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Chemistry Regents June 2010 - Question 12 |
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Written by The Chemistry Wizard
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Answer: 1
Why? Sublimation is the transition of a species from one state to another without passing through the intermediate state. This means the species will go from solid to gas without passing through the liquid phase. Three good examples of this are carbon dioxide, iodine and water.
Carbon dioxide will go from solid to gas without passing through the liquid phase at standard pressure. Water however will sublime, but only under the right temperature and pressure. This is what allows us to freeze-dry products, the temperature is dropped below the melting point of water and the pressure is decreased. This change in temperature and pressure will allow water to transition from solid to gas without transitioning through the liquid phase.
The above equations have I2 in different states, as shown by the letters in (). The (s) represents solid, (l) is liquid and (g) for gas. Sublimation involves the species going from (s) to (g) or vice versa.
Answering the Question:
To answer the question, paying close attention to the states of the element is important. Sublimation would have occurred if the compound transitions from solid to gas or vice versa. Answer (1) shows iodine transitioning from solid to gas as is indicated by the letters in parentheses ().
Where
(s) signifies a solid,
(l) a liquid and
(g) a gas.
Answer (2) has the molecule going from solid to liquid, Answer (3) from liquid to gas and answer (4) liquid to solid. As sublimation requires the movement from one transition to another without passing through the intermediate transition, then answer (1) is the only possible answer. It goes from solid to gas without becoming a liquid. |
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Chemistry Regents June 2010 - Question 15 |
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Written by The Chemistry Wizard
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Answer: 2
Why? An ideal gas is a theoretical concept of how gaseous systems should behave. At standard temperature and pressure, real gases usually behave according to the ideal gas laws. In order for a real gas to behave in an "ideal" manner, it must be in a state that most represents a gas. That is the particles should be in a free state of random motion with as little intermolecular forces impaction on this state as possible. Therefore, to behave most like an ideal gas it must have sufficient energy to overcome any intermolecular forces of attraction and low pressure to facilitate the free random motion. The concept of an ideal gas does not allow for transitioning of states, for example gas to liquid or gas to solid. Hence conditions that would cause a gas to tend towards another phase are contrary to the concept of an ideal gas.
Reason for answer: To answer the question, it is important to understand the difference between an ideal gas and a real gas. With the ideal gas concept in mind, it is now possible to analyze the possible answers. High pressure and low temperature will result in a gas trending towards becoming a solid or liquid. This is the same principle that is used in the process of refrigeration. Where a gas is compressed or placed under high pressure to force it towards becoming a liquid. The question asks the conditions under which a real gas is least like an ideal gas. When the temperature is low, gases tend towards a phase change to a liquid. The same is true, all other conditions remaining constant, when the pressure is high, a gas trends towards a phase change to a liquid or solid. Since both these phases are least like an ideal gas, then the answer would be the one without low pressure and high temperatures, which leaves answer (2) as the only possible answer. |
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Chemistry Regents June 2010 - Question 16 |
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Written by The Chemistry Wizard
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Answer: 2
Why? The separation of substances can occur in many ways, the separation of salt from water by boiling for example. The physical separation of LiCl is possible by utilizing chromatography.
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Chemistry Regents June 2010 - Question 17 |
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Written by The Chemistry Wizard
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Answer: (1)
Why? Many scientists have contributed to the development of what we now know as the ideal gas law. The earliest contributor to it was Robert Boyle in 1622. He stated that at constant temperature, the product of an ideal gas's pressure and volume is always constant. This law was later called Boyle's Law. Charles's Law was the next addition to the ideal gas law by Jacques Charles in 1787. He stated that the volume of an ideal gas is proportional to the absolute temperature when pressure is kept constant. Joseph Louis Gay-Lussac was next in 1809 when he stated that, the pressure exerted on a container's sides by an ideal gas is proportional to the absolute temperature of the gas. Finally Amedeo Avogadro in 1811 stated that an ideal gas at the same temperature, pressure and volume contains the same number of molecules. This implies that the number of particles of a specific volume of gas is independent of the size of the particle.
Based on the stated laws the combined gas laws was formulated and shows the relationship between pressure, temperature, volume and number of molecules, it can be summarized in the following equation.
PV=kNT where:
P=Pressure
V=Volume
k is the Boltzmann constant (1.381x10-23j.k-1)
N= number of molecules or atoms
T= temperature
The above equation is a summery of the relationship between pressure, volume, moles and temperature.
Reason for Answer:
To answer this question it is important to have an understanding of the ideal gas law, as this will be the principle used to determine the number of atoms contained in a liter of gas. Helium at STP is a gas, and so are all the other listed possible answers.
Based on this equation PV = kNT, if the pressure, volume and temperature are kept constant, then the number of molecules of gas will be the same regardless of the gas. The equation does not mention the impact of where the gas falls on the periodic table or that the size of the atom will impact on the number of moles.
Based on the ideal gas laws, whatever gas has the same volume as helium would also have the same number of atoms since all other factors are constant. Therefore, the only possible answer would be Answer (1), 1.0L of Ne. |
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Chemistry Regents June 2010 - Question 18 |
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Written by The Chemistry Wizard
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Answer: (4)
Why? An ideal gas is described as a set of randomly moving non-interacting particles. The ideal gas is composed mostly of empty space such that the volume of the individual particles is negligible. The ideal gas is a theory of what a gas should be like if it were to be the perfect gas. This means it would have to behave most like a gas and least like a liquid or solid. As a result the forces of attraction between the particles would be non-existent in an ideal gas and that would impact its behavior, causing it to behave closer to a liquid or solid.
Answering the Question:
To answer the question it would be easier to have an understanding of what makes a gas a gas. As such answer (1) would not be correct, as the particles are said to be moving in a well-defined, circular path. A well-defined state of movement does not occur for particles occurring in nature. Answer (2) suggests that particles in an ideal gas release energy. This would imply that as gases collide, the temperature of the system would decrease, as energy is lost. A decrease in energy of the system would cause the system to stop behaving like an ideal gas. The closer the energy contained in atoms gets to absolute zero, the more solid the element becomes. Answer (3) suggests that there are forces of attraction between particles. The concept of an ideal gas is that in order for the particles to move randomly and not interact, there should be no forces of attraction between them. Therefore, (4) is the correct answer as it does meet the requirements of an ideal gas, in that the volume of the particle is negligible with respect to the volume of empty space in the system. |
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Chemistry Regents June 2010 - Question 20 |
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Written by The Chemistry Wizard
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Answer: (4)
Why? The equation is that of a reversible reaction in equilibrium, this means that the forward reaction and the reverse reaction are both proceeding at the same rate. This result is no net change in the mass of H2O in either phase. A reversible reaction in equilibrium will have no net change in reactants or products unless some external factor comes to play. For example unless there was a change in temperature, pressure, contaminants or some other such factor.
A reversible reaction in equilibrium however, does not mean that the mass or concentration of the reactant and product are equal, just that they are constant. So there could be significantly more H2O(l) than H2O(s) for example, however the ratio of each remains constant. If either the product or the reactant starts to be produced more rapidly, then the system is no longer in equilibrium.
Answering the Question:
To answer this question all that needs to be understood is the concept of equilibrium. The term equilibrium does not mean equal but that there is no change in the amount of reactant or product. Therefore any answer, which does not describe this, would be the incorrect answer. Answer (1) says the H2O(s) is melting faster than the H2O(l). This means that there is a change in the amount of both forms of H2O, implying that the system is not in equilibrium. Answer (2) has a similar issue to answer (1) in that one part of the process is occurring faster than the other. In this case the H2O(l) is freezing faster than the H2O(s) can melt. This means that eventually only frozen H2O would be present. Answer (3) speaks to equivalence and not equilibrium, leaving answer (4) which says that whatever the amount of each form of H2O there is no change in that mass. |
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Chemistry Regents June 2010 - Question 39 |
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Written by The Chemistry Wizard
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Answer: (1)
Why? Vapor pressure is the pressure of a gas in equilibrium above its liquid or solid phase in a closed system. When a liquid boils, it is because the temperature reached has caused the vapor pressure to equals atmospheric pressure. Vapor pressure increases with increasing temperature in a non-linear manner. However, the degree of increase in vapor pressure can normally be linked to the boiling point of the compound. Such that, compounds with lower boiling points will have a higher vapor pressure with increasing temperature than compounds with higher boiling points. SO compounds with higher boiling points will always have the lower vapor pressure as temperature increases.
Reason for Answer: To answer this question, it is important to know the boiling points of the given compounds. Answer (1) is ethanoic acid, which has a boiling point of 118°C. Answer (2), ethanol, has a boiling point of 79°C. Answer (3), propanone, has a boiling point of 56°C and answer (4), water has a boiling point of 100°C. Ethanoic acid has the highest boiling point of the listed compounds and hence is a safe answer to choose for the lowest vapor pressure at 50°C. Therefore, the answer is answer (1), ethanoic acid. |
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Chemistry Regents June 2010 - Question 41 |
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Written by The Chemistry Wizard
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Answer: (2)
Why? The ideal gas law tries to explain the relationship of different factors on the behavior of the theoretical ideal gas.
In essence pV = nRT, where; p is absolute pressure, V is the volume, n is the number of moles of the gas, T is temperature and R is the universal gas constant.
Based on the relationship defined in the equation pV=nRT, volume (V) and temperature (T) are directly proportional. This means that as volume increases, so will temperature, or as temperature increases so will volume if in both cases pressure remains constant. Because the relationship is a linear one, it also means that, as volume doubles, all other factors being constant, temperature will also double.
Answering the Question:
To answer this question it helps to have an understanding of the behavior of gases and the relationship of pressure, temperature and pressure. Answers (1) and (3), as they propose an inverse relationship between temperature and volume, would not be correct. Based on the equation defining the ideal gas pV=nRT, there is a directly proportional relationship between volume and temperature. The ideal gas law is also based on absolute temperature; the unit for absolute temperature is specified in Kelvin (K). Although Kelvin and Celsius increase by the same degree, they have different values. Using kelvin as the unit means that answer (2) is a better answer than answer (4), as it uses the unit K rather than °C. If knowledge of the gas law and hence equation, was not known, then answers (2) and (4) would have been a good starting point. It is generally known that increasing temperature causes most substances to expand. This could be assumed to hold true for gases, also eliminating answers (1) and (3). This would increase the probability of choosing the correct answer from answers (2) and (4). Answer (2) is the correct answer. |
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Chemistry Regents June 2010 - Question 42 |
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Written by The Chemistry Wizard
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Answer: (4)
Why? Table F lists the solubility guidelines for aqueous solutions. That is it lists compounds that are soluble in aqueous solution and the exceptions, it also lists the common ions are form insoluble compounds in aqueous solution.
Answering the Question:
Utilizing the supplied table;
Answer (1), barium phosphate, the table states that phosphates tend to be insoluble, meaning that answer (1) is not a good option.
Answer (2) calcium sulfate; While sulfates generally are soluble in aqueous solutions, there are a list of exceptions of which the cation Ca2+ is one, eliminating answer 2 as a possible answer.
Answer (3) silver iodide; while halides generally are soluble in aqueous solutions, halides of silver, lead and mercury are not.
Answer (4) sodium perchlorate; perchlorates are soluble in aqueous solutions with no exceptions. Therefore answer (4) is the best possible answer. |
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Last Updated on Friday, 10 June 2011 16:42 |
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Chemistry Regents June 2010 - Question 48 |
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Written by The Chemistry Wizard
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Answer: (3)
Why? An electrolyte is any ionic solution capable of conducting an electrical current. Typically, this means most electrolytes are acids, bases or salts in solution. The main function of an electrolyte is to dissociate to allow for the neutralizing of the electrons being produced at the anode and being consumed at the cathode. In that respect, an electrolyte must be able to dissociate to form an anion (negatively charged ion) and a cation (positively charged ion). Electrolytes will dissociate in water, as water molecules are dipoles and will orient themselves in a manner that is suitable to solvate the ions.
Answering the Question:
To answer this question knowledge of the definition of an electrolyte is helpful. Answer (1) suggests the pH of aqueous potassium chloride as a test to determine whether or not it is an electrolyte. The power of hydrogen (pH) is used as a measure of acidity or basicity in aqueous solutions and not a measure of conductivity. Therefore, answer (2) would also be eliminated as it, too, suggests pH as a method of determination.
Answer (3) suggests the electrical conductivity of aqueous potassium chloride. To determine whether or not potassium chloride is an electrolyte, it would have to be dissolved in water or molten. The (aq) indicates that potassium chloride is dissolved in water. This is important, as an electrolyte will conduct not from the flow of electrons but instead as a result of a chemical reaction. Answer (4) suggests the conductivity of solid potassium chloride. As KCl is a bonded compound with no free electrons (in metals), then it would not be expected to conduct an electrical current. This leaves answer (3) as the only possible answer. |
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