DAT Question of the Day

Correct Answer: E. molecules absorb heat from the environment

When a liquid evaporates, it must have enough energy to break the intermolecular forces holding its particles together. To gain enough energy to do this, energy in the form of heat is absorbed from the surrounding environment. This is an endothermic process. If heat is absorbed from the environment, then the environment is left feeling cool as the liquid molecules begin to break their bonds/attractions and evaporate. It is true that in a solid the molecules remain in fixed locations and vibrate in place and liquid molecules slide across one another, however these do not explain the phenomenon of evaporative cooling.

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Correct Answer: C. Decreasing the OH- concentration in the solvent.

We see from the equation that the solubility is negatively affected by solvent content of OH. The more there is in the solvent, the less solubility the calcium hydroxide is. Thus, we can deduce that decreasing the OH concentration in the solvent will increase solubility.

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Correct Answer: C. 0.333 g/mL

Density is calculated by dividing the grams of a substance by the volume it displaces, in either mL or L. In this case the density would be calculated as: 7.000g / 21.0mL = 0.333 g/mL

Note: The correct answer must have 3 significant digits because the 21.0 mL number used in the calculation had 3 significant digits. The rule for multiplying and dividing significant digits is the answer must have the same number of significant digits as the number with the least amount of significant digits.

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Correct Answer: A. NaI

This is a limiting reagent question. Only one of the reactants can “limit” the amount of CaI2 produced, therefore answer options [C] and [D] are eliminated. Even though the compounds are aqueous, dissolved in water, water is not written in the equation and therefore should not have an effect on the amount of product produced, thus eliminating answer option [E].
Next, we setup two equations to see the amount of CaI2 both reactants can make. The reactant that makes the SMALLEST amount of CaI2 is our limiting reagent.

NaI produces less mols of our product, so it must be the limiting reagent.

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Correct Answer: B. both have 8 protons

¹⁸O and O^-2 are both forms of oxygen. The former is an isotope and the latter is an ion. They do not have the same number of neutrons, as an isotope has a different number of neutrons. Likewise the ion does not have the same number of electrons as the isotope because the ion has gained 2 electrons from its neutral form. The only thing in common between an isotope and an ion is the number of protons. The number of protons within the nucleus of an element does not change.

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Correct Answer: A. NO3^–

The base is a proton acceptor, and would therefore add a hydrogen to its chemical formula. Between the two compounds provided, NH4⁺ would behave as the acid, releasing hydrogen to get back to a zero charge and NO3^– would behave as the base accepting the hydrogen to return to its neutral state of HNO3.

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Correct Answer: B. SO4^2-

The conjugate base, according to Bronsted-Lowry’s definitions, is the compound that results when a H⁺ has been donated to the solution. If HSO4^– is the acid, then it will donate a H+ to form SO4^2-, which would be the conjugate base. It is important to note that the conjugate base would have a -2 charge since it is losing an proton (H⁺) leaving behind a -1 charge. This -1 charge can be added to the already existing -1 charge in the HSO4^– to make the total -2 charge.

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Correct Answer: D. Addition of the sodium acetate adds acetate ions to the solution and the acetic acid equilibrium reacts by reversing to consume some of the introduced acetate ions, thus the pH of the solution will approach 7.

According to Le Chatelier’s principle: “Any change that affects the position of an equilibrium will cause that equilibrium to shift, if possible, in such a way as to partially oppose the effect of that change.”

The addition of the sodium acetate adds acetate ions to the solution increasing the concentration of acetate ions. Thus the acetic acid equilibrium reacts by reversing to consume some of the introduced acetate ions, in doing so hydronium ions are also consumed and thus the solution becomes less acidic and pH of the solution will approach neutral, 7.

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Correct Answer: A.

Molarity = Moles per liter of solution.

Moles = mass/molecular mass = 100/(23.0 + 35.5)

Volume is in liters: 500ml = 0.500 Liters

Molarity =

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Correct Answer: C. HNO3 + H2O -> NO3– + H3O⁺

To act as a Bronsted-Lowry base water must accept a proton from an acid forming the hydronium ion, H3O⁺. This is the case in HNO3 + H2O -> NO3– + H3O⁺.

NH3 + H2O -> NH4⁺ + OH^– – water is acting as a Bronsted-Lowry acid by releasing a H⁺ and forming the hydroxide ion, OH^–.

NaCl(s) + H2O -> Na⁺(aq) + Cl^–(aq) – Water is acting as a solvent.

CuSO4(s) + 5 H2O -> CuSO4.5H2O(s) – Water is incorporating itself into the copper sulfate as water of hydration.

2 H2O -> H3O⁺ + OH^– – Water is undergoing a disproportionation reaction, acting both as an acid and a base to form H3O⁺ and OH^–.

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