Acids, Bases & Salts
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Chemistry Regents June 2010 - Question 25 |
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Written by The Chemistry Wizard
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Answer: (2)
Why? An Arrhenius acid is defined as a molecule that will dissociate in aqueous solution to give a hydrogen ion (H+), now recognized as a hydronium ion (H3O+) as H+ cannot exist on its own. Also the definition of an Arrhenius base is one, which will dissociate in aqueous solution to give a hydroxide ion (OH-). However, this is not the only definition of what defines an acid and what defines a base.
The Bronsted-Lawry definition, developed independently by Johannes Bronstead and Thomas Lawry in 1923, describes as acid as a proton donor. This expands on the Arrhenius definition of acids. Therefore, a Bronsted-Lawry acid can exists even if a hydronium ion is not formed, once a proton is donated to another species.
The Lewis definition of an acid, named after Gilbert Lewis, the scientist who proposed it. His definition of an acid is any species capable of acting as an electron acceptor, meaning a proton does not necessarily have to be donated.
Answering the Question:
To answer this question an understanding of what an Arrhenius acid is, is helpful but not necessary. If you did not know the Arrhenius definition of an acid, then a good guess would be HCl simply as a result of its name, hydrochloric acid. The other categories of compounds could also be analyzed and HCl chosen by elimination.
Answer one (1) is the alcohol methanol, and while methanol does dissociate to give off a H+ in aqueous solution, it does so very weakly. Answer three (3) is sodium chloride, which is a salt, the result of an acid/base reaction, but not in itself an acid. Answer four (4) is sodium hydroxide, a very common base that dissociates to form Na+ and OH- satisfying the Arrhenius definition of a base. Answer two (2) will readily dissociate to give a H+ and as such is a better answer than (1) although technically speaking, both are correct. Therefore, answer (2) HCl is the correct answer |
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Chemistry Regents June 2010 - Question 26 |
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Written by The Chemistry Wizard
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Answer: (2)
Why? There are many definitions as to what defines an acid or a base. The three most used definitions are the Arrhenius acid, Bronsted-Lawry acid and the Lewis acid. Answer (2) agrees with all three definitions. The Arrhenius definition describes an acid as any species that will donate a H+ ion (H3O+) to the reaction. The Bronsted-Lawry definition describes an acid as a species that will donate a proton. The Lewis definition of an acid is any species capable of accepting a pair of electrons.
Analyzing the answers in turn, answer (1) is incorrect as none of our definitions corresponds with an acid being defined as a hydrogen acceptor; this definition is closer to the definition of a base.
Answer (3) and OH- acceptor, while this does sound correct it is also very limiting. Acids according to the Bronsted-Lawry and Lewis definitions are more than simply OH- acceptors. They can react with metals and other bases, which would not involve the transfer of OH- in any way.
For example;
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
Hydrochloric acid + sodium hydroxide → sodium chloride + water
In this example the acid acts as an OH- acceptor.
However, in the following example the acid is not accepting an OH-, but would still be defined as an acid.
2HCl(aq) + 2Na(s) → 2NaCl(aq) + H2(g)
Hydrochloric acid + sodium metal → sodium chloride + hydrogen gas
Answer (4) is suggesting that an acid is an OH- donor, this closer to the Arrhenius definition of a base.
Answering the Question:
Answering the question can be done once you have an idea of what an acid does. Hydrochloric acid (HCl) should be a familiar acid and is composed of hydrogen and chloride ions. Using hydrochloric acid as a guide, the questions can be analyzed in turn. Answer (1) suggests an acid is a hydrogen acceptor. As HCl already has hydrogen present it would make a very weak acceptor of hydrogen. Answer (2) suggests an acid is a H+ donor; HCl would make an excellent H+ donor as in aqueous solution it dissociates to H+ and Cl-. Answer (3) suggests OH- acceptor as a good definition of an acid. Referring to the reaction of HCl + NaOH, HCl will readily react with NaOH to from NaCl and water. However acids will react with metals to produce a salt and hydrogen gas. Therefore, the OH- acceptor definition is a very limiting definition of an acid. Answer (4) suggests OH- donor as a definition for an acid. An OH- donor would be a much better definition of a base. Therefore, answer (2) H+ donor is the most suitable answer. |
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Chemistry Regents June 2010 - Question 49 |
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Written by The Chemistry Wizard
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Answer: (1)
Why? The products and ratios of product, resulting from most chemical reactions can be theorized with the use of chemical equations. The amount of an element in a chemical reaction will be conserved. That is, whatever is used in the reaction will be present in one form or another in the product. For example, the reaction between H2SO4 and NaOH;
H2SO4(aq) + 2NaOH(aq) → Na2SO4(aq) + 2H2O(l)
sulfuric acid + sodium hydroxide → sodium sulfate + water
The amount of each element represented in the equation is equivalent for both reactant and product. There is one (1) unit of S in the reactant and one present in the product, the same holds for Na, where there are two (2) units of Na present in the reactant and product. In order to determine the ratio in which the reactants will react, it is important to have an understanding of the charge of the ion each atom or molecule will form. In the case of H2SO4 , for example:
H2SO4 → 2H+ + SO42- while NaOH → Na+ + OH-.
Since it will take two Na+ to fully react with one SO42-, the ratio of reactants needed for the reaction to theoretically go to completion can be determined.
Answering the Question:
To answer the question writing the chemical equation and determining possible products is a possible method. It is known that an acid and a base will react to form a salt and water. Based on this knowledge a chemical equation for the reaction can be determined.
HCl(aq) + Ca(OH)2 (aq) → CaCl + H2O, however this equation is not balanced or correct in terms of products formed. If the ionic forms of each molecule is analyzed.
HCl → H+ + Cl-
Ca(OH)2 → Ca2+ + 2OH- as calcium will lose 2 electrons to form a stable ion, typical of group 2 metals, while group 1 metals will lose one electron. Therefore, the corrected equation would read:
2HCl(aq) + Ca(OH)2 (aq) → CaCl2 + 2H2O
The equation now takes into consideration the quantities of each element present in the reactant and present in the product. Since CaCl2 is the only salt produced from the chemical reaction, the correct answer would be answer (1) CaCl2. |
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Chemistry Regents June 2010 - Question 77 |
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Written by The Chemistry Wizard
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Answer: The hydrogen ion concentration of tomato juice, which has a pH of 4, would be:
-log10[H+] = 4, therefore, [H+] = 0.0001mol/L.
While the hydrogen ion concentration of milk of magnesia would be:
-log10[H+] = 10. Therefore, [H+] = 0.0000000001 mol/L.
This means that the concentration of hydrogen ions present in tomato juice is far greater than the concentration of hydrogen ions present in milk of magnesia.
Why? pH, sometimes referred to as the power of hydrogen, is a measure of the hydrogen ion concentration in aqueous solutions, or a measure of how acidic or basic a solution is. Solutions with a pH less than 7 are said to be acidic (will turn blue litmus red), and those with a pH above 7 are called basic (will turn red litmus blue). The pH is a measure of the hydrogen ion concentration, where pH = -Log10[H+] and H+ is the molar concentration (mol/L) of hydrogen ions. This means that a solution with a pH of 4 would have a hydrogen ion concentration of 10-4 mol/L or 0.0001 mol/L. A solution with a pH of 10 would have a molar concentration of hydrogen ions of 10-10 mol/L or 0.0000000001 mol/L.
Answering the Question:
To answer the question, it is important to understand the method of calculation used to determine the hydrogen ion concentration from pH.
pH = -log10[H+] |
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Chemistry Regents June 2010 - Question 78 |
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Written by The Chemistry Wizard
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Answer: Mg(OH)2(aq) ↔ Mg2+(aq) + 2OH-(aq)
The negative ion would be OH-(aq)
Answering the Question:
The negative ion would be the one that is negatively charged. To determine the ion with the negative charge, it is necessary to write out the balanced chemical equation for magnesium hydroxide in solution. The balanced equation would help identify the dissociated species involved, in this case magnesium ion and hydroxide ion.
Mg (OH) 2(aq) ↔ Mg2+(aq) + 2OH-(aq)
To determine how magnesium hydroxide will dissociate the periodic table and knowledge of the nature of the hydroxide ion will help in the determination of the dissociation. Magnesium is a group 2 metal and will, therefore, donate 2 electrons when ionizing to achieve a noble gas configuration. It is also known that OH- has a charge of -1, therefore, two OH- ions are necessary for the balancing of the equation, but also OH- is the negative species present in the dissociation. |
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Last Updated on Wednesday, 08 June 2011 09:45 |
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Chemistry Regents June 2010 - Question 79 |
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Written by The Chemistry Wizard
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Answer: Thymol blue indicator would be blue when placed in a sample of milk of magnesia. Milk of magnesia is strongly basic; thymol blue will turn blue in basic solutions with a pH greater than approximately 8.0.
Why? Thymol blue indicator is a alcohol soluble pH indicator. It is red in heavily acidic solutions (pH below 1.7) transition to yellow between pH 1.2-2.8 and remain yellow until another transition phase at about pH 8.9 at which point it will transition to blue. |
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Last Updated on Wednesday, 08 June 2011 09:46 |
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