Science Flashcards

A researcher measures the pH of a solution and finds it to be 3. If the researcher then dilutes the solution by a factor of 1000 with pure water, what is the approximate new pH?

Answer: 6

A pH of 3 means [H⁺] = 10⁻³ M. Diluting by a factor of 1000 (10³) reduces [H⁺] to 10⁻³ / 10³ = 10⁻⁶ M, giving a pH of 6. The solution cannot exceed pH 7 through dilution alone, as pure water is pH 7.

A gas occupies 4.0 L at 300 K and 2.0 atm. If the pressure is doubled and the temperature is increased to 450 K, what is the new volume?

Answer: 3.0 L

Using the combined gas law: V₂ = V₁ × (P₁/P₂) × (T₂/T₁) = 4.0 × (2.0/4.0) × (450/300) = 4.0 × 0.5 × 1.5 = 3.0 L.

Which of the following best explains why water has an unusually high boiling point relative to its molecular weight compared to other hydrides like H₂S and H₂Se?

Answer: Extensive hydrogen bonding between water molecules

Water exhibits extensive hydrogen bonding due to oxygen's high electronegativity and the presence of lone pairs, creating strong intermolecular forces. H₂S and H₂Se cannot form hydrogen bonds to the same extent, so they have much lower boiling points despite their greater molecular weights.

An object is thrown horizontally from the top of a cliff at 20 m/s. If it takes 4 seconds to reach the ground, how far from the base of the cliff does it land? (Ignore air resistance; g = 10 m/s²)

Answer: 80 m

Horizontal and vertical motions are independent. The horizontal distance = horizontal velocity × time = 20 m/s × 4 s = 80 m. The vertical drop = ½gt² = ½ × 10 × 16 = 80 m, confirming the 4-second fall, but the question asks for horizontal distance, which is 80 m.

In a closed ecosystem, a species of hawk (tertiary consumer) is suddenly removed. Which of the following is the most likely long-term consequence?

Answer: An increase in snake populations leading to a decrease in rodents, partially restoring balance

Removing a top predator (trophic cascade) allows its prey — such as snakes — to increase. Snakes then prey more heavily on rodents, reducing rodent populations, which may partially compensate for the hawk's absence. This illustrates indirect trophic cascades and the complexity of food webs.

A radioactive isotope has a half-life of 10 years. A sample initially contains 80 grams of the isotope. After 40 years, how many grams of the original isotope remain?

Answer: 5 g

40 years = 4 half-lives (40 ÷ 10 = 4). After each half-life the amount halves: 80 → 40 → 20 → 10 → 5 g. After 4 half-lives, 80 × (1/2)⁴ = 80/16 = 5 g remains.