Q: A sample contains $$2.0$$ mol of water ($$\mathrm{H_2O}). $$How many molecules are present in the sample? (Use Avogadro's number: $$6.022 \times 10$$^{23}$$ molecules/mol)

$$1.2 \times 10$$^{24}$$ molecules

Question 1

Which of the following best describes a chemical, as used in chemistry?

Accepted Answer

Any substance that has a definite composition and is used in a laboratory.

Question 2

A student measures the mass of a sample as $$12.30 g $$and its volume as $$4.5 mL. $$What is the density of the sample, rounded to the correct number of significant figures?

Accepted Answer

$$2.7\ \mathrm{g/mL}$$

Question 3

Which of the following is an example of a heterogeneous mixture?

Accepted Answer

Salad dressing with oil and vinegar

Question 4

Which statement about the scientific method is correct?

Accepted Answer

It is a systematic approach involving observation, hypothesis, experiment, and conclusion.

Question 5

Convert $$0.000345 to $$scientific notation with the correct number of significant figures.

Accepted Answer

$$3.45 	imes 10$$^{-4}$$

Question 6

Which of the following statements about the Bohr and Quantum-Mechanical models of the atom is true?

Accepted Answer

The Bohr model describes electrons in fixed orbits, while the Quantum-Mechanical model describes electrons in probabilistic orbitals.

Question 7

Which of the following best describes an exothermic process?

Accepted Answer

A chemical reaction that releases energy to the surroundings, causing the temperature to increase.

Question 8

A sample contains $$2.0$$ mol of water ($$\mathrm{H_2O}). $$How many molecules are present in the sample? (Use Avogadro's number: $$6.022 	imes 10$$^{23}$$ molecules/mol)

Accepted Answer

$$1.2 	imes 10$$^{24}$$ molecules

Question 9

Which of the following elements is most likely to form a $$2+$$ ion?

Accepted Answer

Magnesium ($$\mathrm{Mg})$$

Question 10

Which of the following best explains why fluorine ($$\mathrm{F}) $$has the highest electronegativity on the periodic table?

Accepted Answer

Fluorine has a small atomic radius and a high effective nuclear charge, strongly attracting electrons.

Question 11

Which of the following is the correct electron configuration for a neutral atom of sodium ($$\mathrm{Na}$$, atomic number $$11$$)?

Accepted Answer

$$1s^2$$\ $$2s^2$$\ $$2p^6$$\ $$3s^1$$

Question 12

A compound is formed when $$16.0 g of $$oxygen reacts with $$2.0 g of $$hydrogen. What is the mass percent of hydrogen in the compound?

Accepted Answer

$$11\%$$

Question 13

Which of the following best describes a physical change?

Accepted Answer

Ice melting to form liquid water

Question 14

Which of the following is the correct Lewis structure for a chloride ion ($$\mathrm{Cl^-}$$)?

Accepted Answer

A Cl atom surrounded by 8 dots (valence electrons) and a $$-$$ charge outside the brackets

Question 15

Which of the following molecules is non-polar?

Accepted Answer

$$\mathrm{CO_2$}$$

Question 16

A student needs to convert $$25.0\ ^\circ\mathrm{C} to $$Kelvin. What is the correct answer, rounded to the correct number of significant figures?

Accepted Answer

$$298.2\ \mathrm{K}$$

Question 17

Which of the following statements about isotopes is correct?

Accepted Answer

Isotopes of an element have the same number of protons but different numbers of neutrons.

Question 18

Which of the following best describes Hund’s rule?

Accepted Answer

Electrons fill degenerate orbitals singly before pairing up.

Question 19

Which of the following is a correct application of the rules for significant figures in addition?

Accepted Answer

$$12.11 + 0.3 + 1.002 = 13.4$$

Question 20

Which of the following best explains why metals tend to form positive ions?

Accepted Answer

Metals have low ionization energies and lose electrons easily to achieve a stable electron configuration.

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