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Chemistry Regents June 2010 - Question 79 PDF Print E-mail
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.
Last Updated on Wednesday, 08 June 2011 09:46
 
Chemistry Regents June 2010 - Question 78 PDF Print E-mail
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.
Last Updated on Wednesday, 08 June 2011 09:45
 
Chemistry Regents June 2010 - Question 77 PDF Print E-mail
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+]
 
Chemistry Regents June 2010 - Question 49 PDF Print E-mail
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.
 
Chemistry Regents June 2010 - Question 26 PDF Print E-mail
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.
 
Chemistry Regents June 2010 - Question 25 PDF Print E-mail
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
 
Chemistry Regents June 2005 - Question 76 PDF Print E-mail
Written by The Chemistry Wizard   
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Correct Answer: 3.25x10-3M

The above reaction is categorized as acid base reactions. This type of reaction is usually the result of an acid (H+ proton donor) reacting with a base (OH- proton acceptor) to form products and water according to the equation;

Using the mole theory we know that 1 mole of H+ will react completely with 1 mole of OH-. Based on this and the information in the question above, it is then possible to calculate the concentration of H+ ion in the acid rain.

Answering the question:

The molarity of the H+ in the acid rain can be determined by calculating the number of moles of NaOH used to neutralize it.

0.010M of NaOH contains 0.010 moles of NaOH in 1000ml

Therefore 1ml of solution would contain 0.010/1000 moles or 0.00001moles

This means 6.5ml would contain 6.5 x 0.00001 or 0.000065 moles or 6.5x10-5 moles

Now based on the fact that 20ml of acid rain was neutralized by 6.5ml of NaOH, and the relationship of H+ and NaOH follows the ratio;

Thus 1 mole of H+ requires 1 mole of NaOH to completely react.

Therefore 20.0-milliliters of acid rain contains 6.5x10-5 moles of H+ ions. The molarity of H+ is the number of moles in 1000ml is 1000/20 x 6.5x10-5 moles which is equal to 0.00325M or 3.25x10-3M

Giving an answer of 3.25x10-3M

 
Chemistry Regents June 2005 - Question 75 PDF Print E-mail
Written by The Chemistry Wizard   
chemjune05q75

Correct Answer: 6

The pH is a measure of the hydrogen ion (H+) concentration or acidity of a system. The equation below shows the relationship between pH and H+ concentration in moles. If the pH of a system is less than 7 it acidic, if it is greater than 7 then it is basic.

pH = -log[H+]

H+ = 10-pH

Answering the Question:

Since the original pH of the soil was given as 4, the concentration of H+ can be calculated.

H+ = 10-4

Therefore the concentration of H+ = 0.0001M or 1x10-4M

This can be calculated using a scientific calculator by typing -4 and pressing the inv. Log or 2nd func Log key.

After the addition of quicklime the H+ ion concentration fell 100 fold.

Therefore the new soil concentration is 0.0001/100 = 0.000001M or 1x10-6

This mean our new soil pH would be:

pH = -log[1x10-6]

pH = 6

The new pH of the soil sample is 6.

 
Chemistry Regents June 2005 - Question 74 PDF Print E-mail
Written by The Chemistry Wizard   
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Correct answer:

When working on chemical equations, just like mathematical equations, both sides of the equation should be equal. For a chemical equation that means the number of moles of each reacting element must be equal to the number of moles for each element in the product. In order for this to occur it means that not all chemical reactions will occur in a one to one ratio.

Answering the Question:

When balancing a chemical equation it is important to know how electrons are transferred between reactants. An understanding of the trends in the periodic table and knowing the group that elements belong simplifies this process.

For the above equation knowing that Ca belongs to the alkali earth metals and is a part of group 2 of the periodic table is valuable knowledge. This means that Ca has two electrons in its valence orbital and is readily ionized forming Ca2+. However since a chemical equation is given it means the quantity of each elements on the left of the reaction should be equal to the quantity on the right. This means if you have 4 hydrogen atoms on the left of the equation then you should have 4 hydrogen atoms accounted for on the right.

 
Chemistry Regents June 2005 - Question 50 PDF Print E-mail
Written by The Chemistry Wizard   
chemjune05q50

Correct answer: (2)

A thymol blue indicator, also called thymolsulphonephthalein, serves as a pH indicator.  It is in crystalline powder form with its color ranging from brownish-green to reddish-brown.  At pH condition 1.2 to 2.8, it transitions from red to yellow and at pH 8.0 to 9.6, its color changes from yellow to blue.

Answering the question:

First, in order for thymol blue indicator to turn blue, the solution should be basic, as mentioned above, the color change to blue occurs at pH 8.0 to 9.6. We should be familiar with answers (3) and (4) as common acids that are used throughout the laboratory, this would immediately exclude them. Answer (1) known as ethanoic acid (vinegar) may not be as familiar but has the functional group RCOOH, this is the functional group for organic acids, which also have a low pH. Answer (2) is KOH which should also be a familiar base similar to NaOH. Although you may not know the exact pH range it is the only base in the group and therefore the best possible answer.

 
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