1. According to the Arrhenius theory, an acid is a substance that:

A)    Changes litmus from red to blue

B)    Changes phenolphthalein from colorless to pink

C)    Produces hydronium ions as the only positive ions in an aqueous solution

D)    Produces hydroxide ions as the only negative ions in an aqueous solution

Correct Answer: Option C - Produces hydronium ions as the only positive ions in an aqueous solution

According to Arrhenius:

Acid is any substance which delivers hydrogen ion (H+) to the solution.

Here is a generic acid dissociating, according to Arrhenius:

HA à H⁺ + A¯

Hence, options A, B and D are incorrect.

1. An acidic solution could have a pH of:

A)    3.5

B)    7

C)    10

D)    14

Correct Answer: Option A - 3.5

pH is a measure of the acidity or basicity of a solution. It is defined as the cologarithm of the activity of dissolved hydrogen ions (H+). Hydrogen ion activity coefficients cannot be measured experimentally, so they are based on theoretical calculations. The pH scale is not an absolute scale; it is relative to a set of standard solutions whose pH is established by international agreement

Pure water is said to be neutral. The pH for pure water at 25 °C (77 °F) is close to 7.0. Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are said to be basic or alkaline.

Hence, options B, C and D are incorrect.

Base your answers to questions 3 and 4 on the information below:

A 150 gram liquid sample of stearic acid, C₁₇H₃₅COOH, is cooled at a constant rate.

The temperature of the sample is recorded at 2-minute intervals in the data table below.

Cooling data for Stearic Acid

3. Identify the physical change occurring between the time interval 4 minutes to 10 minutes.

Correct Answer: Crystallization / Solidification

The melting point of stearic acid is 69.6^oC. Between the time intervals 4 minutes and 10 minutes the temperature is 69.3^oC which is below the melting point. Hence, in this time interval stearic acid undergoes crystallization / solidification.

Base your answers to questions 3 and 4 on the information below:

A 150 gram liquid sample of stearic acid, C₁₇H₃₅COOH, is cooled at a constant rate.

The temperature of the sample is recorded at 2-minute intervals in the data table below.

Cooling data for Stearic Acid

4. On a grid mark an appropriate scale on the axis labeled "Temperature (°C). Plot the data from the data table.

Correct Answer:

5. What is the pH of a solution with a hydronium ion concentration of 0.001 mole per liter?

A)    5

B)    7

C)    9

D)    3

Correct Answer: Option D - 3

pH is a measure of the acidity or basicity of a solution. It is defined as the co-logarithm of the activity of dissolved hydrogen ions (H+). Hydrogen ion activity coefficients cannot be measured experimentally, so they are based on theoretical calculations.

A 0.001 molar solution has a H+ concentration of 10^-3 (move the decimal point 3 places to the right). The value of the negative exponent (-3) gives a pH of 3.

Hence, options A, B and C are incorrect.

6. Which barium salt is insoluble in water?

A)    BaCO₃

_B) Ba(ClO₄)₂

C)    BaCl₂

_D) Ba(NO₃)₂

Correct Answer: Option A - BaCO₃

Barium carbonate (BaCO₃), also known as witherite, is a chemical compound used in rat poison, bricks, ceramic glazes and cement. It is a white powder with a melting point of 174°C; soluble in acids (except sulfuric acid); used in rodenticides, ceramic flux, optical glass, and television picture tubes.

Witherite crystallizes in the orthorhombic system. The crystals are invariably twinned together in groups of three, giving rise to pseudo-hexagonal forms somewhat resembling bipyramidal crystals of quartz, the faces are usually rough and striated horizontally.

Barium carbonate reacts with many acids to soluble barium salts, for example barium chloride:

BaCO₃(s) + 2 HCl(aq) à BaCl₂(aq) + CO₂(g) + H₂O(l)

However the reaction with sulfuric acid is poor, because barium sulfate is highly insoluble.

Hence, options B, C and D are incorrect.

7. Which substance is an Arrhenius acid?

A)    Ba(OH)₂

_B) H₃PO₄

C)    CH₃COOCH₃

D)    NaCl

Correct Answer: Option B - H₃PO₄

An Arrhenius acid is a substance that when added to water increases the concentration of H¹⁺ ions present. The chemical formulas of Arrhenius acids are written with the acidic hydrogens first.   The H1+ ion produced by an Arrhenius acid is always associated with a water molecule to form the hydronium ion, H3O¹⁺(aq). Arrhenius acids are frequently referred to as proton donors, hydrogen ion donors, or hydronium ion donors, depending on whether we are trying to emphasize the species liberated by the acid (proton or hydrogen ion) or the species present in solution (hydronium ion).

Phosphoric acid, also known as orthophosphoric acid or phosphoric (V) acid, is a mineral (inorganic) acid having the chemical formula H₃PO₄. Orthophosphoric acid molecules can combine with themselves to form a variety of compounds which are also referred to as phosphoric acids, but in a more general way. The term phosphoric acid can also refer to a chemical or reagent consisting of phosphoric acids, usually orthophosphoric acid.

Pure anhydrous phosphoric acid is a white solid that melts at 42.35 °C to form a colorless, viscous liquid. Triprotic means that an orthophosphoric acid molecule can dissociate up to three times, giving up an H⁺ each time, which typically combines with a water molecule, H₂O, as shown in the reaction:

H₃PO₄(s)   + H₂O(l) is in equilibrium with H₃O+(aq) + H₂PO₄^-(aq)

Hence, options A, C and D are incorrect.

8. Which compound releases hydroxide ions in an aqueous solution?

A)    CH₃COOH

B)    HCl

C)    CH₃OH

D)    KOH

Correct Answer: Option D - KOH

Hydronium is the common name for the aqueous cation H3O+, the simplest type of oxonium ion, produced by protonation of water. It is the positive ion present when an Arrhenius acid is dissolved in water, as Arrhenius acid molecules in solution give up a proton (a positive hydrogen ion, H+) to the surrounding water molecules (H2O).The pH of a solution is a measure of its hydrogen ion concentration. As free protons react with water to form hydronium, the acidity of an aqueous solution is determined by its hydronium concentration.

Potassium hydroxide is the inorganic compound with the formula KOH. Along with sodium hydroxide, this colourless solid is a prototypical "strong base". KOH is highly basic, forming strongly alkali solutions in water and other polar solvents.

Dis-association reaction is give by:

KOH → K⁺ + OH^-

Hence, options A, B and C are incorrect.

9. Which indicator, when added to a solution, changes color from yellow to blue as the pH of the solution is changed from 5.5 to 8.0?

A)    Bromocresol green

B)    Litmus

C)    Bromothymol blue

D)    Methyl orange

Correct Answer: Option C - Bromothymol blue

A pH indicator is a halochromic chemical compound that is added in small amounts to a solution so that the pH (acidity or alkalinity) of the solution can be determined visually. Hence a pH indicator is a chemical detector for hydronium ions (H₃O⁺) (or Hydrogen ions (H⁺) in the Arrhenius model). Normally, the indicator causes the color of the solution to change depending on the pH. At 25 degrees Celsius, considered the standard temperature, the pH value of a neutral solution is 7.0. Solutions with a pH value below 7.0 are considered acidic, whereas solutions with pH value above 7.0 are alkali.

Bromothymol blue (also known as bromothymol sulfone phthalein, Bromthymol Blue, and BTB) is a chemical indicator for weak acids and bases. The chemical is also used for observing photosynthetic activities or respiratory indicators (turns green then yellow as CO₂ is added).

Bromothymol blue acts as a weak acid in solution. It can thus be in protonated or deprotonated form, appearing yellow and blue respectively. It is bluish green in neutral solution. It is typically sold in solid form as the sodium salt of the acid indicator. Bromothymol blue is mostly used in measuring substances that would have relatively low acidic or basic levels (near a neutral pH). It is often used in managing the pH of pools and fish tanks, and for measuring the presence of carbonic acid in a liquid.

10. The pH of an aqueous solution changes from 4 to 3 when the hydrogen ion concentration in the solution is

A)    Decreased by a factor of 4/3

B)    Decreased by a factor of 10

C)    Increased by a factor of 3/4

D)    Increased by a factor of 10

Correct Answer: Option D - Increased by a factor of 10

pH is a measure of the acidity or basicity of a solution. It is defined as the cologarithm of the activity of dissolved hydrogen ions (H+). Hydrogen ion activity coefficients cannot be measured experimentally, so they are based on theoretical calculations. The pH scale is not an absolute scale; it is relative to a set of standard solutions whose pH is established by international agreement.

Pure water is said to be neutral. The pH for pure water at 25 °C (77 °F) is close to 7.0. Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are said to be basic or alkaline.

pH is defined as minus the decimal logarithm of the hydrogen ion activity in an aqueous solution. By virtue of its logarithmic nature, pH is a dimensionless quantity.

where a_H is the (dimensionless) activity of hydrogen ions. The reason for this definition is that a_H is a property of a single ion which can only be measured experimentally by means of an ion-selective electrode which responds, according to the Nernst equation, to hydrogen ion activity.

Hence, each time you go down 1 on the pH scale, the concentration of hydrogen/ hydronium ions increases by 10.

Hence, options A, B and C are incorrect.