Chemistry Flashcards

Which substance has the greatest chance of being a solid at STP?

Answer: Na

At Standard Temperature and Pressure (STP), which is 0°C (273.15 K) and 1 atm (101.325 kPa), sodium (Na) is a solid metal. Krypton (Kr) and Xenon (Xe) are noble gases, and ammonia (NH3) is a gas at STP. Therefore, sodium is the substance most likely to be a solid under these conditions.

Which of the following actions might raise a solution's freezing point? I. Divide the mixture between two different containers. II. To boil off some of the solvent, heat the solution. III. Start a reaction that will result in some of the solute precipitating.

Answer: III

The freezing point of a solution is affected by the concentration of the solute. Removing solute from a solution, such as by precipitation (III), effectively lowers the solute concentration. A lower solute concentration means the solution is closer to being pure solvent, which would raise its freezing point towards that of the pure solvent. Boiling off solvent (II) would increase solute concentration, lowering the freezing point, and dividing the mixture (I) doesn't change the concentration.

Cooling occurs to a gas with constant volume. Which gas-related claim must be accurate?

Answer: The gas molecules' kinetic energy has decreased.

Temperature is a direct measure of the average kinetic energy of the molecules within a substance. When a gas is cooled, its temperature decreases, which means the average kinetic energy of its constituent molecules also decreases. This reduction in molecular motion is the fundamental reason for the drop in temperature.

At STP, two moles of hydrogen and one mole of oxygen gas are mixed together in a closed container. Which of the ensuing assertions is accurate?

Answer: Greater than partial pressure of oxygen is partial pressure of hydrogen.

According to Dalton's Law of Partial Pressures, the partial pressure of a gas in a mixture is proportional to its mole fraction. Since there are two moles of hydrogen and one mole of oxygen, hydrogen has a greater mole fraction (2/3 vs 1/3). Therefore, the partial pressure of hydrogen will be greater than the partial pressure of oxygen.

What link between two different materials can be best determined using Graham's law?

Answer: Mass and Diffusion Rate

Graham's Law of Diffusion states that the rate of diffusion or effusion of a gas is inversely proportional to the square root of its molar mass. This law is specifically used to compare the diffusion rates of two different gases based on their masses. Therefore, it best determines the link between mass and diffusion rate for different materials (gases).

Which is the proper sequence for increasing the attraction forces between molecules?

Answer: London dispersion < Dipole - dipole < Hydrogen bonding < Ionic

This sequence correctly orders the intermolecular and intramolecular forces by increasing strength. London dispersion forces are the weakest, present in all molecules. Dipole-dipole forces are stronger and occur between polar molecules. Hydrogen bonding is a particularly strong type of dipole-dipole interaction, and ionic bonds are generally the strongest of these forces, often considered intramolecular but also influencing intermolecular interactions in ionic compounds.

At 25 °C and 101 kPa of pressure, 1,000 mol of He gas is inflated into a weather balloon. What is the weather balloon's volume?

Answer: 24.5 m3

This problem is solved using the Ideal Gas Law, PV = nRT. Given n = 1000 mol, T = 25 °C (298.15 K), and P = 101 kPa, we use the gas constant R = 8.314 kPa·L/(mol·K). Calculating V = nRT/P gives V = (1000 mol * 8.314 kPa·L/(mol·K) * 298.15 K) / 101 kPa = 24518.5 L. Converting this to cubic meters (1 m³ = 1000 L) yields 24.5 m³.

A mole of an ideal gas is heated to 25 °C and compressed to 10 L. What is the gas's pressure?

Answer: 2.4 atm

This problem is solved using the Ideal Gas Law, PV = nRT. Given n = 1 mole, V = 10 L, and T = 25 °C (298.15 K), we need to find P. Using the gas constant R = 0.0821 L·atm/(mol·K), we calculate P = nRT/V = (1 mol * 0.0821 L·atm/(mol·K) * 298.15 K) / 10 L. This results in P = 2.447 atm, which rounds to 2.4 atm.

4 moles of oxygen, 3 moles of nitrogen, and 7 moles of neon are present in a 10 L cylinder. The cylinder's temperature is raised from 20°C to 40°C. Calculate the percentage of the total neon partial pressure in the cylinder.

Answer: 50%

According to Dalton's Law of Partial Pressures, the partial pressure of a gas in a mixture is proportional to its mole fraction. The total number of moles in the cylinder is 4 (oxygen) + 3 (nitrogen) + 7 (neon) = 14 moles. The number of moles of neon is 7, so its mole fraction is 7/14 = 0.5. Therefore, neon's partial pressure constitutes 50% of the total pressure, regardless of the temperature change or volume.

Each flask containing the three liquids X, Y, and Z is suspended in a water bath that is 75°C. Each liquid's boiling point is as follows: Z, 360 K X, 273 K Y, 340 K Which of the three liquids will reach boiling point at a temperature of 75 °C?

Answer: X and Y

A liquid boils when its vapor pressure equals the surrounding atmospheric pressure, which occurs at or above its boiling point. The water bath is at 75°C (348.15 K). Liquid X has a boiling point of 273 K (0°C), and liquid Y has a boiling point of 340 K (67°C); both are below 75°C, so they will boil. Liquid Z has a boiling point of 360 K (87°C), which is above 75°C, so it will not boil.

Which of the following claims regarding liquids' and gases' physical characteristics is accurate? I. Both gases and liquids can be compressed. II. Gases do not flow, but liquids do. III. Gases are incompressible and liquids flow. IV. Gases can be compressed and liquids flow. V. Liquids are incompressible and gases flow.

Answer: IV and V

Gases are highly compressible because their molecules are far apart, allowing for significant reduction in volume under pressure. Liquids, on the other hand, are generally considered incompressible because their molecules are already closely packed, leaving little empty space. Both gases and liquids are fluids, meaning they can flow. Therefore, the accurate claims are that gases can be compressed and liquids flow, and liquids are incompressible and gases flow.

Which of the following assertions best sums up the periodic table of elements' electronegativity trend?

Answer: Electronegativity rises from bottom to top and from left to right.

Electronegativity is a measure of an atom's ability to attract electrons in a chemical bond. On the periodic table, electronegativity generally increases as you move from left to right across a period (due to increasing nuclear charge and decreasing atomic radius) and decreases as you move down a group (due to increasing atomic radius and shielding effect). Therefore, it rises from bottom to top and from left to right, with fluorine being the most electronegative element.

At 110 °C and 1 atm of pressure, a cylinder contains 1 mole of argon and 1 mole of water. The cylinder's temperature drops to -5°C. Which of the following statements regarding the cylinder's contents is truest?

Answer: The partial pressure of argon is substantially greater than that of water, and the pressure inside the cylinder decreases.

When the temperature drops significantly from 110°C to -5°C, water will undergo a phase change, condensing and then freezing into a solid, effectively removing almost all water vapor from the gas phase. Argon, however, remains a gas at -5°C as its boiling point is much lower. Consequently, the partial pressure of argon will be substantially greater than that of water, and the overall reduction in the number of gas moles (from 2 moles to approximately 1 mole of argon) will cause the total pressure inside the cylinder to decrease.

Holding a liquid at its freezing point and allowing it to slowly harden. Which of the following descriptions of this incident is accurate?

Answer: While freezing, heat is given off by the material.

Freezing is an exothermic phase transition where a substance changes from a liquid to a solid state. During this process, the molecules arrange themselves into a more ordered, lower-energy structure, releasing energy in the form of heat to the surroundings. While the material's temperature remains constant at its freezing point during the phase change, heat is continuously given off until all the liquid has solidified.

It is heated from 50° C to 80 ° C. Which of the following assertions regarding the solubility of solids and gases in the liquid is generally accurate?

Answer: The solubility of gases will decline while that of solids will increase.

Generally, the solubility of most solids in a liquid increases with rising temperature. This is because increased kinetic energy helps overcome intermolecular forces in the solid and facilitates better solvation. Conversely, the solubility of gases in a liquid typically decreases as temperature increases, because the higher kinetic energy allows gas molecules to escape from the liquid phase more readily into the atmosphere.

400 mL of solution are created once 100 g of H3PO4 are dissolved in water. What is the solution's normality?

Answer: 7.65 N

Normality (N) is calculated as equivalents of solute per liter of solution. Phosphoric acid (H3PO4) is a triprotic acid, meaning it can donate three H+ ions per molecule, so one mole equals three equivalents. First, calculate moles of H3PO4 (100g / 98 g/mol ≈ 1.02 moles), then multiply by 3 to get equivalents (1.02 moles * 3 = 3.06 equivalents). Finally, divide the equivalents by the volume in liters (3.06 equivalents / 0.4 L) to arrive at 7.65 N.