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NYS Chemistry Regents June 2006
Chemistry Regents June 2006 - Question 84 PDF Print E-mail
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Correct answer: 98.6%

Why?:

Percent error is a relative measure of the deviation of an experimentally obtained value from the true value. This can also serve as a standard for comparison if an experiment can be valid or not. In a measurement, a negative percent error indicates that the experimental value is lower than the true value. A positive percent error indicates that the experimental value is higher than the true value. The difference from the true value is the magnitude of the percent error.

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Chemistry Regents June 2006 - Question 83 PDF Print E-mail
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Correct answer and Why:

The transitions from higher-energy to lower-energy levels by electrons are of different energies for different elements. This means that the emission spectrum of an element is a fingerprint that identifies its presence. By confirming the presence of the spectral lines characteristic of an element, the identity of an element can be confirmed.

A star produces considerable amounts of energy. This energy is exciting the electrons of the elements on the surface of the star to higher energy levels. These electrons are in their excited states and are unstable. For them to be stable, they would have to release the extra amount of energy and go back to their ground states. With this transition, the energy released will be in the form of light. Light is composed of different wavelengths corresponding to the energies in an atom. With the different wavelengths of the light produced, scientists will have a way of identifying the kinds of elements present based on the spectra that will be produced.

Answering this question:

Always remember that a spectrum an element produces is unique to that element. This means that an unknown element can be identified by comparing its spectrum with that of a known element. Recall also that atoms emit light because they have released energy. Light is a form of energy. This only happens if electrons go down from a higher energy level to its ground state. But electrons will only be in a higher energy level if they have absorbed energy. Since a star is home to many reactions that produce great amounts of energy, the elements on its surface can definitely absorb energy and get their electrons excited.

So the spectra of the elements on the surface of a star can be used to determine and confirm their identities.

 
Chemistry Regents June 2006 - Question 82 PDF Print E-mail
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Correct answer: Hydrogen's bright-line spectrum is produced when an excited electron of a hydrogen atom transfers from a higher energy level to a lower energy level.

Why?:

The electric current that is passed through the hydrogen excites electrons. An electron is initially in its lowest energy level. This electron is said to be in its ground state. The electron absorbs energy from the electric current. This electron then climbs up to a higher energy level. After sometime, it will go back to its ground state. On its way to the ground state, it releases energy. This energy is manifested in the form of light. This light corresponds to different wavelengths that also correspond to different energy values. And as we know, different colors of light are characteristic of the energy of the light.

Answering this question:

Recall that light is a form of energy.  When light is produced, energy is released. This released energy is the light. Energy can only be released if the energy of an energy-containing body will decrease. In an atom, energy release can only occur when an electron goes down from a higher energy level to a lower energy level.

So the line spectrum of a hydrogen atom is due to the transition of electrons from a higher energy level to its ground state.

 
Chemistry Regents June 2006 - Question 81 PDF Print E-mail
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Correct answer: In a voltaic cell, the electrochemical reaction is spontaneous. In an electrolytic cell, it is nonspontaneous.

Why?:

A voltaic cell uses the energy produced by a chemical reaction to generate electricity. The chemical reaction in the voltaic cell is spontaneous, and produces an electrical current, converting chemical energy to electrical energy.

In contrast the electrolytic cell used in the Hall process consumes electrical energy to drive a chemical reaction. An amount of energy is still needed to drive the reaction process. It is not spontaneous.

Answering this question:

In answering this question, it is important to have knowledge on how voltaic and electrolytic cells work. A voltaic cell is spontaneous because it occurs without the "push" of an outside force. Moreover, it produces energy instead of consuming it. An electrolytic cell on the other hand needs a driving force before it can start. So it is nonspontaneous.

To sum it up, an electrochemical reaction is spontaneous in a voltaic cell while nonspontaneous in an electrochemical cell.

 
Chemistry Regents June 2006 - Question 80 PDF Print E-mail
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Correct answer: Molten Cryolite conducts electricity because it furnishes charged particles.

Why?:

Cryolite is an ionic compound. When in solution, it ionizes to produce charged particles. These are the sodium cations, aluminum cations, and the fluoride anions. To conduct electricity a substance must have charged particles that are mobile. Molten cryolite has three different ions that can move through the liquid, allowing the flow of charge from one end of a sample of molten cryolite to the other.

Answering this question:

Always remember that electricity is the flow of electrons. Electrons are charged particles. This also means that the ions can conduct electricity if they are moving from one place to another to facilitate the flow of charges.

Therefore, molten cryolite conducts electricity because it furnishes ions in the solution.

 
Chemistry Regents June 2006 - Question 79 PDF Print E-mail
chemjun06q79

Correct answer: Al+3 + 3e- →  Al

Why?:

A half-reaction shows the change in oxidation state of an ion. Reduction is a process wherein an atom or ion gains electrons. If an atom or ion gains electrons, the oxidation state decreases. The number of electrons gained is the same as decrease in the oxidation state.

Answering this question:

Since reduction involves the gain of electrons, there should be a decrease in the oxidation state. And since electrons are gained, they should appear on the reactant side. Write first the equation representing the transformation of Al+3 to Al.

Al3+ Al

This equation is not charge-balanced. We start with a +3 charge on the left side and end up with no charge on the product side. We need to add electrons to the left side to make this equation balanced.

Al3+ + 3e- Al

This is the complete half-reaction equation.

So the correct balance equation is Al+3 + 3e- → Al

 
Chemistry Regents June 2006 - Question 78 PDF Print E-mail
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Correct answer: Na = +1, Al = +3, F = -1

Why?:

Cryolite is an ionic compound with the formula Na3AlF6. Of these elements, fluorine is by far the most electronegative. In fact, it is the most electronegative element. In ionic compounds it will accept one additional electron to fill its octet and achieve a noble gas configuration. This extra electron will give fluorine a -1 charge. This is equivalent to saying that fluorine has a -1 oxidation state.

Sodium on the other hand is a metal in the first column of the periodic table. It loses an electron to form a Na +1 cation. This is equivalent to saying that Na has a +1 oxidation state.

Aluminum belongs to group IIIA or group 13 of the periodic table. It has 5 valence electrons. This means that aluminum is capable of accepting 3 more electrons for a complete octet. Therefore it has an oxidation state of +3.

The molecular formula as a whole is uncharged or neutral. It has an overall charge of zero. We also know that it contains three positive charges from its sodium cations and six negative charges from its fluorine anions. This means that aluminum must have a +3 charge to make the molecule neutral.

Answering this question:

Sodium, aluminum and fluorine are all representative elements. This means that they belong to the s-block and/or p-block of the periodic table. The oxidation states of the representative elements can be predicted through their group numbers. Elements under the group IA or 1 of the periodic table has an oxidation state of +1. Group IIA or 2 has +2. Group IIIA or 13 has +3. Group IVA or 14 has +4 or -4. Group VA or 15 has -3. Group VIA or 16 has -2. Group VIIA or 17 has -1.

Sodium is under group 1 so it has an oxidation state of +1. There are 3 sodium atoms in Cryolite so it would be a total of a +3 oxidation state for all the sodium atoms. Fluorine is under group 17 so it has an oxidation state of -1. There are 6 fluorine atoms, so it would be a total of -6 oxidation state for fluorine. Aluminum is under group 13 so it has an oxidation state of +3. There is only one aluminum atom in Cryolite. To verify, we add all the oxidation state. Since Cryolite is a neutral compound, the sum of the oxidations states must be zero.

So the correct answer is Na = +1, F = -1, and Al = +3.

 
Chemistry Regents June 2006 - Question 77 PDF Print E-mail
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Correct answer: 3.9 mL

Why?:

Density is the ratio of the mass of a substance to its volume. The volume can just be derived if the density and the mass of the substance are known. A given substance has a unique density and this can serve as a qualitative basis for the identity of a substance.

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Chemistry Regents June 2006 - Question 76 PDF Print E-mail
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Correct answer: Density

Why?:

Density is a physical property of any material. It is defined as the mass of a substance per unit volume. Each substance has a unique density. This means a substance can be identified through its density. Moreover, density is an intensive property. This means that it is independent of the size of the substance. No matter how big or how small a gold sample will be, the density will still be the same.

Archimedes measured the mass of the crown. He also determined the volume using water-displacement method. He compared the data obtained to the masses and volumes of gold, tin, and copper. If the mass and the volume of the crown was the same with that of gold, then he could have concluded that the crown is made purely of gold. However, it was found out that the data gathered using the crown were not the same with that of gold. So he concluded that the crown is not made purely of gold.

Answering this question:

Notice that the only physical property Archimedes obtained were the mass and the volume. With the mentioning of the water displaced by the crown, we would have the idea that it was volume that was being measured. It is through water-displacement method. Since density is defined as the mass of a substance per unit volume, then Archimedes used the density of the materials to verify the purity of the gold crown.

So the correct answer is density.

 
Chemistry Regents June 2006 - Question 75 PDF Print E-mail
chemjun06q75

Correct answer: Seawater

Why?:

The pH is a logarithmic scale with base 10. It is calculated as pH = -log[H+]. One common definition of acidity is the ability of a substance to donate H+ ions to solution. So the more acidic the substance, the greater H+ concentration it produces, and the lower its pH (note the negative sign in the equation). Since the equation that defines pH is logarithmic, a factor of 100 in [H+] causes a change in pH level by 2. This means that an acid that is 100 times more acidic than milk of magnesia will have a pH that is 2 lower than the pH of milk of magnesia.

Answering this question:

Locate in the graph milk of magnesia and determine its pH. According to the graph, the pH of milk of magnesia is 10. Anything more acidic than milk of magnesia will have a pH that is lower than 10. Since we are looking for the mixture that is 100 times more acidic than pH 10, we will choose the mixture that has a pH that is lower than 10 by 2. The mixture we are looking for must have a pH of 8 since it is 2 pH values lower than 10. The mixture with a pH of 8 is seawater.

Therefore, seawater is the mixture that is 100 times more acidic than milk of magnesia.

 
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