplets and increasing the final mass of the test tube when the ‘anhydrous ‘anhydrous salt,’ is weighed. The inaccurate reading would would lead to an inaccurate final result.
Materials: 5 g of hydrated copper (II) sulfate 250 mL beaker Tongs Glass stirring rod
Scoop Hot plate Hot pad Electronic Balance
Procedure: 1. Measure the mass of the beaker and the glass rod using electronic balance. In addition, after receiving some hydrated Copper (II) Sulfate from your teacher, measure the mass of the beaker with the chemical in it. 2. Set the hot plate to medium, and heat the beaker with the chemical inside. 3. Heat until the sample turns white and then remove from heat immediately. Place the beaker on a hot pad to cool. 4. Unplug the hot plate. Let the beaker finish cooling on the hot pad, then measure the mass of the beaker. 5. Return the equipment back where they belong. Put the chemical in the disposing container.
Brad Wang, Daniel Kang, Scott Corkum, Sourabh Das
25/01/2014
Observations: Empty Beaker with Stirring Rod (g) Beaker with Stirring Rod + Hydrated Copper (II) Sulfate (g) Beaker with Stirring Rod + Anhydrous Copper (II) Sulfate (g)
Other Observations
① ② ③
130.6 133.3 132.3
Water vapour condensed along inner wall of beaker 2 – 3 minutes into the reaction and then quickly evaporated again There was a clear change of colour from blue to a white powder. Further exposure to heat turned the powder grey then brown (mass was measured before this change)
Analysis: 1. (a) From the masses measured, the mass of hydrated Copper (II) Sulfate is ②-①, which is 2.7 g. The mass of the water molecules is given by ②-③, which is 1.0 g. Hence, the mass percentage of water in Hydrated Copper
(II) Sulfate is
.
(b) The mass percentage of water should approximately be the same among different groups because the percentage refers to the ratio of the mass of water and Copper (II) Sulfate. The ratio is directly correlated to x, the coefficient of the hydrated molecule. Hence the mass of the hydrated Copper (II) Sulfate may vary in individual groups, but the ratio should remain the same.
Brad Wang, Daniel Kang, Scott Corkum, Sourabh Das
25/01/2014
Conclusion: 3. The molar mass of water is 18.02 g/mol, and the molar mass of Copper (II) Sulfate is 159.62 g/mol. We get the number of moles used in the experiment for both water and Copper (II) Sulfate by di viding the mass of each by the molar mass of the substance. Note that the mass of anhydrous Copper (II) Sulfate can be found by subtracting the mass of water from mass of the hydrated Copper (II) Sulfate, which gives 1.7 g.
Given the number of moles for both copper (II) sulfate and water, we determine the ratio of water molecules per one Copper (II) Sulfate, by dividing the number of moles for copper (II) sulfate by the number of moles of water.
Rounding x to the nearest whole number gives us x = 5. Therefore, the chemical formula for hydrated Copper (II) Sulfate is
• 4. Heating a hydrated ionic compound in a test tube, will cause droplets of condensed water vapour to form near the mouth of the test tube. The test tube will be cooler near the mouth as it is receiving less heat and glass does not conduct heat very well. The water evaporating from the hydrated compound will condense when it comes in contact with the glass and form water droplets there. 5. (a) Using the hydrated Copper (II) Sulfate chemical formula, • , we can find the percentage composition.
(b) If we know the anhydrous ionic compound, knowing the percent by mass of water in a hydrate is more useful, in terms of figuring out the chemical formula of hydrate Copper (II) Sulfate. When dealing with percent by
Brad Wang, Daniel Kang, Scott Corkum, Sourabh Das
25/01/2014
mass of water, we only need to figure out the ratio, in contrast to using percentage composition, which will require tedious steps to deduce the empirical formula. 6. At the end of the class, when we compared the results with two other groups, both groups procured a value of x=5. There were differences in the decimal places, but because the differences were minuscule, they could be considered as experimental errors, and rounding annulled the significance of the aberration. 7. (a) If a hydrated compound was not completely converted to its anhydrous form, the mass of water molecules would be significantly less than measured in our experiment. I t would directly affect the percent by mass of water to decrease. (b) With the percent by mass of water decreasing, the calculated ratio of water molecules to copper (II) sulfate would be lower. Hence, the value of x would also be calculated to be lower.
8. (a) If the hydrate spattered out, the net result would be a greater calculated percentage mass of water, as the mass taken for the anhydrous compound would be lower than it should be. Then the difference between the mass of the hydrated compound and the anhydrous compound would be greater leading to a greater calculated mass of water. (b) With the percent by mass of water increasing, the ratio of water molecules to the anhydrous compound molecules would also increase leading to a greater x. 9. (a) Applying too much heat to the compound before cooling it, may cause it to decompose. If any gasses are released, its mass will not be monitored and the total change in mass between the hydrous and anhydrous compound will be calculated to be greater than it is leading to a greater derived x. (b) If there are any impurities in the beaker beforehand, the heat and the water vapour may cause it to react and form a compound which would increase the calculated mass of the anhydrous compound. This would lead to the percent by mass of water to be calculated lower than it is. This would lead to a l ower x.
References:
*1+: "Finding the formula of hydrated copper(II) sulfate | Nuffield Foundation." Nuffield Foundation |. N.p., n.d. Web. 23 Jan. 2012. .
*2+: "hydrate." Encyclopædia Britannica. Encyclopædia Britannica Online . Encyclopædia Britannica Inc., 2012. Web. 23 Jan. 2012. .
