DAT Question of the Day

Correct Answer: D. it increases, then becomes constant.

A pressure cooker fixes the atmospheric pressure to be higher than normal. In order for a liquid to boil, the vapor pressure inside of the container must be at least equal to the atmospheric pressure. For higher atmospheric pressures, like in a pressure cooker, the vapor pressure must also increase. Therefore, it takes longer for the vapor pressure of the gas to accumulate, leaving the liquid to become hotter and hotter. Once the boiling point is reached, the liquid is hotter than normal, cooking the food faster. Once the vapor pressure equals the environmental pressure of the pressure cooker, it remains constant. At this point the liquid is boiling and the vapor pressure of the liquid is equal to the atmospheric pressure of the pressure cooker. In this case, the only answer option that aligns correctly to what occurs in a pressure cooker is [D].

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Correct Answer: C. A pointer on a meter or surface of a liquid in a buret or pipette is not observed from directly straight on.

All options are common laboratory errors, but Parallax Error may occur when a pointer on a meter or surface of a liquid in a buret or pipette is not observed from directly straight on. If they are observed from above or below then the liquid surface or pointer will not be seen to line up with the scale.

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Correct Answer: A. High entropy, exothermic reactions with a low activation energy.

TNT explosions are very fast reactions (the lower the activation energy the less a reaction is impeded) involving the production of enormous quantities of heat (exothermic) and gaseous products (the production of hot gases from a solid explosive dramatically increases the disorder of the system, thus high entropy).

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Correct Answer: E. Volumetric

• Beaker: holds liquids, should not be used for accurate measurement.
• Buret: accurately measures unknown volumes up to 50.00 mL.
• Volumetric Pipette: accurately measures known small volumes like; 5.00 ml, 10.00 mL, 20.00 mL, 25.00 mL and 50.00 mL.
• Measuring cylinder: good for measuring volumes up to a 2 L, but not as accurately as the…
• Volumetric flask: specifically prepared to measure larger known volumes with an accuracy of 1 mL in a Liter.

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Correct Answer: C. 3.

The reagents used are indicative of a hydroboration/oxidation reaction, which results in the net addition of water across a double bond. The boron adds to the less substituted carbon of the alkene, which then places the hydrogen on the more substituted carbon (anti-Markovnikov addition); this eliminates answer (A). Both, the boron and the hydrogen add simultaneously on the same face of the double bond (syn addition)−therefore the methyl group must be trans to the newly introduced borane (eliminate B, D). Answer E is an intermediate in the reaction; note that base and hydrogen peroxide are used to oxidize the borane into the hydroxyl final product (answer C).

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Correct Answer: E. The lower density of ice than the solution from which it forms.

The lowering of vapor pressure, elevation of the boiling point, depression of freezing point and increase in osmotic pressure with increasing solute concentration are all colligative properties of the solution, as they all depend on the concentration of solution only and not the type of solute.

The lower density of ice than the solution it forms from is due to hydrogen bonding between the water molecules in the solid ice holding the water molecules further apart than they are in the liquid state. This is true for all water solutions and pure water and does not depend on concentration.

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Correct Answer: C. 2, 3, 5

The strategy here is to identify planes of symmetry and identify the number of chemically unique carbons. Compounds 2, 3, and 5 have this criterion, and thus are possible structures for the mystery compound.

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Correct Answer: C. Intermolecular hydrogen bonding.

The atoms within water molecules are bonded together by covalent bonding, but it is the intermolecular hydrogen bonding thatprovides the strong forces of attraction between the molecules which gives water its relatively high (for its mass) melting point of 0 ^oC and boiling point of 100 ^oC.

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Correct Answer: A. 6.8 moles

This question requires the use of the ideal gas law: PV = nRT. We are solving for number of moles (n). The temperature in the equation is in Kelvin, so a 22° C room would actually be 295 K approximately. The pressure (P) at sea level is 1 atmosphere. So, (1 atm X 165 L) / (0.082 L atm/K mol X 295 K) = 6.8 moles

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Correct Answer: A. no reaction would occur

Noble gases are inert and do not react under normal conditions. Therefore, the reaction would not take place. In order for a gas to be produced, a chemical reaction would need to occur. The same is true for the color change and the formation of a precipitate as these signal that a chemical change has happened. However, these would not form/occur in this case because the two reactants would not react chemically.

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