Physical Science Answer Key, Part 2
11. The correct answer is choice "A" as Ohm's law states that the amount of current passing through a conductor between two points is directly proportional to the voltage between those two points. Ohm's law also states that the amount of current passing through a conductor between two points is inversely proportional to the resistance between those two points. In other words, as the amount of voltage between two points in a conductor increases, the amount of current between those two points will increase as well. On the other hand, as the amount of resistance between two points in a conductor is increased, the current between those two points will decrease. The concepts stated by Ohm's law are extremely important as they allow electrical engineers and other similar individuals to determine how much resistance or voltage is required in order to achieve the desired level of current within a particular circuit.
12. The correct answer is choice "A" as Kirchhoff's first law, also known as Kirchhoff's current law, states that if an individual chooses any given point in an electrical circuit the sum of the currents flowing towards that point will be equal to the sum of the currents flowing away from that point. However, it is important to note that Kirchhoff's first law is only true if the amount of current remains constant. Choice "B" also refers to one of Kirchhoff's laws as Kirchhoff's voltage law states that the sum of all of the voltage differences in any circuit must equal to zero, but choice "B" is incorrect for this particular question because it relates to Kirchhoff's second law rather than his first law.
13. The correct answer is choice "B" as the pKa of Substance A is approximately 4.20. This is because the pKa of a substance is equal to the -log of the Ka of that substance. In other words, pKa = -log Ka so the pKa of Substance A is equal to the -log of 6.3 X 10-5, which is approximately 4.20. It is important to note that the pKa of a substance is equal to the negative log of the Ka of a substance and not the standard log of the Ka of that substance. In fact, it is extremely important to use a negative log because the standard log will cause the substance to appear significantly stronger or weaker than the substance actually is. This is because the pKa will be positive when it should be negative or negative when it should be positive.
14. The correct answer is choice "C" as the pKa of Substance B is approximately -7.0. This is because the pKa of a substance is equal to the -log of the Ka of that substance. In other words, pKa = -log Ka so the pKa of Substance B is equal to the -log of 107, which is approximately -7.0. It is important to note that the pKa of a substance is equal to the negative log of the Ka of a substance and not the standard log of the Ka of that substance. In fact, it is extremely important to use a negative log because the standard log will cause the substance to appear significantly stronger or weaker than the substance actually is. This is because the pKa will be positive when it should be negative or negative when it should be positive.
15. The correct answer is choice "C" as the pKb of Substance C is approximately 4.75. This is because the pKb of a substance is equal to the -log of the Kb of that substance. In other words, pKb = -log Kb so the pKb of Substance C is equal to the -log of 1.8 x 10-5, which is approximately 4.75. It is important to note that the pKb of a substance is equal to the negative log of the Kb of a substance and not the standard log of the Kb of that substance. In fact, it is extremely important to use a negative log because the standard log will cause the substance to appear significantly stronger or weaker than the substance actually is. This is because the pKb will be positive when it should be negative or negative when it should be positive.
16. The correct answer is choice "C" as the pKa of Substance C is approximately 9.25. This is because the pKa of a substance is equal to 14 minus the pKb of that substance. In other words, pKa = 14 - pKb so the pKa of Substance C is equal to 14 - 4.75 or 9.25. It may seem strange to subtract the pKb from14 as 14 might seem to be simply a random number, but it is actually not as random as it appears. In fact, the dissociation constant of water, also referred to as pKw, is 14 and the pKb and the pKa of a substance must equal the dissociation constant of water so pKb + pKa = pKw and pKw = 14 at 25°C.
17. The correct answer is choice "B" as substance A is actually a weak acid. The tests performed by the laboratory indicate that the substance is an acid because it has a pH of less than 7 and any substance that has a pH lower than the pH of water is considered to be an acid. Since water has a pH of 7 and substance A has a pH lower than 7, substance A is acidic. The tests also make it clear that the acid is a weak acid because the pKa of the substance, 4.20 is greater than -1.74 and acidic substances that have a pKa of less than -1.74 are considered to be strong acids and any acidic substance with a lower pKa is considered to be weak.
18. The correct answer is choice A" as substance B is actually a strong acid. The tests performed by the laboratory indicate that the substance is an acid because it has a pH of less than 7 and any substance that has a pH lower than the pH of water is considered to be an acid. Since water has a pH of 7 and substance B has a pH lower than 7, substance B is acidic. However, the tests also indicate that the acid is a strong acid because the pKa of the substance, -7.0, is less than -1.74 and acidic substances that have a pKa of less than -1.74 are considered to be strong acids and any acidic substance with a lower pKa is considered to be weak.
19. The correct answer is choice "D" as substance C is actually a weak base. The tests performed by the laboratory indicate that the substance is a base because it has a pH of greater than 7 and any substance that has a pH greater than the pH of water is considered to be a base. Since water has a pH of 7 and substance C has a pH greater than 7, substance C is a base. However, the tests also indicate that the base is weak because the pKa of the substance is less than 13 and basic substances that have a pKa of greater than 13 are considered to be strong bases and any basic substance with a lower pKa is considered to be weak.
20. The correct answer is choice "D" as it is impossible to determine which substance was used from the information provided. The tests that the forensics laboratory performed all indicate that substance B is a strong acid. In fact, based upon the pKa of the substance, it is possible that substance B is some form of hydrochloric acid, which could certainly have been used to destroy the lock. However, it is impossible to determine which substance was used and whether substance B is actually hydrochloric acid or not from the information provided. This is because these tests do not necessarily indicate that a strong acid was used to corrode the lock and they cannot indicate exactly what type of substance each substance is based solely on the information provided. It is actually a common misconception that all strong acids are corrosive and some weak acids are actually more corrosive than certain strong acids. This means that Substance B may actually be corrosive, but it is impossible to determine if substance B was used from the information provided.
