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AP Chemistry Formula Sheet 2014, Lab Reports of Chemistry

Chemical formula detective: determine the empirical formula of hydrate and draw the flow charts.

Typology: Lab Reports

2021/2022

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Bellevue College | CHEM& 161
Chemical Formula Detective:
Determining the empirical formula of a hydrate
Background
Different elements can form chemical bonds to create compounds. For example, sodium and
chlorine combine to form sodium chloride, NaCl. In the chemical formula NaCl, there is a 1:1
ratio of sodium ions:chloride ions. However, not all compounds form in a 1:1 ratio of their
constituent elements. If they did, John Dalton would have been correct in 1803 when he
proposed the chemical formula of water as HO. Of course, we now know that the correct
chemical formula of water is H2O, in which there is a 2:1 ratio of hydrogen atoms to oxygen
atoms. Since a mole is Avogadro’s number of atoms, H2O is also a 2:1 ratio of moles of
hydrogen to moles of oxygen. Thus, the atom ratio is equivalent to the mole ratio (not a mass
ratio) in a given chemical formula.
As chemistry students you have learned how to predict chemical formulas of ionic compounds
based on periodic trends and nomenclature rules, but it hasn’t always been that way. For
hundreds of years, the chemical composition of compounds was studied experimentally, and the
results generalized into the nomenclature rules used today. These rules allow the accurate
prediction of chemical formulas for many ionic compounds without doing any experimentation.
For example, the nomenclature rules can be used to correctly predict the formula of magnesium
iodide as MgI2 rather than MgI. The curious student of chemistry will wonder how such a
prediction could be verified by experimental means.
Your task is to determine the chemical formula of an unknown copper chloride hydrate by
experiment. An ionic hydrate is an ionic compound that has water molecules trapped within its
crystal lattice (refer to the index/glossary of your textbook for more information). For example,
Epsom salt (MgSO4·7H2O) is a heptahydrate of magnesium sulfate: within one mole of
magnesium sulfate heptahydrate there are seven moles of water. This water can be driven off by
heat to form the anhydrous (dehydrated) ionic compound, magnesium sulfate (MgSO4).
The chemical formula of your unknown copper chloride hydrate is in the general form of
CuxCly·zH2O. Your objective is to determine what the actual formula is (what are the integers x,
y, and z?) You will be required to make careful mass measurements and make calculations based
on these.
The Overall Strategy
John Dalton (1766-1844) made an assumption that when only one compound was formed from two elements, they did so in the simplest ratio,
1:1. (Water was the only known compound formed from hydrogen and oxygen at the time. Hydrogen peroxide, H2O2, was not discovered until
1815.) Since the mass ratio of oxygen to hydrogen in water is 8:1, he assigned the mass of hydrogen (the lightest element) to be 1 and , assuming
the formula HO, assigned the value 8 to oxygen. The correct formula of water a nd the relative atomic mass of oxygen as 16 was a puzzle that
would not be solved for another fifty years, despite evidence on the combining volumes of hydrogen and oxygen gas in a 2:1 ratio. Avodgadro’s
hypothesis would later be used to interpret this evidence correctly.
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Bellevue College | CHEM& 161

Chemical Formula Detective:

Determining the empirical formula of a hydrate

Background

Different elements can form chemical bonds to create compounds. For example, sodium and chlorine combine to form sodium chloride, NaCl. In the chemical formula NaCl, there is a 1: ratio of sodium ions:chloride ions. However, not all compounds form in a 1:1 ratio of their constituent elements. If they did, John Dalton would have been correct in 1803 when he proposed the chemical formula of water as HO∗. Of course, we now know that the correct chemical formula of water is H 2 O, in which there is a 2:1 ratio of hydrogen atoms to oxygen atoms. Since a mole is Avogadro’s number of atoms, H 2 O is also a 2:1 ratio of moles of hydrogen to moles of oxygen. Thus, the atom ratio is equivalent to the mole ratio (not a mass ratio) in a given chemical formula. As chemistry students you have learned how to predict chemical formulas of ionic compounds based on periodic trends and nomenclature rules, but it hasn’t always been that way. For hundreds of years, the chemical composition of compounds was studied experimentally, and the results generalized into the nomenclature rules used today. These rules allow the accurate prediction of chemical formulas for many ionic compounds without doing any experimentation. For example, the nomenclature rules can be used to correctly predict the formula of magnesium iodide as MgI 2 rather than MgI. The curious student of chemistry will wonder how such a prediction could be verified by experimental means. Your task is to determine the chemical formula of an unknown copper chloride hydrate by experiment. An ionic hydrate is an ionic compound that has water molecules trapped within its crystal lattice (refer to the index/glossary of your textbook for more information). For example, Epsom salt (MgSO 4 ·7H 2 O) is a heptahydrate of magnesium sulfate: within one mole of magnesium sulfate heptahydrate there are seven moles of water. This water can be driven off by heat to form the anhydrous (dehydrated) ionic compound, magnesium sulfate (MgSO 4 ). The chemical formula of your unknown copper chloride hydrate is in the general form of Cu x Cl y · z H 2 O. Your objective is to determine what the actual formula is (what are the integers x , y , and z ?) You will be required to make careful mass measurements and make calculations based on these.

The Overall Strategy

∗ (^) John Dalton (1766-1844) made an assumption that when only one compound was formed from two elements, they did so in the simplest ratio, 1:1. (Water was the only known compound formed from hydrogen and oxygen at the time. Hydrogen peroxide, H 2 O 2 , was not discovered until 1815.) Since the mass ratio of oxygen to hydrogen in water is 8:1, he assigned the mass of hydrogen (the lightest element) to be 1 and , assuming the formula HO, assigned the value 8 to oxygen. The correct formula of water and the relative atomic mass of oxygen as 16 was a puzzle that would not be solved for another fifty years, despite evidence on the combining volumes of hydrogen and oxygen gas in a 2:1 ratio. Avodgadro’s hypothesis would later be used to interpret this evidence correctly.

The formula will be determined by careful mass measurements. Remember, you are starting with Cu x Cl y · z H 2 O. You will decompose this into several components, taking mass measurements along the way. The first step is to gently dehydrate a known mass of your sample. The resulting dehydrated sample will be weighed to determine the amount of water lost (this is the z H 2 O part). The dehydrated copper chloride (now just Cu x Cl y ) will be made into a solution, dissolving the sample into water, making a mixture of copper ions and chloride ions. The copper ions will be reduced∗^ to copper metal, which will be collected, dried, and weighed (now just Cu x ). The remaining task is to determine the mass of chloride∗∗^ in the compound, which can easily be done by mass difference. (The masses of the initial sample, water lost, and copper were determined in the previous steps.) These steps should give you enough data to figure out the chemical formula of the unknown copper chloride hydrate.

A flow chart for today’s experiment:

∗ (^) Reduction of copper means that copper ions gain electrons to form copper metal. These electrons will be provided by the oxidation (loss of electrons) of an aluminum wire in the solution. ∗∗ (^) The mass of chloride (Cl-^ ion) is being determined. The difference between the mass of chlorine (Cl) and chloride (Cl-^ ion) is negligible. (Why?)

Cu x Cl y · z H 2 O (hydrate crystal)

(goal = find x, y, z)

Remove water

Cu x Cl y (anhydrous crystal)

Reduction

Cu (reduced copper)

  1. The reaction will slow down as the surface of aluminum is reduced. Use a glass stirring rod to scrape the copper from the wire as completely as possible, exposing more of the surface for reaction. Record your observations. What changes do you observe as the reaction slows down? How will you know when it is over? With your partner, determine when the reaction is finished.
  2. After the reaction is finished, remove the aluminum wire from the beaker with forceps and dispose of it in the waste container. Add 5 drops of 6M HCl to dissolve any insoluble aluminum salts and clear up the solution.
  3. In the next steps, you will be collecting the copper by filtration. Secure a filtration flask (Erlenmeyer flask with a side port) to a ring stand using a large clamp. Attach the side port of filtration flask to the vacuum line using a thick-walled rubber hose. Obtain a 7.5-cm diameter circular filter paper and write your name on it using a pencil. Record the mass of the filter paper. The filter paper collects any solid material and allows liquid to pass through, thus separating a solid from a liquid. It will be difficult to scrape the solid copper off the filter paper without losing some of it. (In a later step you will weigh the filter paper with your copper, once it has dried, and then calculate the mass difference to determine the mass of copper. This method is called “weighing by difference”.)
  4. Place your filter paper in a Büchner funnel. Place a rubber gasket on top of the filter flask. Insert the Büchner funnel. Pour some distilled water onto the filter paper to make the filter paper lay flat on the bottom of the Büchner funnel. Open the vacuum valve and adjust to obtain light suction. Pour all of the copper mixture into the center of the funnel. Use distilled water as necessary for the transfer and also to rinse the solid copper. Turn off the suction. Add 10 mL of acetone (a drying agent) to the funnel. Turn on the suction again and leave it on for 5 minutes. Sketch the **setup with labels in your notebook.
  5. Record the mass of a watch glass.** Use forceps to carefully transfer the filter paper with the copper to the weighed watch glass. Dry the copper by placing it (on the filter paper on the watch glass) in the oven, at 110°C, for about 1 5 minutes. Record the total mass (mass of the watch glass + filter paper + dried Cu). Keep your copper until your calculations are finished! Complete these calculations in your notebook. It is crucial that you carry extra digits through each step and only round at the end.
  1. Determine the number of moles of water lost from the hydrated unknown.
  2. Determine the number of moles of copper collected.
  3. Determine the number of moles of chloride there must have been in the compound. 4 ) Use the answers from above to determine the chemical formula of the copper chloride hydrate. (Note: It is possible that your chemical formula may look strange due to experimental error.) 5) Show your calculations to your instructor BEFORE you discard your copper.

Data Mass of crucible Mass of crucible + CuxCly•zH 2 O Color of CuxCly•zH 2 O crystals Mass of crucible + CuxCly Color of CuxCly solid Initial color of CuxCly solution (before adding Al) Final color of CuxCly solution (after reaction with Al) Mass of watchglass + filter paper Mass of watchglass + filter paper + dried copper Results mass (g) moles CuxCly•zH 2 O CuxCly water released copper chlorine Empirical formula of the copper chloride hydrate: ______________________ Report Sheets Name________________________

Chemical Formula Detective Lab partner_______________ Section______

  1. What effect would each of the following situations have on the calculation of the number of moles of copper in this experiment? Would the moles of copper increase, or decrease, or stay the same? Give a very brief explanation for each. a) You removed the aluminum wire while the mixture was still blue or contained bubbles. b) You added twice as much aluminum wire as necessary to the copper chloride. (Hint: Is aluminum a limiting reactant or in excess?) c) You couldn’t scrape all of the copper off the aluminum wire. d) The dehydration was not complete because your crystals were still green. You did not reheat to complete the dehydration.

  2. List at least two reasonable sources of error in this experiment. DO NOT list human error ( e.g. spilling chemicals), miscalculations, or significant figures/rounding errors.

Bellevue College | CHEM& 161

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